Chapter IV…

Chapter IV

USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD--CULTIVATION OF THE OYSTER, MUSSEL, AND SNAIL--SNAILS AS MEDICINE--PRICES GIVEN FOR SHELLS

The employment of shells as a medium of exchange was exceedingly common amongst uncivilised tribes in all parts of the world, and has by no means yet become obsolete. One of the commonest species thus employed is the ‘money cowry’ (Cypraea moneta, L.), which stands almost alone in being used entire, while nearly all the other forms of shell money are made out of portions of shells, thus requiring a certain amount of labour in the process of formation.

One of the earliest mentions of the cowry as money occurs in an ancient Hindoo treatise on mathematics, written in the seventh century A.D. A question is propounded thus: ‘the ¼ of 1/16 of ⅕ of ¾ of ⅔ of ½ a dramma was given to a beggar by one from whom he asked an alms; tell me how many cowry shells the miser gave.’ In British India about 4000 are said to have passed for a shilling, but the value appears to differ according to their condition, poor specimens being comparatively worthless. According to Reeve[204] a gentleman residing at Cuttack is said to have paid for the erection of his bungalow entirely in cowries. The building cost him 4000 Rs. sicca (about £400), and as 64 cowries = 1 pice, and 64 pice = 1 rupee sicca, he paid over 16,000,000 cowries in all.

Cowries are imported to England from India and other places for the purposes of exportation to West Africa, to be exchanged for native products. The trade, however, appears to be greatly on the decrease. At the port of Lagos, in 1870, 50,000 cwts. of cowries were imported.[205]

A banded form of Nerita polita was used as money in certain parts of the South Pacific. The sandal-wood imported into the China market is largely obtained from the New Hebrides, being purchased of the natives in exchange for Ovulum angulosum, which they especially esteem as an ornament. Sometimes, as in the Duke of York group, the use of shell money is specially restricted to certain kinds of purchase, being employed there only in the buying of swine.

Among the tribes of the North-West coasts of America the common Dentalium indianorum used to form the standard of value, until it was superseded, under the auspices of the Hudson’s Bay Company, by blankets. A slave was valued at a fathom of from 25 to 40 of these shells, strung lengthwise. Inferior or broken specimens were strung together in a similar way, but were less highly esteemed; they corresponded more to our silver and copper coins, while the strings of the best shells represented gold.

The wampum of the eastern coast of North America differed from all these forms of shell money, in that it required a laborious process for its manufacture. Wampum consisted of strings of cylindrical beads, each about a quarter of an inch in length and half that breadth. The beads were of two colours, white and purple, the latter being the more valuable. Both were formed from the common clam, Venus mercenaria, the valves of which are often stained with purple at the lower margins, while the rest of the shell is white. Cut small, ground down, and pierced, these shells were converted into money, which appears to have been current along the whole sea-board of North America from Maine to Florida, and on the Gulf Coast as far as Central America, as well as among the inland tribes east of the Mississippi. Another kind of wampum was made from the shells of Busycon carica and B. perversum. By staining the wampum with various colours, and disposing these colours in belts in various forms of arrangement, the Indians were able to preserve records, send messages, and keep account of any kind of event, treaty, or transaction.

Another common form of money in California was Olivella biplicata, strung together by rubbing down the apex. Button-shaped disks cut from Saxidomus arata and Pachydesma crassatelloides, as well as oblong pieces of Haliotis, were employed for the same purpose, when strung together in lengths of several yards.

“There is a curious old custom,” writes Mr. W. Anderson Smith,[206] “that used formerly to be in use in this locality [the western coast of Scotland], and no doubt was generally employed along the sea-board, as the most simple and ready means of arrangement of bargains by a non-writing population. That was, when a bargain was made, each party to the transaction got one half of a bivalve shell--such as mussel, cockle, or oyster--and when the bargain was implemented, the half that fitted exactly was delivered up as a receipt! Thus a man who had a box full of unfitted shells might be either a creditor or a debtor; but the box filled with fitted shells represented receipted accounts. Those who know the difficulty of fitting the valves of some classes of bivalves will readily acknowledge the value of this arrangement.”

Shells are employed for use and for ornament by savage--and even by civilised--tribes in all parts of the world. The natives of Fiji thread the large Turbo argyrostoma and crenulatus as weights at the edge of their nets, and also employ them as sinkers. A Cypraea tigris cut into two halves and placed round a stone, with two or three showy Oliva at the sides, is used as a bait for cuttles. Avicula margaritifera is cut into scrapers and knives by this and several other tribes. Breast ornaments of Chama, grouped with Solarium perspectivum and Terebra duplicata are common among the Fijians, who also mount the Avicula on a backing of whales’ teeth sawn in two, for the same purpose. The great Orange Cowry (Cypraea aurantiaca) is used as a badge of high rank among the chieftains. One of the most remarkable Fijian industries is the working of whales’ teeth to represent this cowry, as well as the commoner C. talpa, which is more easily imitated.

Among the Solomon islanders, cowries are used to ornament their shields on great field days, and split cowries are worn as a necklace, to represent human teeth. Small bunches of Terebellum subulatum are worn as earrings, and a large valve of Avicula is employed as a head ornament in the centre of a fillet. The same islanders ornament the raised prows of their canoes, as well as the inside of the stern-post, with a long row of single Natica.

The native Papuans employ shells for an immense variety of purposes. Circlets for the head are formed of rows of Nassa gibbosula, rubbed down till little but the mouth remains. Necklaces are worn which consist of strings of Oliva, young Avicula, Natica melanostoma, opercula of Turbo, and valves of a rich brown species of Cardium, pendent at the end of strings of the seeds known as Job’s tears. Struthiolaria is rubbed down until nothing but the mouth is left, and worn in strings round the neck. This is remarkable, since Struthiolaria is not a native Papuan shell, and indeed occurs no nearer than New Zealand. Sections of Melo are also worn as a breast ornament, dependent from a necklace of cornelian stones. Cypraea erosa is used to ornament drinking bowls, and Ovulum ovum is attached to the native drums, at the base of a bunch of cassowary feathers, as well as being fastened to the handle of a sago-beater.

In the same island, the great Turbo and Conus millepunctatus are ground down to form bracelets, which are worn on the biceps. The crimson lip of Strombus luhuanus is cut into beads and perforated for necklaces. Village elders are distinguished by a single Ovulum verrucosum, worn in the centre of the forehead. The thick lip of Cassis cornuta is ground down to form nose pieces, 4½ inches long. Fragments of a shell called Kaïma (probably valves of a large Spondylus) are worn suspended from the ears, with little wisps of hair twisted up and thrust through a hole in the centre. For trumpets, Cassis cornuta, Triton tritonis, and Ranella lampas are used, with a hole drilled as a mouthpiece in one of the upper whorls. Valves of Batissa, Unio, and Mytilus are used as knives for peeling yams. Spoons for scooping the white from the cocoa-nut are made from Avicula margaritifera. Melo diadema is used as a baler in the canoes.[207]

In the Sandwich Islands Melampus luteus is worn as a necklace, as well as in the Navigator Islands. A very striking necklace, in the latter group, is formed of the apices of a Nautilus, rubbed down to show the nacre. The New Zealanders use the green opercula of a Turbo, a small species of Venus, and Cypraea asellus to form the eyes of their idols. Fish-hooks are made throughout the Pacific of the shells of Avicula and Haliotis, and are sometimes strengthened by a backing made of the columella of Cypraea arabica. Small axe-heads are made from Terebra crenulata ground down (Woodlark I.), and larger forms are fashioned from the giant Tridacna (Fiji).

Shells are used to ornament the elaborate cloaks worn by the women of rank in the Indian tribes of South America. Specimens of Ampullaria, Orthalicus, Labyrinthus, and Bulimulus depend from the bottom and back of these garments, while great Bulimi, 6 inches long, are worn as a breast ornament, and at the end of a string of beads and teeth.[208]

The chank-shell (Turbinella rapa) is of especial interest from its connexion with the religion of the Hindoos. The god Vishnu is represented as holding this shell in his hand, and the sinistral form of it, which is excessively rare, is regarded with extraordinary veneration. The chank appears as a symbol on the coins of some of the ancient Indian Empires, and is still retained on the coinage of the Rajah of Travancore.

The chief fishery of the chank-shell is at Tuticorin, on the Gulf of Manaar, and is conducted during the N. E. monsoon, October-May. In 1885–86 as many as 332,000 specimens were obtained, the net amount realised being nearly Rs.24,000. In former days the trade was much more lucrative, 4 or 5 millions of specimens being frequently shipped. The government of Ceylon used to receive £4000 a year for licenses to fish, but now the trade is free. The shells are brought up by divers from 2 or 3 fathoms of water. In 1887 a sinistral specimen was found at Jaffna, which sold for Rs.700.[209] Nearly all the shells are sent to Dacca, where they are sliced into bangles and anklets to be worn by the Hindoo women.

Perhaps the most important industry which deals only with the shells of Mollusca is that connected with the ‘pearl-oyster.’ The history of the trade forms a small literature in itself. It must be sufficient here to note that the species in question is not an ‘oyster,’ properly so called, but an Avicula (margaritifera Lam.). The ‘mother-of-pearl,’ which is extensively employed for the manufacture of buttons, studs, knife-handles, fans, card-cases, brooches, boxes, and every kind of inlaid work, is the internal nacreous laminae of the shell of this species. The most important fisheries are those of the Am Islands, the Soo-loo Archipelago, the Persian Gulf, the Red Sea, Queensland, and the Pearl Islands in the Bay of Panama. The shell also occurs in several of the groups of the South Pacific--the Paumotu, Gambier and Navigator Islands, Tahiti being the centre of the trade--and also on the coasts of Lower California.[210]

Pearls are the result of a disease in the animal of this species of Avicula and probably in all other species within which they occur. When the Avicula is large, well formed, and with ample space for individual development, pearls scarcely occur at all, but when the shells are crowded together, and become humped and distorted, as well as affording cover for all kinds of marine worms and parasitic creatures, then pearls are sure to be found. Pearls of inferior value and size are also produced by Placuna placenta, many species of Pinna, the great Tridacna, the common Ostrea edulis, and several other marine bivalves. They are not uncommon in Unio and Anodonta, and the common Margaritana margaritifera of our rapid streams is still said to be collected, in some parts of Wales, for the purpose of extracting its small ‘seed-pearls.’ Pink pearls are obtained from the giant conch-shell of the West Indies (Strombus gigas), as well as from certain Turbinella.

In Canton, many houses are illuminated almost entirely by skylights and windows made of shells, probably the semitransparent valves of Placuna placenta. In China lime is commonly made of ground cockle-shells, and, when mixed with oil, forms an excellent putty, used for cementing coffins, and in forming a surface for the frescoes with which the gables of temples and private houses are adorned. Those who suffer from cutaneous diseases, and convalescents from small-pox, are washed in Canton with the water in which cockles have been boiled.[211]

A recent issue of the Peking Gazette contains a report from the outgoing Viceroy of Fukhien, stating that he had handed over the insignia of office to his successor, including inter alia the conch-shell bestowed by the Throne. A conch-shell with a whorl turning to the right, i.e. a sinistral specimen, is supposed when blown to have the effect of stilling the waves, and hence is bestowed by the Emperor upon high officers whose duties oblige them to take voyages by sea. The Viceroy of Fukhien probably possesses one of these shells in virtue of his jurisdiction over Formosa, to which island periodical visits are supposed to be made.[212]

Shells appear to be used occasionally by other species besides man. Oyster-catchers at breeding time prepare a number of imitation nests in the gravel on the spit of land where they build, putting bits of white shell in them to represent eggs.[213] This looks like a trick in order to conceal the position of the true nest. According to Nordenskjöld, when the eider duck of Spitzbergen has only one or two eggs in its nest, it places a shell of Buccinum glaciale beside them. The appropriation of old shells by hermit-crabs is a familiar sight all over the world. Perhaps it is most striking in the tropics, where it is really startling, at first experience, to meet--as I have done--a large Cassis or Turbo, walking about in a wood or on a hill side at considerable distances from the sea. A Gephyrean (Phascolion strombi) habitually establishes itself in the discarded shells of marine Mollusca. Certain Hymenoptera make use of dead shells of Helix hortensis in which they build their cells.[214] Magnus believes that in times when heavy rains prevail, and the usual insects do not venture out, certain flowers are fertilised by snails and slugs crawling over them, e.g. Leucanthemum vulgare by Limax laevis.[215]

=Mollusca as Food for Man.=--Probably there are few countries in the world in which less use is made of the Mollusca as a form of food than in our own. There are scarcely ten native species which can be said to be at all commonly employed for this purpose. Neighbouring countries show us an example in this respect. The French, Italians, and Spanish eat Natica, Turbo, Triton, and Murex, and, among bivalves, Donax, Venus, Lithodomus, Pholas, Tapes, and Cardita, as well as the smaller Cephalopoda. Under the general designation of clam the Americans eat Venus mercenaria, Mya arenaria, and Mactra solidissima. In the Suez markets are exposed for sale Strombus and Melongena, Avicula and Cytherea. At Panama Donax and Solen are delicacies, while the natives also eat the great Murex and Pyrula, and even the huge Arca grandis, which lives embedded in the liquid river mud.

The common littoral bivalves seem to be eaten in nearly all countries except our own, and it is therefore needless to enumerate them. The Gasteropoda, whose habits are scarcely so cleanly, seem to require a bolder spirit and less delicate palate to venture on their consumption.

The Malays of the East Indian islands eat Telescopium fuscum and Pyrazus palustris, which abound in the mangrove swamps. They throw them on their wood fires, and when they are sufficiently cooked, break off the top of the spire and suck the animal out through the opening. Haliotis they take out of the shell, string together, and dry in the sun. The lower classes in the Philippines eat Arca inaequivalvis, boiling them as we do mussels.[216] In the Corean islands a species of Monodonta and another of Mytilus are quite peppery, and bite the tongue; our own Helix revelata, as I can vouch from personal experience, has a similar flavour. Fusus colosseus, Rapana bezoar, and Purpura luteostoma are eaten on the southern coasts of China; Strombus luhuanus, Turbo chrysostomus, Trochus niloticus, and Patella testudinaria, by the natives of New Caledonia; Strombus gigas and Livona pica in the West Indies; Turbo niger and Concholepas peruvianus on the Chilian coasts; four species of Strombus and Nerita, one each of Purpura and Turbo, besides two Tridacna and one Hippopus, by the natives of British New Guinea. West Indian negroes eat the large Chitons which are abundant on their rocky coasts, cutting off and swallowing raw the fleshy foot, which they call ‘beef,’ and rejecting the viscera. Dried cephalopods are a favourite Chinese dish, and are regularly exported to San Francisco, where the Chinamen make them into soup. The ‘Challenger’ obtained two species of Sepia and two of Loligo from the market at Yokohama.

The insipidity of fresh-water Mollusca renders them much less desirable as a form of food. Some species of Unionidae, however, are said to be eaten in France. Anodonta edulis is specially cultivated for food in certain districts of China, and the African Aetheriae are eaten by negroes. Navicella and Neritina are eaten in Mauritius, Ampullaria and Neritina in Guadeloupe, and Paludina in Cambodia.

The vast heaps of empty shells known as ‘kitchen-middens,’ occur in almost every part of the world. They are found in Scotland, Denmark, the east and west coasts of North America, Brazil, Tierra del Fuego, Australia and New Zealand, and are sometimes several hundred yards in length. They are invariably composed of the edible shells of the adjacent coast, mixed with bones of Mammals, birds, and fish. From their great size, it is believed that many of them must have taken centuries to form.

Pre-eminent among existing shell-fish industries stands the cultivation of the oyster and the mussel, a more detailed account of which may prove interesting.

The cultivation of the oyster[217] as a luxury of food dates at least from the gastronomic age of Rome. Every one has heard of the epicure whose taste was so educated that

“he could tell At the first mouthful, if his oysters fed On the Rutupian or the Lucrine bed Or at Circeii.”[218]

The first artificial oyster-cultivator on a large scale appears to have been a certain Roman named Sergius Orata, who lived about a century B.C. His object, according to Pliny the elder,[219] was not to please his own appetite so much as to make money by ministering to the appetites of others. His vivaria were situated on the Lucrine Lake, near Baiae, and the Lucrine oysters obtained under his cultivation a notoriety which they never entirely lost, although British oysters eventually came to be more highly esteemed. He must have been a great enthusiast in his trade, for on one occasion when he became involved in a law-suit with one of the riparian proprietors, his counsel declared that Orata’s opponent made a great mistake if he expected to damp his ardour by expelling him from the lake, for, sooner than not grow oysters at all, he would grow them upon the roof of his house.[220] Orata’s successors in the business seem to have understood the secret of planting young oysters in new beds, for we are told that specimens brought from Brundisium and even from Britain were placed for a while in the Lucrine Lake, to fatten after their long journey, and also to acquire the esteemed “Lucrine flavour.”

Oysters are ‘in season’ whenever there is an ‘r’ in the month, in other words, from September to April. ‘Mensibus erratis,’ as the poet has it, ‘vos ostrea manducatis!’ It has been computed that the quantity annually produced in Great Britain amounts to no less than sixteen hundred million, while in America the number is estimated at five thousand five hundred million, the value being over thirteen million dollars, and the number of persons employed fifty thousand. Arcachon, one of the principal French oyster-parks, has nearly 10,000 acres of oyster beds, the annual value being from eight to ten million francs; in 1884–85, 178,359,000 oysters were exported from this place alone. In the season 1889–90, 50,000 tons of oysters were consumed in London.

Few will now be found to echo the poet Gay’s opinion:

“That man had sure a palate covered o’er With brass or steel, that on the rocky shore First broke the oozy oyster’s pearly coat, And risq’d the living morsel down his throat.”

There were halcyon days in England once, when oysters were to be procured at 8d. the bushel. Now it costs exactly that amount before a bushel, brought up the Thames, can even be exposed for sale at Billingsgate (4d. porterage, 4d. market toll), and prime Whitstable natives average from 3½d. to 4d. each. The principal causes of this rise in prices, apart from the increased demand, are (1) over-dredging; (2) ignorant cultivation, and to these may be added (3) the effect of bad seasons in destroying young oysters, or preventing the spat from maturing. Our own principal beds are those at Whitstable, Rochester, Colchester, Milton (famous for its ‘melting’ natives), Faversham, Queenborough, Burnham, Poole, and Carlingford in Co. Down, and Newhaven, near Edinburgh.

The oyster-farms at Whitstable, public and private, extend over an area of more than 27 square miles. The principal of these is a kind of joint-stock company, with no other privilege of entrance except birth as a free dredgeman of the town. When a holder dies, his interest dies with him. Twelve directors, known as “the Jury,” manage the affairs of the company, which finds employment for several thousand people, and sometimes turns over as much as £200,000 a year. The term ‘Natives,’ as applied to these Whitstable or to other English oysters, requires a word of explanation. A ‘Native’ oyster is simply an oyster which has been bred on or near the Thames estuary, but very probably it may be developed from a brood which came from Scotland or some other place at a distance. For some unexplained reason, oysters bred on the London clay acquire a greater delicacy of flavour than elsewhere. The company pay large sums for brood to stock their own grounds, since there can be no certainty that the spat from their own oysters will fall favourably, or even within their own domains at all. Besides purchases from other beds, the parks are largely stocked with small oysters picked up along the coast or dredged from grounds public to all, sometimes as much as 50s. a bushel being paid for the best brood. It is probably this system of transplanting, combined with systematic working of the beds, which has made the Whitstable oyster so excellent both as to quality and quantity of flesh. The whole surface of the ‘layings’ is explored every year by the dredge, successive portions of the ground being gone over in regular rotation, and every provision being made for the well-being of the crop, and the destruction of their enemies. For three days of every week the men dredge for ‘planting,’ i.e. for the transference of suitable specimens from one place to another, the separation of adhering shells, the removal of odd valves and of every kind of refuse, and the killing off of dangerous foes. On the other three days they dredge for the market, taking care only to lift such a number as will match the demand.

The Colne beds are natural beds, as opposed to the majority of the great working beds, which are artificial. They are the property of the town of Colchester, which appoints a water-bailiff to manage the concern. Under his direction is a jury of twelve, who regulate the times of dredging, the price at which sales are to be made, and are generally responsible for the practical working of the trade. Here, and at Faversham, Queenborough, Rochester, and other places, ‘natives’ are grown which rival those of Whitstable.

There can be no question, however, that the cultivation of oysters by the French is far more complete and efficient than our own, and has reached a higher degree of scientific perfection combined with economy and solid profits. And yet, between 40 and 50 years ago, the French beds were utterly exhausted and unproductive, and showed every sign of failure and decay. It was in 1858 that the celebrated beds on the Ile de Ré, near Rochelle, were first started. Their originator was a certain shrewd stone-mason, by name Boeuf. He determined to try, entirely on his own account, whether oysters could not be made to grow on the long muddy fore-shore which is left by the ebb of the tide. Accordingly, he constructed with his own hands a small basin enclosed by a low wall, and placed at the bottom a number of stones picked out of the surrounding mud, stocking his ‘parc’ with a few bushels of healthy young brood. The experiment was entirely successful, in spite of the jeers of his neighbours, and Boeuf’s profits, which soon began to mount up at an astonishing rate, induced others to start similar or more extensive farms for themselves. The movement spread rapidly, and in a few years a stretch of miles of unproductive mud banks was converted into the seat of a most prosperous industry. The general interests of the trade appear to be regulated in a similar manner to that at Whitstable; delegates are appointed by the various communities to watch over the business as a whole, while questions affecting the well-being of oyster-culture are discussed in a sort of representative assembly.

At the same time as Boeuf was planting his first oysters on the shores of the Ile de Ré, M. Coste had been reporting to the French government in favour of such a system of ostreiculture as was then practised by the Italians in the old classic Lakes Avernus and Lucrinus. The principle there adopted was to prevent, as far as possible, the escape of the spat from the ground at the time when it is first emitted by the breeding oyster. Stakes and fascines of wood were placed in such a position as to catch the spat and give it a chance of obtaining a hold before it perished or was carried away into the open sea. The old oyster beds in the Bay of St. Brieuc were renewed on this principle, banks being constructed and overlaid with bundles of wood to prevent the escape of the new spat. The attempt was entirely successful, and led to the establishment or re-establishment of those numerous parcs, with which the French coast is studded from Brest to the Gironde. The principal centres of the industry are Arcachon, Auray, Cancale, and la Teste.

It is at Marennes, in Normandy, that the production of the celebrated ‘green oyster’ is carried out, that especial luxury of the French epicure. Green oysters are a peculiarly French taste, and, though they sometimes occur on the Essex marshes, there is no market for them in England. The preference for them, on the continent, may be traced back as early as 1713, when we find a record of their having been served up at a supper given by an ambassador at the Hague. Green oysters are not always green, it is only after they are placed in the ‘claires,’ or fattening ponds, that they acquire the hue; they never occur in the open sea. The green colour does not extend over the whole animal, but is found only in the branchiae and labial tentacles, which are of a deep blue-green. Various theories have been started to explain the ‘greening’ of the mollusc; the presence of copper in the tanks, the chlorophyll of marine algae, an overgrowth of some parasite, a disease akin to liver complaint, have all found their advocates. Prof. Lankester seems to have established[221] the fact,--which indeed had been observed 70 years before by a M. Gaillon,--that the greening is due to the growth of a certain diatom (Navicula ostrearia) in the water of the tanks. This diatom, which is of a deep blue-green colour, appears from April to June, and in September. The oyster swallows quantities of the Navicula; the pigment enters the blood in a condition of chemical modification, which makes it colourless in all the other parts of the body, but when the blood reaches the gills the action of the secretion cells causes the blue tint to be restored. The fact that the colour is rather green than blue in the gills, which are yellowish brown, is due to certain optical conditions.

Not till the young white oyster has been steeped for several years in the muddy waters of the ‘claires’ does it acquire the proper tint to qualify it for the Parisian restaurant. The ‘claires’ are each about 100 feet square, surrounded by low broad banks of earth, about 3 feet high and 6 feet thick at the base. Before the oysters are laid down, the gates which admit the tide are carefully opened and shut a great many times, in order to collect a sufficient amount of the Navicula. When this is done, the beds are formed, and are not again overflowed by the sea, except at very high tides. The oysters are shifted from one ‘claire’ to another, in order to perfect the ‘greening’ process. About fifty million of these ‘huitres de Marennes’ are produced annually, yielding a revenue of 2,500,000 francs.

It appears, from the experience of one of the most enthusiastic of French oyster-growers (Dr. Kemmerer), that oysters grow best in muddy water, and breed best in clear water. Thus the open sea is the place where the spat should fall and be secured, and, as soon as it is of a suitable size, it should be transferred to the closed tank or reservoir, where it will find the quiet and the food (confervae, infusoria, minute algae) which are so requisite for its proper growth. In muddy ground the animal and phosphorous matter increases, and the flesh becomes fatter and more oily. A sudden change from the clear sea-water to the muddy tank is inadvisable, and thus a series of shiftings through tanks with water of graduated degrees of nourishment is the secret of proper oyster cultivation.

The American oyster trade is larger even than the French. The Baltimore oyster beds in the Chesapeake River and its tributaries cover 3000 acres, and produce an annual crop of 25 million bushels, as many as 100,000 bushels being sometimes taken from Chesapeake Bay in a single day. Baltimore is the centre of the tinned oyster trade, while that in raw oysters centres in New York. Most of the beds whose produce is carried to New York are situated in New Jersey, Connecticut, Delaware, or Virginia. The laws of these states do not allow the beds to be owned by any but resident owners, and the New York dealers have consequently to form fictitious partnerships with residents near the various oyster beds, supply them with money to buy the beds and plant the oysters, and then give them a share in the profits. It has been estimated that from the Virginia beds 4,000,000 bushels of oysters are carried every year to Fair Haven in New England, 4,000,000 to New York, 3,000,000 to Providence, and 2,000,000 each to Boston, Philadelphia, and Baltimore. The American ‘native’ (O. virginica) is a distinct species from our own, being much larger and longer in proportion to its breadth; it is said to be also much more prolific.

According to Milne-Edwards,[222] in the great oyster parks on the coasts of Calvados, the oysters are educated to keep their shells closed when out of water, and so retain water enough inside to keep their gills moist, and arrive at their destination in good condition. As soon as an oyster is taken out of the sea, it closes its shells, and keeps them closed until the shock of removal has passed away, or perhaps until the desirability of a fresh supply of water suggests itself. The men take advantage of this to exercise the oysters, removing them from the sea for longer and longer periods. In time this has the desired effect; the well-educated mollusc learns that it is hopeless to ‘open’ when out of the water, and so keeps his shell closed and his gills moist, and his general economy in good condition.

Oysters have been known to live entirely out of water for a considerable time. Prof. Verrill once noticed[223] a large cluster of oysters attached to an old boot, hanging outside a fish-shop in Washington. They had been taken out of the water on about 10th December, and on 25th February following some of the largest were still alive. It was noticed that all those which survived had the hinge upward and the ventral edge downward, this being the most favourable position possible for the retention of water within the gill-cavity, since the edge of the mantle would pack against the margins of the shell, and prevent the water from leaking away.

Such a succulent creature as the oyster has naturally many enemies. One of the worst of these is the ravenous Starfish, or Five-finger. His omnivorous capacities are well described by a clever writer and shrewd observer of nature: “Here is one doubled up like a sea-urchin, brilliant of hue, and when spread out quite 16 inches in diameter; where, and oh where, can you obtain a prey? The hoe we carry is thrust out and the mass dragged shorewards, when the rascal disgorges two large dog-whelks he has been in the process of devouring. We feel a comfortable glow of satisfaction to think that this enemy of our oyster-beds is also the enemy of our other enemy, this carnivorous borer. Here, quite close alongside, is another, only inferior in size, and we drag him ashore likewise, to find that the fellow has actually had the courage and audacity to suck the contents out of a large horse-mussel (Modiola), the strong muscle alone remaining undevoured. We proceed along but a short way when we meet with still another in the curled-up condition in which they gorge themselves, and as we drag it shorewards the shell of a Tapes pullastra drops from the relaxing grasp of the ogre. Slowly the extended stomach returns to its place, and the monster settles back to an uncomfortable after-dinner siesta on an exposed boulder; for the starfish wraps its turned-out stomach around the prey it has secured, in place of attempting to devour the limey covering in which most of its game is protected. Once the mouth of the shell is enclosed in the stomach of the starfish, the creature soon sickens, the hinge-spring relaxes its hold, and the shell opening permits the starfish to suck out the gelatinous contents, and cast free the calcareous skeleton.”[224]

According to other observers the starfish seizes the oyster with two of his fingers, while with the other three he files away the edge of the flat or upper valve until the points of contact with the round valve are reduced almost to nothing; then he can introduce an arm, and the rest is easy work. Others suggest that the starfish suffocates the oyster by applying two of its fingers so closely to the edge of the valves that the oyster is unable to open them; after a while the vital powers relax and the shell gapes. The Rev. J. G. Wood holds[225] that the starfish pours a secretion from its mouth which “paralyses the hinge muscle and causes the shell to open.” Sometimes in a single night a whole bed of oysters will be totally destroyed by an invasion of starfish. Another dreaded enemy is the ‘whelk,’ a term which includes Purpura lapillus, Murex erinaceus, Buccinum undatum, and probably also Nassa reticulata. All these species perforate the shell with the end of their radula, and then suck out the contents through the neatly drilled hole. Skate fish are the cause of terrible destruction in the open beds, and a scarcely less dangerous visitant is the octopus. Crabs crush the young shells with their claws, and are said to gather in bands and scratch sand or mud over the larger specimens, which makes them open their shells. Yet another, and perhaps unconscious, foe is found in the common mussel, which takes up room meant for the young oysters, grows over the larger individuals, and harbours all sorts of refuse between and under its closely packed ranks. Cliona, a parasitic sponge, bores in between the layers of the oyster’s shell, pitting them with tiny holes (corresponding to its oscula), and disturbing the inmate, who has constantly to construct new layers of shell from the inside. Weed, annelids, ‘blubber,’ shifting sand or mud, sewage or any poisoning of the water, are seriously harmful to the oyster’s best interests. A very severe winter is often the cause of wholesale destruction in the beds. According to the Daily News of 26th March 1891, the Whitstable oyster companies lost property to the value of £30,000 in the exceptionally cold winter of 1890–91, when, on the coast of Kent, the surface temperature of the sea sank below 32°, and the advancing tide pushed a small ice-floe before it. Two million oysters were laid down in one week of the following spring, to make up for the loss. During the severe winter of 1892–93 extraordinary efforts were made at Hayling I. to protect the oysters from the frost. Twenty million oysters were placed in ponds for the winter, and a steam-engine was for days employed to keep the ponds thawed and supplied with water, while large coal and coke fires were kept burning at the edge of the ponds.[226] On the other hand, the unusually warm and sunny summer of 1893 is said to have resulted in the finest fall of spat known in Whitstable for fifty years.[227]

The reproductive activity of the oyster is supposed to commence about the third year. Careful research has shown[228] that the sexes in the English oyster are not separate, but that each individual is male as well as female, producing spermatozoa as well as ova in the same gland. Here, however, two divergent views appear. Some authorities hold that the oyster does not fecundate its own eggs, but that this operation is performed by spermatozoa emitted by other specimens. It is believed that, in each individual, the spermatozoa arrive at maturity first, and that the ova are not produced until after the spermatozoa have been emitted; thus the oyster is first male and then female, morphologically hermaphrodite, but physiologically unisexual. Others are of opinion that the oyster does fecundate its own eggs, ova being first produced, and passed into the infrabranchial chamber--the ‘white-sick’ stage--and then, after an interval, spermatozoa being formed and fecundating these ova--the ‘black-sick’ stage. In this latter view the oyster is first female and then male, and is, both morphologically and physiologically, hermaphrodite. The old view, that ‘black-sick’ oysters are the male, and ‘white-sick’ the female, is therefore quite incorrect.

The ova, in their earliest stage, consist of minute oval clusters of globules floating in a transparent mucus. They pass from the ovary into the gills and folds of the mantle, and are probably fecundated within the excretory ducts of the ovary, before arriving in the mantle chamber. In this stage the oyster is termed ‘white-sick.’ In about a fortnight, as the course of development proceeds, the fertilised ova become ciliated at one end (the so-called veliger stage, p. 131), and soon pigment appears in various parts of the embryos, giving them a darker colour, which varies from grayish to blue, and thus the white-sick oyster becomes ‘black-sick.’ When the black spat emerge, they are still furnished with cilia for their free-swimming life. This is of very short duration, for unless the embryo finds some suitable ground on which to affix itself within forty-eight hours, it perishes. As the spat escapes from the parent oyster, which slightly opens its valves and blows the spat out in jets, it resembles a thick cloud in the water, and is carried about at the mercy of wind and tide. April to August are the usual spawning months, warm weather being apparently an absolute necessity to secure the adhering of the spat. A temperature of 65° to 72° F. seems requisite for their proper deposit. Thus on a fine, warm day, with little wind or tide running, the spat will fall near the parents and be safely secured, while in cold, blustering weather it will certainly be carried off to a distance, and probably be altogether lost. The number of young produced by each individual has been variously estimated at from 300,000 to 60,000,000. Either extreme seems enormous, but it must be remembered that besides climatal dangers, hosts of enemies--other Mollusca, fish, and Crustacea--beset the opening career of the young oyster.

As soon as the spat has safely ‘fallen,’ it adheres to some solid object, and loses the cilia which were necessary for its swimming life. It begins to grow rapidly, increasing from about 1/20 inch in diameter to about the size of a threepenny piece in five or six months, and in a year to one inch in diameter. Roughly speaking, the best guide to an oyster’s age is its size; it is as many years old as it measures inches across.

The oyster is at its prime at the age of five; its natural life is supposed to be about ten years. The rings, or ‘shoots’ on a shell are not--as is frequently supposed--marks of annual growth; cases have been noticed where as many as three ‘shoots’ were made during the year.

An oyster is furnished, on the protruding edges of the mantle, with pigmented spots which may be termed ‘visual organs,’ though they hardly rise to the capacities and organisation of real ‘eyes.’ But there is no doubt that they are sufficiently sensitive to the action of light to enable the oyster to apprehend the approach of danger, and close his doors accordingly. ‘How sensitive,’ notes Mr. W. Anderson Smith,[229] ‘the creatures are to the light above them; the shadow of the iron as it passes overhead is instantaneously noted, and snap! the lips are firmly closed.’

The geographical distribution of Ostrea edulis extends from Tränen, in Norway, close to the Arctic circle, to Gibraltar and certain parts of the Mediterranean, Holland, and N. Germany to Heligoland, and the western shores of Sleswick and Jutland. It occurs in Iceland, but does not enter the Baltic, where attempts to colonise it have always failed. Some authorities regard the Mediterranean form as a distinct species.

The literature of oyster-cookery may be passed over in silence. The curious may care to refer to M. S. Lovell’s Edible British Mollusks, where no less than thirty-nine different ways of dressing oysters are enumerated. It may, however, be worth while to add a word on the subject of poisonous oysters. Cases have been known where a particular batch of oysters has, for some reason, been fatal to those who have partaken of them. It is possible that this may have been due, in certain instances, to the presence of a superabundance of copper in the oysters, and there is no doubt that the symptoms detailed have often closely resembled those of copper poisoning. Cases of poisoning have occurred at Rochefort through’ the importation of ‘green oysters’ from Falmouth. It would no doubt be dangerous ever to eat oysters which had grown on the copper bottom of a ship. But copper is present, in more or less minute quantities, in very many Mollusca, and it is more probable that a certain form of slow decomposition in some shell-fish develops an alkaloid poison which is more harmful to some people than to others, just as some people can never digest any kind of shell-fish.[230] These alkaloid developments from putrescence are called ptomaines. In confirmation of this view, reference may be made to a case, taken from an Indian Scientific Journal, in which an officer, his wife, and household ate safely of a basket of oysters for three days at almost every meal. The basket then passed out of their hands, not yet exhausted of its contents, and a man who had already eaten of these oysters at the officer’s table was afterwards poisoned by some from the same basketful.

The cultivation of the common mussel (Mytilus edulis L.) is not practised in this country, although it is used as food in the natural state of growth all round our coasts. The French appear to be the only nation who go in for extensive mussel farming. The principal of these establishments is at a little town called Esnaudes, not far from La Rochelle, and within sight of the Ile de Ré and its celebrated oyster parks. The secret of the cultivation consists in the employment of ‘bouchots,’ or tall hurdles, which are planted in the mud of the fore-shore, and upon which the mussel (la moule, as the French call it) grows. The method is said[231] to have been invented as long ago as 1235 by a shipwrecked Irishman named Walton. He used to hang a purse net to stakes, in the hope of capturing sea birds. He found, however, that the mussels which attached themselves to his stakes were a much more easily attainable source of food, and he accordingly multiplied his stakes, out of which the present ‘bouchot’ system has developed. The shore is simply a stretch of liquid mud, and the bouchots are arranged in shape like a single or double V, with the opening looking towards the sea. The fishermen, in visiting the bouchots, glide about over the mud in piroques or light, flat-bottomed boats, propelling them by shoving the mud with their feet. Each bouchot is now about 450 yards long, standing 6 feet out of the mud, making a strong wall of solid basket-work, and as there are altogether at least 500 bouchots, the total mussel-bearing length of wall is nearly 130 miles.

The mussel-spat affixes itself naturally to the bouchots nearest the sea, in January and February. Towards May the planting begins. The young mussels are scraped off these outermost bouchots, and placed in small bags made of old canvas or netting, each bag holding a good handful of the mussels. The bags are then fastened to some of the inner bouchots, and the mussels soon attach themselves by their byssus, the bag rotting and falling away. They hang in clusters, increasing rapidly in size, and at the proper time are transplanted to bouchots farther and farther up the tide level, the object being to bring the matured animal as near as possible to the land when it is time for it to be gathered. This process, which aims at keeping the mussel out of the mud, while at the same time giving it all the nutrition that comes from such a habitat, extends over about a year in the case of each individual. Quality, rather than quantity, is the aim of the Esnaudes boucholiers. The element of quantity, however, seems to come in when we are told that each yard of the bouchots is calculated to yield a cartload of mussels, value 6 francs, and that the whole annual revenue is at least £52,000.

In this country, and especially in Scotland, mussels are largely used as bait for long-line fishing. Of late years other substances have rather tended to take the place of mussels, but within the last twenty years, at Newhaven on the Firth of Forth, three and a half million mussels were required annually to supply bait for four deep-sea craft and sixteen smaller vessels. According to Ad. Meyer,[232] boughs of trees are laid down in Kiel Bay, and taken up again, after three, four, or five years, between December and March, when they are found covered with fine mussels. The boughs are then sold, just as they are, by weight, and the shell-fish sent into the interior of Germany.

Mussels are very sensitive to cold weather. In 1874, during an easterly gale, 195 acres of mussels at Boston, in Lincolnshire, were killed in a single night. They soon affix themselves to the bottom of vessels that have lain for any length of time in harbour or near the coast. The bottom of the Great Eastern steamship was at one time so thickly coated with mussels that it was estimated that a vessel of 200 tons could have been laden from her. In some of our low-lying coast districts mussels are a valuable protection against inundation. “An action for trespass was brought some time ago for the purpose of establishing the right of the lord of the manor to prevent the inhabitants of Heacham from taking mussels from the sea-shore. The locality is the fore-shore of the sea, running from Lynn in a north-westerly direction towards Hunstanton in Norfolk; and the nature of the shore is such that it requires constant attention, and no little expenditure of money, to maintain its integrity, and guard against the serious danger of inundations of the sea. Beds of mussels extend for miles along the shore, attaching themselves to artificial jetties running into the sea, thereby rendering them firm, and thus acting as barriers against the sea [and as traps to catch the silt, and thus constantly raise the level of the shore]. Therefore, while it is important for the inhabitants, who claim a right by custom, to take mussels and other shell-fish from the shore, it is equally important for the lord of the manor to do his utmost to prevent these natural friends of his embankments and jetties from being removed in large quantities.”[233]

The fable that Bideford Bridge is held together by the byssi of Mytilus, which prevent the fabric from being carried away by the tide, has so often been repeated that it is perhaps worth while to give the exact state of the case, as ascertained from a Town Councillor. The mussels are supposed to be of some advantage to the bridge, consequently there is a by-law forbidding their removal, but the corporation have not, and never had, any boat or men employed in any way with regard to them.

Poisoning by mussels is much more frequent than by oysters. At Wilhelmshaven,[234] in Germany, in 1885, large numbers of persons were poisoned, and some died, from eating mussels taken from the harbour. It was found that when transferred to open water these mussels became innocuous, while, on the other hand, mussels from outside, placed in the harbour, became poisonous. The cause obviously lay in the stagnant and corrupted waters of the harbour, which were rarely freshened by tides. It was proved to demonstration that the poison was not due to decomposition; the liver of the mussels was the poisonous part. In the persons affected, the symptoms were of three kinds, exanthematous (skin eruptions), choleraic, and paralytic. Cases of similar poisoning are not unfrequent in our own country, and the circumstances tend to show that, besides the danger from mussels bred in stagnant water, there is also risk in eating them when ‘out of season’ in the spawning time.

Whelks are very largely employed for bait, especially in the cod fishery. The whelk fishery in Whitstable Bay, both for bait and for human food, yields £12,000 a year. Dr. Johnston, of Berwick, estimated that about 12 million limpets were annually consumed for bait in that district alone. The cockle fishery in Carmarthen Bay employs from 500 to 600 families, and is worth £15,000 a year, that in Morecambe Bay is worth £20,000.

=Cultivation of Snails for Food; Use as Medicine.=--It was a certain Fulvius Hirpinus who, according to Pliny the elder,[235] first instituted snail preserves at Tarquinium, about 50 B.C. He appears to have bred several species in his ‘cochlearia,’ keeping them separate from one another. In one division were the albulae, which came from Reate; in another the ‘very big snails’ (probably H. lucorum), from Illyria; in a third the African snails, whose characteristic was their fecundity; in a fourth those from Soletum, noted for their ‘nobility.’ To increase the size of his snails, Hirpinus fed them on a fattening mixture of meal and new wine, and, says the author in a burst of enthusiasm, ‘the glory of this art was carried to such an extent that a single snail-shell was capable of holding eighty sixpenny pieces.’ Varro[236] recommends that the snaileries be surrounded by a ditch, to save the expense of a special slave to catch the runaways. Snails were not regarded by the Romans as a particular luxury. Pliny the younger reproaches[237] his friend Septicius Clarus for breaking a dinner engagement with him, at which the menu was to have been a lettuce, three snails and two eggs apiece, barley water, mead and snow, olives, beetroot, gourds and truffles, and going off somewhere else where he got oysters, scallops, and sea-urchins. In Horace’s time they were used as a gentle stimulant to the appetite, for

“’Tis best with roasted shrimps and Afric snails To rouse your drinker when his vigour fails.”[238]

Escargotières, or snail-gardens, still exist in many parts of Europe, e.g. at Dijon, at Troyes and many other places in central and southern France, at Brunswick, Copenhagen, and Ulm. The markets at Paris, Marseilles, Bordeaux, Toulouse, Nantes, etc., are chiefly supplied by snails gathered from the open country, and particularly from the vineyards, in some of which Helix pomatia abounds. In the Morning Post of 8th May 1868 there is an account of the operation of clearing the celebrated Clos de Vougeot vineyard of these creatures. No less than 240 gallons were captured, at a cost in labour of over 100 francs, it being estimated that these snails would have damaged the vines to an extent represented by the value of 15 to 20 pipes of wine, against which may be set the price fetched by the snails when sold in the market.

It is generally considered dangerous to eat snails at once which have been gathered in the open country. Cases have occurred in which death by poisoning has resulted from a neglect of this precaution, since snails feed on all manner of noxious herbs. Before being sent to table at the restaurants in the great towns, they are fattened by being fed with bran in the same way as oysters.

The Roman Catholic Church permits the consumption of snails during Lent. Very large numbers are eaten in France and Austria at this time. At the village of Cauderon, near Bordeaux, it is the proper thing to end Carnival with especial gaiety, but to temper the gaiety with a dish of snails, as a foretaste of Lenten mortification.

The following species appear to be eaten in France at the present day: H. pomatia, aspersa, nemoralis, hortensis, aperta, pisana, vermiculata, lactea. According to Dr. Gray, the glassmen at Newcastle used to indulge in a snail feast once a year, and a recent writer informs us that H. aspersa is still eaten by working people in the vicinity of Pontefract and Knottingley.[239] But in this country snails appear to be seldom consciously used as an article of food; the limitation is necessary, for Lovell tells us that they are much employed in the manufacture of cream, and that a retired (!) milkman pronounced it the most successful imitation known.

Preparations made from snails used to be highly esteemed as a cure for various kinds of diseases and injuries. Pliny the elder recommends them for a cough and for a stomach-ache, but it is necessary “to take an uneven number of them.”[240] Five African slugs, roasted and beaten to a powder, with half a drachm of acacia, and taken with myrtle wine, is an excellent remedy for dysentery. Treated in various ways, snails have been considered, in modern times, a cure for ague, corns, web in the eye, scorbutic affections, hectic fevers, pleurisy, asthma, obstructions, dropsy, swelling of the joints, headache, an impostume (whitlow), and burns. One of Pliny’s remedies for headache, which competes with the bones of a vulture’s head or the brain of a crow or an owl, is a plaister made of slugs with their heads cut off, which is to be applied to the forehead. He regards slugs as immature snails, whose growth is not yet complete (nondum perfectae). Lovell states that “a large trade in snails is carried on for Covent Garden market in the Lincolnshire fens, and that they are sold at 6d. per quart, being much used for consumptive patients and weakly children.”

The custom still seems to linger on in some parts of the country. Mr. E. Rundle, of the Royal Cornwall Infirmary, gives his experience in the following terms: “I well remember, some twelve years since, an individual living in an adjoining parish [near Truro] being pointed out to me as ‘a snail or slug eater.’ He was a delicate looking man, and said to be suffering from consumption. Last summer I saw this man, and asked him whether the statement that he was a ‘snail eater’ was true: he answered, ‘Yes, that he was ordered small white slugs--not snails--and that up till recently he had consumed a dozen or more every morning, and he believed they had done him good.’ There is also another use to which the country people here put snails, and that is as an eye application. I met with an instance a few weeks since, and much good seemed to have followed the use.”[241]

A reverend Canon of the Church of England, whose name I am not permitted to disclose, informs me that there was a belief among the youth of his native town (Pontypool, in Monmouthshire) that young slugs were ‘good for consumption,’ and that they were so recommended by a doctor who practised in the town. The slugs selected were about ¾ inch long, “such as may be seen crawling on the turf of a hedge-bank after a shower of rain.” They were “placed upon the tongue without any previous preparation, and swallowed alive.” My informant himself indulged in this practice for some time, “not on account of any gustatory pleasure it afforded, but from some vague notion that it might do him good.”

A colleague of mine at King’s College tells me that the country people at Ponteland, near Morpeth, habitually collect Limax agrestis and boil it in milk as a prophylactic against consumption. He has himself frequently devoured them alive, but they must be swallowed, not scrunched with the teeth, or they taste somewhat bitter.

Snails have occasionally fallen, with other noxious creatures, under the ban of the Church. In a prayer of the holy martyr Trypho of Lampsacus (about 10th cent. A.D.) there is a form of exorcism given which may be used as occasion requires. It runs as follows: “O ye Caterpillars, Worms, Beetles, Locusts, Grasshoppers, Woolly-Bears, Wireworms, Longlegs, Ants, Lice, Bugs, Skippers, Cankerworms, Palmerworms, Snails, Earwigs, and all other creatures that cling to and wither the fruit of the grape and all other herbs, I charge you by the many-eyed Cherubim, and by the six-winged Seraphim, which fly round the throne, and by the holy Angels and all the Powers, etc. etc., hurt not the vines nor the land nor the fruit of the trees nor the vegetables of ---- the servant of the Lord, but depart into the wild mountains, into the unfruitful woods, in which God hath given you your daily food.”

=Prices given for Shells.=--Very high prices have occasionally been given for individual specimens, particularly about thirty or forty years ago, when the mania for collecting was at its height. In those days certain families, such as the Volutidae, Conidae, and Cypraeidae, were the especial objects of a collector’s ardour, and he spared no expense to make his set of the favourite genus as complete as possible. Thus at Stevens’ auction-rooms in Covent Garden, on 21st July 1854, one specimen of Conus cedo nulli fetched £9: 10s., and another £16, a C. omaicus 16 guineas, C. victor £10, and C. gloria maris, the greatest prize of all, £43: 1s. At the Vernède sale, on 14th Dec. 1859 two Conus omaicus fetched £15 and £22, and a C. gloria maris £34. At the great Dennison sale, in April 1865, the Conidae fetched extravagant prices, six specimens averaging over £20 apiece. Conus cedo nulli went for £18 and £22, C. omaicus for £12, C. malaccanus for 10 guineas (this and one of the cedo nulli being the actual specimens figured in Reeve’s Conchologia Iconica), C. cervus for £19 and C. gloria maris for £42. On 9th May 1866 a Cypraea Broderipii was sold at Stevens’ auction-rooms for £13, and at the Dennison sale a Cypraea princeps fetched £40, and C. guttata £42. The Volutidae, although not quite touching these prices, have yet done fairly well. Mr. Dennison’s Voluta fusiformis sold for £6: 15s., V. papillaris for £5, V. cymbiola for £5: 15s., V. reticulata for eight guineas, and two specimens of the rarest of all Volutas, V. festiva, for £14 and £16, both being figured in the Conchologia. At the same sale, two unique specimens of Oniscia Dennisoni fetched £17 and £18 respectively, and, at the Vernède sale, Ancillaria Vernèdei was bought for £6: 10s., and Voluta piperata for £7: 10s.

A unique specimen of a recent Pleurotomaria (quoyana F. and B.) was purchased by Miss de Burgh in 1873 for 25 guineas, and another species of the same genus (adansoniana Cr. and F.), of extraordinary size and beauty, is now offered for sale for about £100.

Bivalves have never fetched quite such high prices as univalves, but some of the favourite and showy genera have gone near to rival them. On 22nd June 1869, at Stevens’, Pecten solaris fetched £4: 5s., P. Reevii £4: 8s., and Cardita varia 5 guineas. Mr. Dennison’s specimens of Pecten subnodosus sold for £7, of Corbula Sowerbyi for £10, of Pholadomya candida for £8 and £13, while at the Vernède sale a Chama damicornis fetched £7.

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Chapter V

REPRODUCTION--DEPOSITION OF EGGS--DEVELOPMENT OF THE FERTILISED OVUM--DIFFERENCES OF SEX--DIOECIOUS AND HERMAPHRODITE MOLLUSCA--DEVELOPMENT OF FRESH-WATER BIVALVES

Reproduction in the Mollusca invariably takes place by means of eggs, which, after being developed in the ovary of the female, are fertilised by the spermatozoa of the male. As a rule, the eggs are ‘laid,’ and undergo their subsequent development apart from the parent. This rule, however, has its exceptions, both among univalve and bivalve Mollusca, a certain number of which hatch their young from the egg before expelling them. Such ovoviviparous genera are Melania, Paludina, Balea, and Coeliaxis among land and fresh-water Mollusca, and Cymba and many Littorina amongst marine. The young of Melania tuberculata, in Algeria, have been noticed to return, as if for shelter, to the branchial cavity of the mother, some days after first quitting it. Isolated species among Pulmonata are known to be ovoviviparous, e.g. Patula Cooperi, P. Hemphilli, and P. rupestris, Acanthinula harpa, Microphysa vortex, Pupa cylindracea and muscorum, Clausilia ventricosa, Opeas dominicensis, Rhytida inaequalis, etc. All fresh-water Pelecypoda yet examined, except Dreissensia, are ovoviviparous.

The number of eggs varies greatly, being highest in the Pelecypoda. In Ostrea edulis it has been estimated at from 300,000 to 60,000,000; in Anodonta from 14,000 to 20,000; in Unio pictorum 200,000. The eggs of Doris are reckoned at from 80,000 to 600,000, of Loligo and Sepia at about 30,000 to 40,000. Pulmonata lay comparatively few eggs. Arion ater has been observed to lay 477 in forty-eight days (p. 42). Nests of Helix aspersa have been noticed, in which the number of eggs varied from about 40 to 100. They are laid in little cup-shaped hollows at the roots of grass, with a little loose earth spread over them. The eggs of Testacella are rather large, and very elastic; if dropped on a stone floor they will rebound sharply several inches. The Cochlostyla of the Philippines lay their eggs at the tops of the great forest trees, folding a leaf together to serve as a protection.

The eggs of the great tropical Bulimus and Achatina, together with those of the Macroön group of Helix (Helicophanta, Acavus, Panda) are exceedingly large, and the number laid must be decidedly less than in the smaller Pulmonata. Bulimus oblongus, for instance, from Barbados, lays an egg about the size of a sparrow’s (Fig. 38), Achatina sinistrorsa as large as a pigeon’s. The Cingalese Helix Waltoni when first hatched is about the size of a full-grown H. hortensis. There is, in the British Museum, a specimen of the egg of a Bulimus from S. America (probably maximus or popelairanus) which measures exactly 1¾ inch in length.

The Limnaeidae deposit their eggs in irregular gelatinous masses on the under side of the leaves of water-plants, and on all kinds of débris.

The Rachiglossa or marine carnivorous families lay their eggs in tough leathery or bladdery capsules, which are frequently joined together in shapes which differ with the genus. Each capsule contains a varying number of ova. The cluster of egg-capsules of Buccinum undatum is a familiar object on all our sandy coasts. The capsules of Purpura lapillus are like delicate pink grains of rice, set on tiny stalks. They are not attached to one another, but are set closely together in groups in sheltered nooks of the rocks. A single Purpura has been observed to produce 245 capsules! Busycon lays disc-shaped capsules which are all attached at a point in the edge to a cartilaginous band nearly 3 feet in length, looking like a number of coins tied to a string at equal distances from one another. In Murex erinaceus the egg-capsules are triangular, with a short stalk. They are deposited separately in clusters of from 15 to 150, there being about 20 ova in each capsule. It appears that all the species of the same genus have by no means the same method of depositing their eggs, nor do they always produce eggs of at all similar size or shape. Thus, of two British species of Nassa, N. reticulata lays egg-capsules in shape like flattened pouches with a short stalk, and fastens them in rows to the leaves of Zostera; M. incrassata, on the other hand, deposits solitary capsules, which are shaped like rounded oil-flasks. Neptunea antiqua lays its eggs in bunched capsules, like Bucc. undatum (Fig. 40), but the capsules of N. gracilis are solitary.

In Natica the eggs are deposited in what looks like a thick piece of sand-paper, curled in a spiral form (Fig. 41). The sand is agglutinated by copious mucus into a sort of sheet, and the eggs are let into this, sometimes (N. heros) in regular quincunx form. Ianthina attaches its eggs to the under side of its float (Fig. 42). The Trochidae deposit their eggs on the under side of stones and sea-weeds, each ovum being contained in a separate capsule, and all the capsules glued together into an irregular mass of varying size. The female of Galerus chinensis hatches her eggs by keeping them between her foot and the stone she adheres to. They are laid in from 6 to 10 capsules, connected by a pedicle and arranged like the petals of a rose, with 10 to 12 eggs in each capsule. Those Littorina which are not ovoviviparous deposit their spawn on sea-weeds, rocks, and stones. The eggs are enveloped in a glairy mass which is just firm enough to retain its shape in the water; each egg has its own globule of jelly and is separated from the others by a very thin transparent membrane.[242]

Chiton marginatus, when kept in captivity, has been noticed[243] to elevate the posterior part of the girdle, and to pour out a continuous stream of flaky white matter like a fleecy cloud, which proved to be of a glutinous nature. It then discharged ova, at the rate of one or two every second, for at least fifteen minutes, making a total of 1300 to 1500, each being about 1/100 inch diameter. The ova were shot into the glutinous cloud, which seemed to serve as a sort of nidus to entangle the ova and prevent them being carried away. The subsequent development was rapid, and in seven days the young Chiton was hatched, being then about 1/20 inch long. Lovén has described the same species as laying its eggs, loosely united in clusters of seven to sixteen, upon small stones. There is probably some mistake about the identification, but the observation illustrates the varying methods of oviposition among allied forms.

Not very much is known with regard to the ovipositing of the Cephalopoda, especially those which inhabit deep water. Masses of ova arranged in very various forms have occasionally been met with floating in the ocean, but it is next to impossible to determine to what species, or even genus, they belong.[244]

In Loligo punctata the ova are contained in small cylindrical cases measuring 3 to 4 in. by ½ in., to the number of about 250 ova in each case. Hundreds of these cases are attached together like a bundle of sausages or young carrots, and the movements of the embryos within can be distinctly noted. Sepia officinalis lays large black pear-shaped capsules, each of which is tied to some place of attachment by a kind of ribbon at the upper end of the capsule, the whole forming a large group like a bunch of grapes. Octopus vulgaris deposits thousands of small berry-shaped ova, attached to a string which runs along the centre of the mass (Fig. 43).

The so-called shell of the female Argonauta is nothing more than a form of protection for the ova, and is in no sense homologous to the ordinary molluscan shell. The ova consist of a large granulated mass, attached to a many branched stem; they are contained in the spire of the shell, in contact with the posterior part of the body of the mother, but sometimes project externally beyond the coil of the spire.

Certain species possess the curious property of laying their eggs on the outside of their own shells. Buccinopsis Dalei is not unfrequently found decorated with its own egg-capsules. Possibly this species, which lives on oozy ground, finds this the only secure place of attachment for its progeny. Neritina fluviatilis has a similar habit, and so have many other species of Neritina and Navicella. It is not quite clear, in the latter cases, whether the eggs are laid by the specimens on whose shell they are found, or whether they are deposited by others. In either case, perhaps the shell is the safest place for them in the rapid streams which both genera frequent. Specimens of Hydrobia ulvae taken on the wet sands at the mouth of the Dee, are found to have several little rounded excrescences scattered over the surface of the shell. These, on examination, are found to be little masses of small sand-grains, in the centre of which is a clear jelly containing segmenting ova or young embryos. Here again, in all probability, the shell is the only comparatively stable object, in the expanse of shifting sands, on which the eggs can be laid.[245]

The pulmonate genus Libera, which occurs on a few of the island groups in the Central Pacific, is remarkable for the habit of laying its eggs within its own cavernous umbilicus, which is narrowed at the lower part. The eggs number from four to six, or the same number of very young shells may be seen closely packed in the cavity, each being in shape exactly like a young Planorbis. This constriction of the umbilicus does not occur till the formation of the last two whorls, i.e. till the animal is sexually mature. Some species, but not all, provide for the safety of their eggs more completely by forming a very thin shelly plate, which nearly closes the umbilical region, and breaks away or is absorbed to facilitate the escape of the young shells.[246]

=Union of Limax.=--With regard to the act of union itself, the method in certain species of Limax deserves special notice. L. maximus has been observed at midnight to ascend a wall or some perpendicular surface. A pair then crawl round and round one another emitting a quantity of mucus which at length forms a patch, 2 to 2½ inches in diameter. When this acquires consistency the pair begin to twist round each other in corkscrew form, and detach themselves from the wall, hanging by a cord of the thickened mucus, about 8–15 inches long, and still twisting round each other. The external generative organs are then protruded and copulation takes place, after which the bodies untwist, separate, and crawl up the cord again to the wall.[247]

=Periodicity in Breeding.=--In the marine Mollusca, the winter months appear to be the usual time for the deposition of eggs. Careful observations have been made on the Mollusca occurring at Naples,[248] and the general result seems to be that for all Orders alike the six winter months from November to April, roughly speaking, are the breeding time. Scarcely any forms appear to breed habitually in August, September, or October. On our own coasts, Nudibranchiata come in shore to deposit their ova from January to April. Purpura lapillus may be observed depositing ova all the year round, but is most active from January to April. Buccinum undatum breeds from October to May; Littorina all the year round.

The land Mollusca exhibit rather more periodicity than the marine. In temperate climates they breed exclusively in the summer months. In the tropics their periods are determined by the dry and rainy seasons, where such occur, otherwise they cohabit all the year round. According to Karl Semper, the snails of the warm Mediterranean region arrive at sexual maturity when they are six months old, i.e. before they are fully grown. After a rest of about three months during the heat of summer, a second period of ovipositing occurs.[249] Helix hortensis and H. nemoralis ascend trees, sometimes to a height of forty feet, when pairing.[250]

=Hybridism= as the result of union between different species of Mollusca is exceedingly rare. Lecoq once noticed[251] on a wall at Anduze (Gard) as many as twenty specimens of Pupa cinerea united with Clausilia papillaris. No offspring seem to have resulted from what the professor calls ‘this innocent error,’ for the wall was carefully scrutinised for a long time, and no hybrid forms were ever detected.

The same observer noticed, in the Luxembourg garden at Paris, and M. Gassies has noticed[252] at various occasions, union between Helix aspersa and nemoralis, H. aspersa and vermiculata, between Stenogyra decollata and a Helix (sp. not mentioned), H. variabilis and pisana, H. nemoralis and hortensis. In the two latter cases a hybrid progeny was the result. It has been noticed that these unions generally took place when the air was in a very electric condition, and rain had fallen, or was about to fall, abundantly.

Of marine species Littorina rudis has been noticed[253] in union both with L. obtusata and with L. littorea, but no definite facts are known as to the result of such unions.

=Self-impregnation= (see p. 44).

=Development of the Fertilised Ovum.=--The first stages in the development of the Mollusca are identical with those which occur in other classes of animals. The fertilised ovum consists of a vitellus or yolk, which is surrounded with albumen, and is either contained in a separate capsule, or else several, sometimes many, ova are found in the same capsule, only a small proportion of which ultimately develop. The germinal vesicle, which is situated at one side of the vitellus, undergoes unequal segmentation, the result of which is usually the formation of a layer of small ectoderm cells overlying a few much larger cells which contain nearly the whole of the yolk. The large cells are then invaginated, or are simply covered by the growth of the ectoderm cells. The result in either case is the formation of an area, the blastopore, where the inner cells are not covered by the ectoderm. The blastopore gradually narrows to a circular opening, which, in the great majority of cases, eventually becomes the mouth. The usual differentiation of germinal layers takes place, the epiblast eventually giving rise to the epidermis, nervous system, and special sense organs, the hypoblast to the liver and to the middle region of the alimentary tract, the mesoblast to the muscles, the body cavity, the vascular, the excretory and reproductive systems. The next, or trochosphere (trochophora) stage, involves the formation of a circlet of praeoral cilia, dividing the still nearly spherical embryo into two unequal portions, the smaller of which consists simply of the prostomium, or part in front of the mouth, the larger bearing the mouth and anus.

So far the series of changes undergone by the embryo are not peculiar to the Mollusca; we now come to those which are definitely characteristic of that group. The stage next succeeding the development of the trochosphere is the definitive formation of the velum, a process especially characteristic of the Gasteropoda and Pelecypoda, but apparently not occurring in the great majority of land Pulmonata.

The circlet of cilia becomes pushed more and more towards the anterior portion of the embryo, the cilia themselves become longer, while the portion of the body from which they spring becomes elevated into a ridge or ring, which, as a rule, develops on each side a more or less pronounced lobe. The name velum is applied to this entire process of ciliated ring and lobes, and to the area which they enclose.

In this so-called veliger stage, the velum serves, in the first place, to cause rotation of the larva within the egg-capsules, and, after hatching, as an organ of locomotion. As a rule, the velum disappears entirely in the adult mollusc after the free-swimming stage is over, but in the common Limnaea stagnalis it persists, losing its cilia, as the very prominent circum-oral lobes. Simultaneously with the development of the velum, and in some cases earlier, appear the rudiments of the shell-gland and of the foot, the latter being situated on the ventral side, between the mouth and anus, the former on the dorsal side, behind the velum, and above the surface of the eventual visceral sac. Thus the prime characteristics of the veliger stage, subsequent to the appearance of the velum itself, are the development of the visceral sac and shell-gland on the upper, and of the foot on the under side. According to Lankester the primitive shell-gland does not, as a rule, directly give rise to the shell of the adult mollusc, but becomes filled up by a horny substance, and eventually disappears; the permanent shell then forms over the surface of the visceral hump from the original centre of the shell-gland. It is only in Chiton, and possibly in Limax, that the primitive shell-sac is retained and developed into the final shell-forming area, which is much wider, and extends to the edges of the mantle. Within the velar area first appear the rudiments of the tentacles and eyes; when these become developed the velum atrophies and disappears.

Several of these veligers when captured in the open sea have been mistaken for perfect forms, and have been described as such. Thus the larva of Dolium has been described as Macgillivrayia, that of a Purpura as Chelotropis and Sinusigera, that of Aporrhais pes pelecani as Chiropteron, that of Marsenia conspicua as Brownia, Echinospira, and Calcarella.

Cephalopoda.--The embryonic development of the Cephalopoda is entirely distinct from that of all other Mollusca. The segmentation of the vitellus is partial, and the embryo is furnished with a vitelline sac, which is very large in the majority of cases (Fig. 48). There is no free-swimming stage, but the embryo emerges from the egg fully developed.

=Differences of Sex.=--In the Mollusca there are two main types of sexual difference: (i) sexes separate (dioecious type), (ii) sexes united in the same individual (hermaphrodite type).

In some cases--e.g. certain Pelecypoda--what is practically a third type occurs. The animal is hermaphrodite, but the male and female elements are not developed simultaneously, i.e. the same individual is at one time female, at another male.

1. The sexes are separate in

All Cephalopoda.

Gasteropoda Amphineura (except Neomeniidae).

Gasteropoda Prosobranchiata (except Valvata and some species of Marsenia).

Scaphopoda.

Many Pelecypoda.

2. The sexes are united in

Gasteropoda Opisthobranchiata.

Gasteropoda Pulmonata.

Certain Pelecypoda.[254]

In the dioecious Mollusca, sexual union is the rule, but is by no means universal. In some instances,--e.g. Vermetus, Magilus, Patella, Haliotis, Crepidula, Chiton, the Scaphopoda--the form and habits of the animal do not admit of it; in others (many Trochus) a male copulative organ is wanting. When this is the case, the male scatters the spermatozoa freely; the majority must perish, but some will be carried by currents in the direction of the female.

When the sexes are separate, the female is frequently larger than the male. This is markedly the case in Littorina, Buccinum, and all the Cephalopoda; in Argonauta the difference is extreme, the male not being more than ¼ the size of the female.

Those hermaphrodite Mollusca which are capable of sexual union (Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently divided into two sections, according as (1) there are separate orifices for the male and female organs, or (2) one orifice serves for both. To the former section (Digonopora[2]) belong the Limnaeidae, Vaginulidae, and Onchidiidae, and many Opisthobranchiata, including all the Pteropoda; to the latter (Monogonopora[255]) nearly all the Nudibranchiate Opisthobranchiata, and all the rest of the Pulmonata. In the latter case during union, mutual impregnation takes place, and each of the two individuals concerned has been observed (compare p. 42) to deposit eggs. In the former, however, no such reciprocal act can take place, but the same individual can play the part of male to one and female to another, and we sometimes find a string of Limnaea thus united, each being at once male and female to its two adjacent neighbours.

=The Reproductive System.=--Broadly speaking, the complicated arrangements which are found in Mollusca resolve themselves into modifications of three important factors:--

(a) The gonads or germ-glands, in which are developed the ova and the spermatozoa. These glands are generally known as the ovary in the female, the sperm-gland or testis in the male.

(b) The channels which provide for the passage of the seminal products; namely, the oviduct in the female, the vas deferens or sperm-duct in the male.

(c) The external generative organs.

=Dioecious Mollusca.=--The common Littorina obtusata will serve as a typical instance of a dioecious prosobranchiate, exhibiting the simplest form of organs. In the female the ovary, a lobe-shaped body, is embedded in the liver. An oviduct with many convolutions conveys the ova into the uterus, an oblong chamber which consists simply of a dilatation of the oviduct. The ova descend into the uterus, which is sometimes furnished with a seminal pouch. In this seminal pouch, or above it, in the oviduct, the ova come into contact with the spermatozoa. The lower part of the uterus secretes a gelatinous medium (or capsule, as the case may be) in which the fertilised ova become enclosed previous to exclusion. In position the oviduct abuts on the kidney, while the uterus is in close proximity to the rectum, and the female external orifice is found close to the anus, within the branchial cavity.

The male organs of Littorina are more simple. The testis is lodged, like the ovary, in the liver; the vas deferens is, like the oviduct, convoluted, and eventually traverses the right side of the neck, emerging near the right tentacle, and terminating in the penis or external copulative organ (Fig. 50).

This system prevails, with but slight modifications in detail, throughout the prosobranchiate Gasteropoda. The most important modification is the passage of the seminal products in certain cases (many of the Diotocardia) through the right kidney, with which the oviduct and vas deferens always stand in close relation. The same arrangement occurs in the Scaphoda and some Pelecypoda.

The penis varies greatly in form and size. In the Strombidae (see Fig. 99) and Buccinidae (Fig. 62) it is very large and prominent; in Littorina it is somewhat spinulose at one side; in Paludina a portion of it is lodged in the right tentacle, which becomes atrophied and much more obtuse than the tentacle on the left side.

=Spermatozoa.=--The shape of the spermatozoa and of the ova in Mollusca is of the usual type. In Paludina Ampullaria, and certain species of Murex two types of spermatozoa occur, one hair-like, the other worm-like, three times as long as the former, and not tapering at one end. The former type alone take part in fertilisation, and penetrate the ovum. It has been suggested that these worm-like spermatozoa are a kind of incipient ova, and indicate a possible stage in commencing hermaphroditism. And, since the nearest allies of the Prosobranchiata (in which these types occur) are hermaphrodite (i.e. the Opisthobranchiata and Pulmonata), it is not unreasonable to suppose that the Prosobranchiata should show some tendency towards hermaphroditism in their genital glands.[256]

=Cephalopoda.=--The special characteristic of the reproductive organs in female Cephalopoda is the development of various glands, some of considerable size, in connexion with the ovary and oviduct. Sepia, Loligo, and Sepiola are furnished with two large nidamental glands, which open into the mantle cavity independently of the oviduct. Their purpose is to produce a viscid mucus, which envelops the ova at the moment of their emission and eventually hardens into the egg-capsules. A pair of accessory nidamental glands occur in Sepia, as well as a pair of smaller glands situated on the oviduct itself.

In many of the male Cephalopoda the vas deferens is long and dilated at its outer end into a glandular reservoir, within which are formed the spermatophores, or narrow cylindrical packets which contain a very large number of spermatozoa. When charged, the spermatophores pass into what is known as Needham’s sac, where they remain until required for use. These spermatophores are a very characteristic part of the reproductive arrangements in the Cephalopoda. The male of Sepia has been noticed to deposit them, during union, upon the buccal membrane of the female. During the emission of the ova by the female, the spermatophores, apparently through the agency of a kind of spring contained at one end, burst, and scatter the spermatozoa over the ova.

=The Hectocotylus Arm.=--Perhaps the most remarkable feature in the sexual relations of all the Mollusca is the so-called hectocotylus of the Cephalopoda. In the great majority of the male Cephalopoda, one of the ‘arms,’ which is modified for the purpose in various ways and to a greater or less extent, becomes charged with spermatophores, and sometimes, during union, becomes detached and remains within the mantle of the female, preserving for some considerable time its power of movement.

The hectocotylus is confined to the dibranchiate Cephalopoda, and its typical form, i.e. when part of the arm becomes disengaged and left with the female, occurs only in three genera of the Octopodidae, viz.[Argonauta, Ocythoe (Philonexis), and Tremoctopus. In all of these, the male is many sizes smaller than the female. In Argonauta the third arm on the left side becomes hectocotylised. At first it is entirely enveloped in a kind of cyst, in such a way that only a small portion of the tip projects; subsequently the cyst parts asunder, and allows the arm to become expanded to its full length, which considerably exceeds that of the other arms. At a certain point the acetabula or suckers terminate, and the remainder of the arm consists of a very long, tapering, sometimes thread-like filament, which is pointed at the extreme tip. It is not yet known how the spermatophores find their way into the hectocotylus, or how the hectocotylus impregnates the ova of the female. The arm thus affected is not always the same. In Tremoctopus it is the third of the right side, in the Decapoda the modification usually affects the fourth of the left.

This singular property of the male Cephalopoda has only recently been satisfactorily explained. It is true that Aristotle, more than twenty-two centuries ago, distinctly stated that certain of the arms were modified for sexual purposes. Speaking of what he calls the polypus (which appears to represent the Octopus vulgaris of the Mediterranean), he says: ‘It differs from the female in having what the fishermen call the white sexual organ on its arm;’ again, ‘Some say that the male has something of a sexual nature (αὶδοιῶδές τι) on one of its arms, that on which the largest suckers occur; that this is a kind of muscular appendage attached to the middle of the arm, and that it is entirely introduced within the funnel of the female’. Unfortunately the word translated by introduced is corrupt, and can only be restored conjecturally. He again remarks, ‘The last of the arms, which tapers to a fine point and is the only whitish arm, it uses in sexual union.’[257]

The typical hectocotylus seems to have entirely escaped notice until early in the present century, when both Delle Chiaje and Cuvier described it, as detected within the female, as a parasite, the latter under the name of Hectocotylus octopodis. Kölliker, in 1845–49, regarded the Hectocotylus of Tremoctopus as the entire male animal, and went so far as to discern in it an intestine, heart, and reproductive system. It was not until 1851 that the investigations of Vérany and Filippi confirmed a suggestion of Dujardin,[258] while H. Müller, in 1853, completed the discovery by describing the entire male of Argonauta.

In all genera of dibranchiate Cephalopoda except Argonauta, Ocythoe, and Tremoctopus, one of the arms is sexually modified in various ways, but never becomes so much prolonged, and is never detached and left with the female. In Loligo Forbesii Stp. the fourth arm on the left has 23 pairs of regularly developed acetabula, which then lessen in size and disappear, being replaced by long pedunculated papillae, of which there are about 40 pairs. In Loligo vulgaris Lam. and L. Pleii Orb. 18 or 19 pairs of acetabula are regularly formed, and then occur 40 pairs of papillae, as in Forbesii. In other species of Loligo (gahi Orb., brevis Bl., brasiliensis Orb.) only the outer row of suckers becomes modified into papillae after about the 20th to the 22nd pair. In Sepioteuthis sepioides the modification is the same as in the Loligo last mentioned, but the corresponding arm on the right side is so covered with acetabula towards its extreme end, that it is thought that it in some way co-operates with the hectocotylised left arm.

In Octopus, the third arm on the right side is subject to modification. This arm is always shorter than the corresponding arm on the other side, and carries fewer suckers, but is furnished at the extreme tip with a peculiar kind of plate, which connects with the membrane at the base of the arm by a channel of skin, which probably conveys the spermatophores up to the tip.

In Octopus vulgaris, the species referred to by Aristotle, the hectocotylised arm is short, thin in its outer half and pointed at the extremity, while the fold of skin is very white, and gives the arm an appearance of being divided by a cleft at the side. At the same time, an unusual development of one or two suckers on the arm is not uncommon.[259]

It is believed that in the Tetrabranchiate Cephalopoda (Nautilus) a union of the four inner ventral arms may correspond functionally to the hectocotylising of the arm in the Dibranchiates.

=Hermaphrodite Mollusca.=--(a) Monogonopora.--The reproductive system in the hermaphrodite Mollusca is far more complicated than in the dioecious, from the union of the male and female organs in the same individual. As a type of the Monogonopora, in which a single orifice serves for both male and female organs, may be taken the common garden snail (Helix aspersa), the accompanying figure of which is drawn from two specimens found in the act of union (Fig. 53).

Beginning from the inside and proceeding outwards we have firstly the hermaphrodite gland or ovo-testis (H.G.), a yellowish white mass of irregular shape, embedded in the liver (L.) and forming part of its spiral but not reaching quite to the apex. Within this gland are developed the ova and spermatozoa. The former are rather large round cells, produced within the outer wall of the gland, while the spermatozoa, which are produced in the more central part, are thread-like bodies, generally aggregated in small bundles. From the hermaphrodite gland the ova and spermatozoa pass through the upper part of the hermaphrodite duct (H.D.), which is always more or less convoluted. Below the convoluted portion, the duct opens into the albumen gland (A.G.), a large linguiform mass of tissue which becomes dilated at the time of pairing, and secretes a thick viscid fluid which probably serves to envelop the ova. Up to this point both the male and female elements follow the same course, but on their exit from the albumen gland they diverge. The hermaphrodite duct becomes greatly enlarged, and is partially divided by a kind of septum into a male and female portion. These run parallel to one another, the larger or female portion (H.DF.), through which the ova pass (and which is sometimes termed the uterus) being dilated into a number of puckered folds, while the smaller or male portion (H.DM.) is comparatively narrow, and not dilated. At their anterior end, the two portions of the duct separate completely from one another, the female portion being then termed the oviduct (OV.) and the male portion the vas deferens (V.D.).

Following first the oviduct, we find that it soon widens into the vagina (V.), which is furnished with a pair of mucous glands (M.G.), one on each side. These are much branched, and resemble little bunches of whitish seaweed. A little above the mucous glands a long tube diverges from the vagina, which is furnished with a produced coecum (C.) and a pouch, the spermatheca (SP.) at the extreme end. In this pouch, and in the duct leading to it, is stored the spermatophore received in union with another snail. Just below the mucous glands the vagina is joined by the dart sac (D.S.), which is more fully described below. Finally, at its lower end the vagina unites with the penis sac at a point just posterior to the common orifice.

Returning now to the male organs, we find that the vas deferens is the continuation of the male portion of the hermaphrodite duct, after its final separation from the female portion. It passes under the retractor muscle of the upper right tentacle, which has been cut away in the specimen figured, to dissect it out. Just before the vas deferens widens into the penis sac, it branches off into a long and tapering tube, the flagellum, in which the spermatozoa are stored and become massed together in the long packet known as the spermatophore. The penis sac (P.S.) is the continuation of the vas deferens beyond the point at which the flagellum diverges. It joins the vagina at its extreme anterior end, uniting with it to form the common genital aperture, which cannot be exactly represented in the figure. The penis itself lies in the interior of the penis sac, and is a rather long muscular tube which is protruded during union, but at other times remains retracted within the sac.

In the Helicidae generally, the form of the generative organs varies with each separate species, sometimes merely as regards the size of the different parts, at others in the direction of greater simplicity or complication. The mucous glands may be absent, and the flagellum greatly reduced in size, or absent altogether.

=The Dart Sac.=--A remarkable part of the reproductive system in many of the true Helicidae is the so-called dart, Liebespfeil, or telum veneris. It consists of ‘a straight, or curved, sometimes slightly twisted tubular shaft of carbonate of lime, tapering to a fine point above, and enlarging gradually, more often somewhat abruptly, to the base.’ The sides of the shaft are sometimes furnished with two or more blades; these are apparently not for cutting purposes, but simply to brace the stem. The dart is contained in a dart sac, which is attached as a sort of pocket to the vagina, at no great distance from its orifice. There are four different forms of sac. It may be single or double, and each of these divisions may be bilobed, each lobe containing one dart at a time. In Helix aspersa the dart is about 5/16 in. in length, and ⅛ in. in breadth at its base (see Fig. 54).

It appears most probable that the dart is employed as an adjunct to the sexual act. Besides the fact of the position of the dart sac anatomically, we find that the darts are extruded and become embedded in the flesh just before or during the act of copulation. It may be regarded, then, as an organ whose punctures induce excitement preparatory to sexual union. It only occurs in well-grown specimens. When once it begins to form, it grows very rapidly, perhaps not more than a week being required for its entire formation.

The dart is almost confined to Helicidae, a certain number of exceptions being known which border on Helix. Hyalinia nitida and excavata are the only British species, not Helices, which are known to possess it. It has not been noticed to occur in the slugs, except in the N. American genus Tebennophorus. About one-third of the British Helices are destitute of the dart.[260] H. rufescens possesses a double bilobed sac, but only two darts, which lie in the lower lobes. It does not use the darts, and could not do so, from the relative sizes of dart and sac; it has often been watched when uniting, but the use of the darts has never been observed. From this it has been inferred that the darts are degenerate weapons of defence, and that they were in fact at one time much stronger organs and more often used.[261] This theory, however, does not seem consistent with the whole circumstances of the occurrence, position, and present use of the darts.

=Hermaphrodite Mollusca.=--(b) Digonopora.--As an example of the Digonopora or hermaphrodite Mollusca with separate generative apertures for the male and female organs, we may take the common Limnaea stagnalis (Fig. 55). It will be seen from the figure that the relative positions of the hermaphrodite gland and duct, and of the albumen gland, are the same as in Helix. When the oviduct parts company from the vas deferens, it becomes furnished with several accessory glands, one of which (GL.E.) probably serves as a reservoir for the ova, and answers more or less to a uterus. The tube leading to the spermatheca is short, and there is no divergent caecum. The female orifice lies near to the external opening of the branchial cavity. The vas deferens, which is very long, is furnished with a large prostate gland. The penis sac is greatly dilated, and there is no flagellum. The male orifice is behind the right tentacle, slightly in advance of the female orifice (compare Fig. 102).

Most of the Opisthobranchiata, but not all, have separate sexual orifices. Numerous variations from the type just described will be found to occur, particularly in the direction of the development of accessory glands, which are sometimes very large, and whose precise purpose has in many cases not been satisfactorily determined.

Pelecypoda.--In the dioecious Pelecypoda, which form the great majority, the reproductive system is simple, and closely parallel in both sexes. It consists of a pair of gonads, which are either ovaries or testes, and a pair of oviducts or sperm-ducts which lead to a genital aperture. The gonads are usually placed symmetrically at the sides or base of the visceral mass. The oviduct is short, and the genital aperture is usually within the branchial chamber, thus securing the fertilisation of the ova by the spermatozoa, which are carried into the branchial chamber with the water which passes through the afferent siphon.

Hermaphrodite Pelecypoda are rare, the sexes being usually separate. The following are assured instances: Pecten glaber, P. jacobaeus, P. maximus, Ostrea edulis, Cardium norvegicum, Pisidium pusillum, Cyclas cornea, Pandora rostrata, Aspergillum dichotomum, and perhaps Clavagella. The greater number of these have only a single genital gland (gonad) on each side, with a single efferent duct from each, but part of the gland is male and part female, e.g. in the Pectens above mentioned. Pandora and Aspergillum have two distinct glands, respectively male and female, on each side, each of the two glands possessing its separate duct, and the two ducts from each side eventually opening near one another. It appears probable that the Septibranchiata (Cuspidaria, Poromya, Lyonsiella, etc.) must also be added to the number of hermaphrodite Pelecypoda which have separate male and female glands.

It is worthy of remark that all the hermaphrodite Pelecypoda belong to forms decidedly specialised, while forms distinctly primitive, such as Nucula, Solenomya, Arca, and Trigonia are all dioecious. In Gasteropoda similarly, the least specialised forms (the Amphineura, with the exception of the Neomeniidae, and the Rhipidoglossa) are dioecious. It is possible therefore that in the ancestors of the Mollusca the separation of the sexes had already become the normal type of things, and that hermaphroditism in the group is, to a certain extent, a sign or accompaniment of specialisation.[262]

=Development of Fresh-water Bivalves.=--The vast majority of fresh-water bivalves either pass the larval stage entirely within the mother, and do not quit her except in a perfectly developed form (Cyclas, Pisidium), or assume a mode of development in which free larvae indeed occur, but are specially modified for adaptation to special circumstances (Unio). Cyclas and Pisidium, and no doubt all the kindred genera, preserve their ova in a sort of brood-pouch within the gills, in which the ova pass the earlier stages of their development. But, even so, the larva of these genera retains some traces of its original free-swimming habits, for a rudimentary velum, which is quite useless for its present form of development, has been detected in Cyclas.

The larva of Dreissensia (see Fig. 47, A), so far as is at present known, stands alone among fresh-water bivalves in being free-swimming, and to this property has been attributed, no doubt with perfect justice, the fact of the extraordinarily rapid spread of Dreissensia over the continent of Europe (chap. xvi.). In expelling the ova, the parent slightly opens the shells and then quickly closes them, shooting out a small point of white slime, which is in fact a little ball of eggs. The general course of development is precisely parallel to that of marine Pelecypoda, greatly resembling, so far as form is concerned, certain stages in the growth of the larvae of Modiolaria and Cardium, as figured by Lovén.[263]

In June and July the larvae appear in large numbers on the surface of the water, when in spite of their exceedingly small size, they can be captured with a fine hand-net. They pass about eight days on the surface, feeding apparently on minute floating algae. During this time, the principal change they undergo is in the formation of the foot, which first appears as a small prominence midway between the mouth and anus, and gradually increases in length and flexibility. When the larva sinks to the bottom, the velum soon disappears entirely, the foot becomes exceedingly long and narrow, while the shell is circular, strongly resembling a very young Cyclas.

=Larvae of Unionidae.=--The early stages of the development of Unio and Anodonta (so far as the species of North America, Europe, and Asia are concerned) is of extreme interest, from the remarkable fact that the young live for some time parasitically attached to certain species of fresh-water fishes. In order to secure this attachment, the larva, which is generally known as Glochidium, develops a long filament which perhaps renders it aware of the neighbourhood of a fish, and also a larval shell furnished with strong hooks by which it fastens itself to the body of its unconscious host (Fig. 56). According to some interesting observations made by Mr. O. H. Latter,[264] the ova pass into the external gill of the mother, in which is secreted a nutritive mucus on which they are sustained until they arrive at maturity and a suitable opportunity occurs for their ‘being born.’ If this opportunity is deferred, and the Glochidia mature, their so-called ‘byssus’ becomes developed, and by being entangled in the gill filaments of the parent, prevents their escaping. It is interesting to notice that, when the nutritive mucus of the parent is used up, it becomes, as it were, the turn of the children to provide for themselves a secondary mode of attachment.

The mother Anodonta does not always retain the Glochidium until fish are in her neighbourhood. Gentle stirring of the water caused them to emit Glochidium in large masses, if the movement was not so violent as to cause alarm. The long slimy masses of Glochidium were observed to be drawn back again within the shell of the mother, even after they had been ejected to a distance of 2 or 3 inches.

It is a mistake to assert that the young Glochidium can swim. When they finally quit the mother, they sink to the bottom, and there remain resting on their dorsal side, with the valves gaping upwards and the so-called byssus streaming up into the water above them. There they remain, until a convenient ‘host’ comes within reach, and if no ‘host’ comes within a certain time, they perish. They are evidently peculiarly sensitive to the presence of fish, but whether they perceive them by smell or some other sense is unknown. “The tail of a recently killed stickleback thrust into a watch-glass containing Glochidium throws them all into the wildest agitation for a few seconds; the valves are violently closed and again opened with astonishing rapidity for 15–25 seconds, and then the animals appear exhausted and lie placid with widely gaping shells--unless they chance to have closed upon any object in the water (e.g. another Glochidium), in which case the valves remain firmly closed.”

In about four weeks after the Glochidium has quitted its host, and the permanent shell has made its appearance within the two valves of the Glochidium, the projecting teeth of the latter press upon the ventral edge of the permanent shell, at a point about half way in its lengthward measurement, retarding the growth of the shell at that particular point, and indenting its otherwise uninterrupted curve with an irregular notch or dent. As growth proceeds, this dent becomes less and less perceptible on the ventral margin of the shell itself, but its effects may be detected, in well-preserved specimens, by the wavy turn in the lines of growth, especially near the umbones of the young shell.

Mr. Latter found that all species of fish with which he experimented had a strong dislike to Glochidium as an article of food. Sometimes a fish would taste it “just to try,” but invariably spit it out again in a very decided manner. The cause of unpleasantness seemed not to be the irritation produced in the mouth of the fish by the attempt of the Glochidium to attach itself, but was more probably due to what the fish considered a nasty taste or odour in the object of his attentions.

* * * * *

The following works will be found useful for further study of this portion of the subject:--

=F. M. Balfour=, Comparative Embryology, vol. i. pp. 186–241.

=F. Blochmann=, Ueber die Entwickelung von Neritina fluviatilis Müll.: Zeit. wiss. Zool. xxxvi. (1881), pp. 125–174.

=L. Boutan=, Recherches sur l’anatomie et le développement de la Fissurelle: Arch. Zool. exp. gén. (2) iii. suppl. (1885), 173 pp.

=W. K. Brooks=, The development of the Squid (Loligo Pealii Les.): Anniv. Mem. Bost. Soc. Nat. Hist. 1880.

„ „ The development of the oyster: Studies Biol. Lab. Johns Hopk. Univ. i. (1880), 80 pp.

=R. von Erlanger=, Zur Entwickelung von Paludina vivipara: Morph. Jahrb. xvii. (1891), pp. 337–379, 636–680.

„ „ Zur Entwickelung von Bythinia tentaculata: Mitth. Zool. Stat. Neap, x. (1892), pp. 376–406.

=H. Fol=, Sur le développement des Ptéropodes: Arch. Zool. exp. gén. iv. (1875), pp. 1–214.

„ Etudes sur le développement des Mollusques. Hétéropodes: ibid v. (1876), pp. 105–158.

„ Etudes sur le développement des Gastéropodes pulmonés: ibid. viii. (1880), pp. 103–232.

=H. Grenacher=, Zur Entwickelungsgeschichte der Cephalopoden: Zeit. wiss. Zool. xxiv. (1874), pp. 419–498.

=B. Hatschek=, Ueber Entwickelungsgeschichte von Teredo: Arb. Zool. Inst. Univ. Wien, iii. (1881), pp. 1–44.

=R. Horst=, On the development of the European oyster: Quart. Journ. Micr. Sc. xxii. (1882), pp. 339–346.

=E. Korschelt= and =K. Heider=, Lehrbuch der vergleichenden Entwickelungsgeschichte der wirbellosen Thiere, Heft iii. (1893), pp. 909–1177 (the work is in process of translation into English).

=A. Kowalewsky=, Embryogénie du Chiton polii avec quelques remarques sur le développement des autres Chitons: Ann. Mus. Hist. Nat. Mars. Zool. i. (1883), v.

=E. Ray Lankester=, Contributions to the developmental history of the Mollusca: Phil. Trans. Roy. Soc. vol. 165 (1875), pp. 1–31.

„ „ Observations on the development of the pond-snail (Lymnaeus stagnalis), and on the early stages of other Mollusca: Quart. Journ. Micr. Sc. xiv. (1874), pp. 365–391.

„ „ Observations on the development of the Cephalopoda: ibid. xv. (1875), pp. 37–47.

=W. Patten=, The embryology of Patella: Arb. Zool. Inst. Univ. Wien, vi. (1886), pp. 149–174.

=M. Salensky=, Études sur le développement du Vermet: Arch. Biol. vi. (1885), pp. 655–759.

=L. Vialleton=, Recherches sur les premières phases du développement de la Seiche (Sepia officinalis): Ann. Sc. Nat. Zool. (7) vi. (1888), pp. 165–280.

=S. Watase=, Observations on the development of Cephalopods: Stud. Biol. Lab. Johns Hopk. Univ. iv. (1888), pp. 163–183.

„ „ Studies on Cephalopods: Journ. Morph. iv. (1891), pp. 247–294.

=E. Ziegler=, Die Entwickelung von Cyclas cornea Lam.: Zeit. wiss. Zool. xli. (1885), pp. 525–569.

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VI. RESPIRATION AND CIRCULATION--THE MANTLE

The principle of respiration is the same in the Mollusca as in all other animals. The blood is purified by being brought, in successive instalments, into contact with pure air or pure water, the effect of which is to expel the carbonic acid produced by animal combustion, and to take up fresh supplies of oxygen. Whether the medium in which a mollusc lives be water or air, the effect of the respiratory action is practically the same.

Broadly speaking, Mollusca whose usual habitat is the water ‘breathe’ water, while those whose usual habitat is the land ‘breathe’ air. But this rule has its exceptions on both sides. The great majority of the fresh-water Mollusca which are not provided with an operculum (e.g. Limnaea, Physa, Planorbis), breathe air, in spite of living in the water. They make periodic visits to the surface, and take down a bubble of air, returning again for another when it is exhausted. On the other hand many marine Mollusca which live between tide-marks (e.g. Patella, Littorina, Purpura, many species of Cerithium, Planaxis, and Nerita) are left out of the water, through the bi-diurnal recess of the tide, for many hours together. Such species invariably retain several drops of water in their branchiae, and, aided by the moisture of the air, contrive to support life until the water returns to them. Some species of Littorina (e.g. our own L. rudis and many tropical species) live so near high-water mark that at neap-tides it must frequently happen that they are untouched by the sea for several weeks together, while they are frequently exposed to a burning sun, which beats upon the rocks to which they cling. In this case it appears that the respiratory organs will perform their functions if they can manage to retain an extremely small amount of moisture.[265]

The important part which the respiratory organs play in the economy of the Mollusca may be judged from the fact that the primary subdivision of the Cephalopoda into Dibranchiata and Tetrabranchiata is based upon the number of branchiae they possess. Further, the three great divisions of the Gasteropoda have been named from the position or character of the breathing apparatus, viz. Prosobranchiata, Opisthobranchiata and Pulmonata, while the name Pelecypoda has hardly yet dispossessed Lamellibranchiata, the more familiar name of the bivalves.

Respiration may be conducted by means of--(a) Branchiae or Gills, (b) a Lung or Lung-cavity, (c) the outer skin.

In the Pelecypoda, Cephalopoda, Scaphopoda, and the great majority of the Gasteropoda, respiration is by means of branchiae, also known as ctenidia[266], when they represent the primitive Molluscan gill and are not ‘secondary’ branchiae (pp. 156, 159).

In all non-operculate land and fresh-water Mollusca, in the Auriculidae, and in one aberrant operculate (Amphibola), respiration is conducted by means of a lung-cavity, or rarely by a true lung, whence the name Pulmonata. The land operculates (Cyclophoridae, Cyclostomatidae, Aciculidae, and Helicinidae) also breathe air, but are not classified as Pulmonata, since other points in their organisation relate them more closely to the marine Prosobranchiata. Both methods of respiration are united in Ampullaria, which breathes indifferently air through a long siphon which it can elevate above the surface of the water, and water through a branchia (see p. 158). Siphonaria (Fig. 57) is also furnished with a lung-cavity as well as a branchia. Both these genera may be regarded as in process of change from an aqueous to a terrestrial life, and in Siphonaria the branchia is to a great extent atrophied, since the animal is out of the water, on the average, twenty-two hours out of the twenty-four. In the allied genus Gadinia, where there is no trace of a branchia, but only a lung-cavity, and in Cerithidea obtusa, which has a pulmonary organisation exactly analogous to that of Cyclophorus,[267] this process may be regarded as practically completed.

Respiration by means of the skin, without the development of any special organ, is the simplest method of breathing which occurs in the Mollusca. In certain cases, e.g. Elysia, Limapontia, and Cenia among the Nudibranchs, and the parasitic Entoconcha and Entocolax, none of which possess breathing organs of any kind, the whole outer surface of the body appears to perform respiratory functions. In others, the dorsal surface is covered with papillae of varied size and number, which communicate with the heart by an elaborate system of veins. This is the case with the greater number of the Aeolididae (Fig. 58, compare Fig. 5, C), but it is curious that when the animal is entirely deprived of these papillae, respiration appears to be carried on without interruption through the skin.

In the development of a distinct breathing organ, it would seem as if progress had been made along two definite lines, each resulting in the exposure of a larger length of veins, i.e. of a larger amount of blood, to the simultaneous operation of fresh air or fresh water. Either (a) the skin itself may have developed, at more or less regular intervals, elevations, or folds, which gradually took the form of papillae, or else (b) an inward folding, or ‘invagination,’ of the skin, or such a modification of the mantle-fold as is described below (p. 172) may have taken place, resulting in the formation of a cavity more or less surrounded by walls, within which the breathing organs were ultimately developed. Sometimes a combination of both processes seems to have occurred, and after a papilliform organ has been produced, an extension or prolongation of the skin has taken place, in order to afford a protection to it. Respiration by means of a lung-cavity is certainly subsequent, in point of time, to respiration by means of branchiae.

The branchiae seem to have been originally paired, and arranged symmetrically on opposite sides of the body. It is not easy to decide whether the multiple form of branchia which occurs in Chiton (Fig. 59), or the simple form as in Fissurella (Fig. 60), is the more primitive. Some authorities hold that the multiple branchia has gradually coalesced into the simple, others that the simple form has grown, by serial repetition, into the multiple. There appears to be no trace of any intermediate forms, and, as a matter of fact, the multiple branchia is found only in the Amphineura, while one or rarely two (never more) pairs of branchiae, occur, with various important modifications, in the vast majority of the Mollusca.

Amphineura.--In Chiton the branchiae are external, forming a long row of short plumes, placed symmetrically along each side of the foot. The number of plumes, at the base of each of which lies an osphradial patch, varies from about 70 to as few as 6 or 7. When the plumes are few, they are confined to the posterior end, and thus approximate to the form and position of the branchiae in the other Amphineura. In Chaetoderma, the branchiae consist of two small feather-shaped bodies, placed symmetrically on either side of the anus, which opens into a sort of cloaca within which the branchiae are situated. In Neomenia the branchiae are still further degraded, consisting of a single bunch of filaments lying within the cloaca, while in Proneomenia there is no more than a few irregular folds on the cloaca-wall (Fig. 61).

In the Prosobranchiata, symmetrically paired branchiae occur only in the Fissurellidae, Haliotidae, and Pleurotomariidae, in the former of which two perfectly equal branchiae are situated on either side of the back of the neck. These three families taken together form the group known as Zygobranchiata.[268] In all other families the asymmetry of the body has probably caused one of the branchiae, the right (originally left), to become aborted, and consequently there is only one branchia, the left, in the vast majority of marine Prosobranchiata, which have been accordingly grouped as Azygobranchiata. Even in Haliotis the right branchia is rather smaller than the left, while the great size of the attachment muscle causes the whole branchial cavity to become pushed over towards the left side. In those forms which in other respects most nearly approach the Zygobranchiata, namely, the Trochidae, Neritidae, and Turbinidae, the branchia has two rows of filaments, one on each side of the long axis, while in all other Prosobranchiata there is but one row (see Fig. 79, p. 169).

In the great majority of marine Prosobranchiata the branchia is securely concealed within a chamber or pouch (the respiratory cavity), which is placed on the left dorsal side of the animal, generally near the back of the neck. For breathing purposes, water has to be conveyed into this chamber, and again expelled after it has passed over the branchia. In the majority of the vegetable-feeding molluscs (e.g. Littorina, Cerithium, Trochus) water is carried into the chamber by a simple prolongation of one of the lobes or lappets of the mantle, and makes its exit by the same way, the incoming and outgoing currents being separated by a valve-like fringe depending from the lobe. In the carnivorous molluscs, on the other hand, a regular tube, the branchial siphon, which is more or less closed, has been developed from a fold of the mantle surface, for the special purpose of conducting water to the branchia. After performing its purpose there, the spent water does not return through the siphon, but is conducted towards the anus by vibratile cilia situated on the branchiae themselves. In a large number of cases, this siphon is protected throughout its entire length by a special prolongation of the shell called the canal. Sometimes, as in Buccinum and Purpura, this canal is little more than a mere notch in the ‘mouth’ of the shell, but in many of the Muricidae (e.g. M. haustellum, tenuispina, tribulus) the canal becomes several inches long, and is set with formidable spines (see Fig. 164, p. 256). In Dolium and Cassis the canal is very short, but the siphon is very long, and is reflected back over the shell.

The presence or absence of this siphonal notch or canal forms a fairly accurate indication of the carnivorous or vegetarian tendencies of most marine Prosobranchiata, which have been, on this basis, subdivided into Siphonostomata and Holostomata. But this classification is of no particular value, and is seriously weakened by the fact that Natica, which is markedly ‘holostomatous,’ is very carnivorous, while Cerithium, which has a distinct siphonal notch, is of vegetarian tendencies.

In the Zygobranchiata the water, after having aerated the blood in the branchiae, usually escapes by a special hole or holes in the shell, situated either at the apex (Fissurella) or along the side of the last whorl (Haliotis). In Pleurotomaria the slit answers a similar purpose, serving as a sluice for the ejection of the spent water, and thus preventing the inward current from becoming polluted before it reaches the branchiae (see Fig. 179, p. 266).

In Patella the breathing arrangements are very remarkable. In spite of their apparent external similarity, this genus possesses no such symmetrically paired plume-shaped branchiae as Fissurella, but we notice a circlet of gill-lamellae, which extends completely round the edge of the mantle. It has been shown by various authorities that these lamellae are in no sense morphologically related to the paired branchiae in other Mollusca, but only correspond to them functionally. The typical paired branchiae, as has been shown by Spengel, exist in Patella in a most rudimentary form, being reduced to a pair of minute yellow bodies on the right and left sides of the back of the ‘neck.’ A precisely similar abortion of the true branchiae, and special development of a new organ to perform their work, is shown in Phyllidia and Pleurophyllidia (see below under Opisthobranchiata). This circlet of functional gills in Patella has therefore little systematic value, being only developed in an unusual position, like the eyes on the mantle in certain Pelecypoda, to supply the place of the true organs which have fallen into disuse. Accordingly Cuvier’s class of Cyclobranchiata, which included Patella and Chiton, has no value, and has indeed long been discarded. In Chiton the gills never extend completely round the animal, but are always more or less interrupted at the head and anus. They are the true gills, the plumes being serially repeated in the same way as the shell plates.

In the land Prosobranchiata (Cyclostomatidae, Cyclophoridae, Aciculidae, Helicinidae) which, having exchanged a marine for an aerial life, breathe air instead of water, the branchia has completely disappeared, and breathing is conducted, as in the Pulmonata, by a lung-cavity. In certain genera of land operculates, e.g. Pupina, Cataulus, Pterocyclus, a slight fissure or tube in the last whorl (see Fig. 180, p. 266) serves to introduce air into the shell, which is perhaps otherwise closed to air by the operculum. In Aulopoma, which has no tube, the operculum admits free circulation of air. In certain other Cyclostomatidae the apex is truncated, and air can enter there. De Folin closed with wax the aperture of Cycl. elegans, and found that on placing it in a pneumatic machine, the shell gave off air through its whole surface. On the other hand, Cylindrella and Stenogyra decollata, on being submitted to the same test, showed that the truncated part alone was permeable by air.

Fischer and Bouvier have made some interesting observations on the breathing of a species of Ampullaria (insularum Orb.). The species has, in common with all Ampullaria, two siphons, but while the right siphon is but slightly developed, the left is very long, almost twice as long as the shell (see Fig. 65). The animal, when under the water, lengthens its siphon, brings the orifice to the surface, and by alternately raising and depressing its head produces in the pulmonary sac movements of ex- and inspiration; these are repeated about ten or fifteen times at regular intervals of from six to eight seconds, a method of respiration strongly resembling that of the Cetacea. At the same time, branchial respiration takes place. If powdered carmine is added to water, the particles are seen to enter the branchial cavity by the siphon and pass out by the short right siphon. Sometimes the animal remains under water for hours without rising to the surface to inspire air. In Valvata (Fig. 66) the branchia is very large, and projects like a leaf or fan above the shell on the left side; on the corresponding position on the right side is a long filiform appendage, which some have regarded as representing the other branchia.

Opisthobranchiata.--A true branchia occurs only in the Tectibranchiata and the Ascoglossa. It lies on the right side, and is usually more or less external, being partly covered sometimes by the shell (as in Umbrella, Fig. 5), sometimes by a fold of the mantle. In the Pteropoda (which are probably derived from the Tectibranchiata), all the Thecosomata, with the exception of Cavolinia, have no specialised branchia, but probably respire through portions or the whole of the integument. In the Gymnosomata an accessory branchia has in many cases been developed at the posterior end of the body. Pneumodermon alone has both lateral and posterior branchiae well developed, Clione and Halopsyche are destitute of either, while the four remaining families have one branchia, sometimes lateral, sometimes posterior.[269]

Certain of the Nudibranchiata possess no special breathing organs, and probably respire through the skin (Elysia, Limapontia, Cenia, Phyllirrhoë). The majority, however, have developed secondary branchiae, in the form of prominent lobes or leaf-like processes (the cerata), which are carried upon the back, without any means of protection. These cerata are, as a rule, of extreme beauty and variety of form, consisting sometimes of long whip-like tentaculae, in other cases of arborescent plumes of fern-like leafage, in others of curious bead-like appendages of every imaginable shape and colour. In Doris they lie at the posterior end of the body, in a sort of rosette, which is generally capable of retraction into a chamber. In Phyllidia and Pleurophyllidia these secondary branchiae lie, as in Patella, on the lateral portions of the mantle.

The Scaphopoda in all probability possess neither true nor secondary branchiae.

Pulmonata.--When we use the term ‘lung,’ it must be remembered that this organ in the Mollusca does not correspond, morphologically, with the spongy, cellular lung of vertebrates; it simply performs the same functions. The ‘lung,’ in the Mollusca, is a pouch or cavity, lined with blood-vessels which are disposed over its vaulted surface in various patterns of network. The pulmonary sac or cavity is therefore a better name by which to denote this organ.

It seems probable, as has been already shown (pp. 18–22), that all Pulmonata are ultimately derived from marine forms which breathed water by means of branchiae. Thus we find intermediate forms, such as Siphonaria, possessed of both a branchia and a pulmonary sac, the former being evanescent, while in Gadinia and Amphibola it has quite disappeared. In the vast majority of Pulmonata no trace of a branchia remains; its function is performed by a chamber, always situated at the right side of the animal, and generally more or less anterior, admitting air by a narrow aperture which is rhythmically opened and closed. In Arion and Geomalacus (Fig. 69) this aperture is in the front of the right side of the ‘shield,’ in Limax (Fig. 71) in the hinder part, in Testacella (Fig. 20) it is near the extremity of the tail, under the spire of the shell; in Janella it is on the middle of the right edge of the shield (Fig. 70). If a specimen of Helix aspersa, or better, of H. pomatia, is held up to the light, the beautiful arborescent vessels, with which the upper part of the pulmonary chamber is furnished, can be clearly seen by looking through the aperture as it dilates. It is only in the Auriculidae that an actual spongy mass of lung material appears to exist. When in motion, a Helix inspires air much more frequently than when at rest. Temperature, too, seems to affect the number of inspirations; it appears doubtful whether, during hibernation, a snail breathes at all. In any case, the amount of air required to sustain life must be small.

With regard to the respiration of fresh-water Pulmonata there appears to be some difference of opinion. It is held, on the one hand, that the Limnaeidae only respire air, making periodic visits to the surface to procure it, and that they perish, if prevented from doing so, by asphyxiation. If, we are told,[270] as a Limnaea is floating on the surface of the water in a glass jar, a morsel of common salt be dropped upon its outstretched foot, it will sink heavily to the bottom, emitting a stream of air from its pulmonary orifice. On recovering from the shock, it will anxiously endeavour to regain the surface, but will have some difficulty in doing so, owing to its now much greater specific gravity. When it succeeds, it creeps almost out of the water, and exposes its respiratory orifice freely to the air. If the experiment is repeated several times on the same individual, it becomes so much weakened that it has to be taken out of the water to save its life. Moquin-Tandon, on the other hand, is strongly of opinion[271] that there is no absolute necessity for Limnaea to obtain air by rising to the surface, and that, if prevented from emerging, it can obtain air from the water. When covered in by a roof of ice, Limnaea has not been observed to suffer any inconvenience. Moquin-Tandon kept L. glabra and Planorbis rotundatus in good health under 20 mm. of water for eighteen and nineteen days, and relates a case in which Physa was kept alive under water for four days, and Planorbis for twelve. Young specimens, both of Limnaea and Planorbis, do not rise to the surface for a supply of air; they are hatched with the pulmonary cavity full of water.

It is probable, therefore, that Limnaeidae are capable, on occasion, of respiration through the skin. Some authorities are of opinion that certain long and narrow lamellae, situated within the pulmonary sac, are employed for the purpose of aqueous respiration. Ancylus, which never makes periodic excursions to the surface, perhaps respires by receiving into its pulmonary chamber the minute quantities of oxygen given off by the vegetation on which it feeds.

Limnaeidae taken from a great depth of water, e.g. from 130 fathoms in the lake of Geneva, have been examined by Forel.[272] The pulmonary sac is full of water, but there is no transformation of organs, no appearance of a branchia, to meet the changed circumstances of their environment. Doubtless a good deal of respiration is done by the skin; being soft and vascular, it respires the air dissolved in the water. Forel cites cases of Limnaea living at much shallower depths, which come to the surface once, and then remain below for months. The oxygen of this supply must soon have become exhausted, and the animals, discontinuing for a time the use of the pulmonary chamber, must have respired through the skin. Shallow-water Limnaea, according to the same authority, remain beneath the surface during cold weather; when warm weather returns they rise to the surface to take in a supply of air. Since the water at great depths is always very cold, there is no need for the Limnaea living there to rise to the surface at all.

It is a curious fact that Limnaea, which have been respiring by the skin for the whole winter, should suddenly, on the first warm days of summer, take to rising to the surface and breathing air. But exactly the same phenomenon is shown in the case of Limnaea from great depths. Placed in an aquarium, they immediately begin rising to the surface and inspiring air; in other words, they experience instantaneously a complete transformation of their respiratory system.

In Onchidium, a land pulmonate which has retrogressed to an amphibious or quasi-marine mode of life, there is no organ which represents the pulmonary or branchial cavity, the so-called lung being only a cavity of the kidney. Respiration is, however, conducted by the skin as well, and by the dorsal papillae.[273]

Land Mollusca can sustain, for a considerable time, complete deprivation of atmospheric air. Helices placed in an exhausted receiver show no signs of being inconvenienced for about 20 hours, and are able to survive for about two or three days. If detained under water, they are very active for about 6 hours, then become motionless, the body swells, owing to the water absorbed, and death ensues in about 36 hours. Immersion for only 24 hours is generally followed by recovery. In the latter case, the cause of death is not so much deprivation of air as compulsory absorption of water by the skin. The amount of water thus taken up is surprising. Spallanzani found that a Helix which weighed 18 grammes increased in weight by 13½ grammes after a prolonged immersion. Even slugs enclosed in moist paper gained more than 2 grammes in the course of half an hour. Experiment has shown that the amount of carbonic acid gas produced by respiration stands in direct relation to the amount of food consumed. Four pairs of snails were taken which had recently awakened from their winter sleep and had eaten heartily, and an equal number, under the same circumstances, which had been prevented from eating. It was found that the first four pairs produced, in consuming a given amount of oxygen, 11, 9, 10, and 13 parts respectively of carbonic acid, while the second set produced, in consuming the same amount of oxygen, only 4, 8, 7, and 9 parts of carbonic acid.[274] Hibernating Helices, if weighed in December and again in April, will be found to have lost weight, due to the expiration of carbonic acid. Owing to the difficulty of experiment, opinions vary as to the absolute temperature of snails. It appears to be established that several snails, if placed together in a tube, raise the temperature one or two degrees C., but as a rule, the temperature of a solitary Helix differs very slightly from that of the surrounding air. Increased activity, whether in respiration or feeding, is found to raise the temperature.

W. H. Dall, writing of the branchia in Pelecypoda, remarks[275] that there can be no doubt that its original form was a simple pinched-up lamella or fold of the skin or mantle. This, elongated, becomes a filament. Filaments united by suitable tissue, trussed, propped, and stayed by a chitinous skeleton, result in the forms, wonderful in number and complexity, which puzzle the student to describe, much more to classify.

In Pelecypoda the branchiae are placed on each side of the body, between the mantle and the visceral mass. They lie in a chamber known as the branchial cavity. Leading into this cavity, and behind it, are, as a rule, two tubes or siphons, one of which conducts water to the branchiae, while the other carries it away after it has passed over them. The lower is known as the branchial or afferent siphon, the upper as the anal or efferent siphon (see Figs. 72 and 73). The action of these siphons can readily be observed by placing a little carmine in water, near to the siphonal apertures of an Anodonta or Unio. In many cases (e.g. Psammobia, Tellina, Mya, genera which burrow deeply in sand) both the siphons are exceedingly long, sometimes considerably longer than the whole shell. In some cases the two tubes are free throughout their entire length, in others they become fused together before their entrance within the shell (Fig. 74). In other genera, which do not burrow (e.g. Ostrea, Pecten, Arca, Mytilus), the siphons are rudimentary or altogether absent (Fig. 75).

The number and arrangement of the branchiae varies considerably. It appears probable that the different degrees of complication of the gill indicate degrees of specialisation in the different groups of Pelecypoda, in other words, assuming that a simpler form of gill precedes, in point of development, a more complicated form, the nature of the gill may be taken as indicating different degrees of removal from the primitive form of bivalve.

1. The simplest form of gill (Nucula, Leda, Solenomya, etc.) is that which consists (Fig. 76, A, compare Fig. 100, p. 201) of two rows of very short, broad, not reflected filaments, the rows being placed in such a way that they incline at right angles to one another from a common longitudinal axis. The filaments are not connected with one another, nor are the two leaves of each gill united at any point. (Protobranchiata.)

2. In the Anomiidae, Arcadae, Trigoniidae, and Mytilidae each gill consists of two plates or rows of much longer filaments, which consequently occupy a much larger space in the mantle cavity (Fig. 76, B). Unable to extend beyond the limits of the mantle, filaments are reflected or doubled back upon one another, those of the external plate being reflected towards the outside, those of the internal plate towards the inside. Each separate filament is not connected with the filament next adjacent, except by surface cilia situated on small projections on the sides of the filaments, and interlocking with the cilia of the adjacent filament. The two superposed plates or leaves of the gill may or may not be united by cords running between the two parts of a filament. (Filibranchiata.)

3. In the Pectinidae, Aviculidae, and Ostreidae a further development takes place. The filaments of each gill are reflected in the same way as in the Filibranchiata, but the part thus reflected may become completely united or ‘concresce’ with the mantle on the exterior and with the base of the foot on the interior side. The leaves of each gill plate, which have thus become doubled (the gills being apparently two instead of one on each side), are folded or crumpled, and the filaments are modified at the re-entrant angles of the fold. (Pseudolamellibranchiata.)

4. In all the remaining Pelecypoda, except class 5, in other words, in the very large majority of families, the filaments are either reflected, as in (3), or simple; but the process of concrescence is so far advanced that the adjacent filaments are always intimately connected with one another in such a way as to admit the passage of the blood; and the leaves of each gill-plate (Fig. 76, C) are united by cross channels in a similar way. (Eulamellibranchiata.)

5. In certain of the Anatinacea alone (Cuspidaria, Lyonsiella, Poromya, Silenia) the gills are transformed into a more or less muscular partition, extending from one adductor muscle to the other (Fig. 76, D), and separating off the pallial chamber into two distinct divisions, which communicate by means of narrow slits in the partition. (Septibranchiata.)

Thus the process of gill development in the Pelecypoda appears to lead up from a simple to a very complex type. In its original form, at all events in the most primitive form known to us, the gill is a series of short filaments, quite independent of one another, strung in two rows; then the filaments become longer and double back, while at the same time they begin to show signs of adhesion, as yet only superficial, to one another. In a further stage, the reflected portions become fused to the adjacent surfaces of the foot and mantle, while the interlamellar junctions serve to lock the two gill-plates together; finally, the mere ciliary junction of adjacent filaments is exchanged for intimate vascular connection, while the gill-plates as a whole become closely fused together in a similar manner.

This theory of origin is strengthened by closer observation of the phenomena of a single group. Taking the Septibranchiata as an instance, we find that in Lyonsiella the branchiae unite with the mantle in such a way as to form two large pallial chambers, the structure of the branchiae being preserved, and their lamellae covering the partition. A further stage is observed in Poromya. There, a similar partition exists, but it has become muscular, preserving, however, on each side two groups of branchial lamellae, separated one from the other by a series of slits, which form a communication between the two pallial chambers. A further stage still is seen in Silenia. There the same muscular partition exists, but the branchial lamellae on either side have disappeared, the slits between the two chambers, which occur in Poromya, still persisting, but separated into three groups. Cuspidaria represents the last stage in the development. In the ventral chamber there appears nothing at all corresponding to a branchia; the surface of the partition appears perfectly uniform, but on careful examination three little separate orifices, remains of the three groups of orifices in Silenia, are observed.[276]

Relation between Branchiae and Heart.--The object of the branchiae being, as has been already stated, to aerate the blood on its way to the heart, we find that the heart and the branchiae stand in very important structural relations to one another. When the branchiae are in pairs, we find that the auricles of the heart are also paired, the auricle on the right and left sides being supplied by the right and left branchiae respectively. This is the case with the Dibranchiate Cephalopods (Argonauta, Octopus, Loligo, etc.), the Zygobranchiate Prosobranchs (Fissurella, Haliotis), and all Pelecypoda. In the Amphineura (Chiton, etc.) there are two auricles corresponding to the two sets of multiple branchiae. In the case of the Tetrabranchiate Cephalopods (Nautilus) there are four auricles corresponding to each of the four branchiae. Compare Fig. 79, A, B, C, D, E.

On the other hand, when the branchia is single, or when both branchiae are on the same side, and one is aborted and functionless, the auricle is single too, and on the same side as the branchia. This is the case with the Tectibranchiate Opisthobranchs (Philine, Scaphander, etc.), all the Pectinibranchiate Prosobranchs (Rachiglossa, Taenioglossa, and Ptenoglossa), and the other Azygobranchiate Prosobranchs (Trochidae, Neritidae, etc.). In the last case the right auricle exists, as well as the left, but is simply a closed sac, the coalescing of the two gills on the left side having thrown all the work upon the left auricle. Compare Fig. 79, F, G, H.

=Circulatory System=

All Mollusca, without exception, possess a circulatory system of more or less complexity. The centre of the system is the heart, which receives the aerated blood from the breathing organs, and propels it to every part of the body. In the Scaphopoda alone there appears to be no distinct heart.

The heart may consist simply of a single auricle and ventricle, and an aorta opening out of the ventricle. From the aorta the blood is conveyed to the various parts of the body by arteries. Veins convey the blood back to the breathing organs, after passing over which it returns by the branchial or pulmonary vein to the heart, thus completing the circuit.

As regards position, the heart is situated within the pericardium, a separate chamber which in the Pelecypoda, Cephalopoda, and the bilaterally symmetrical Gasteropoda lies on the median line, while in the asymmetrical Gasteropoda it is on one or other of the sides of the body, usually the right. The veins connected with the branchiae, and consequently the auricle into which they open, are situated behind the ventricle in the Opisthobranchiata (whence their name), while in the Prosobranchiata they are situated in front of the ventricle.

The number of auricles corresponds to the number of branchiae. Thus there is only one auricle in the great majority of Prosobranchiata (which are accordingly classified as Monotocardia), and also in the Opisthobranchiata, while the Pulmonata have a single auricle corresponding to the pulmonary chamber. There are two auricles in the Amphineura, in a small group of Gasteropoda, hence known as Diotocardia, in all Pelecypoda, and in the Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda alone there are four auricles corresponding to the four branchiae.

A single aorta occurs only in the Amphineura and in the Tetrabranchiate Cephalopoda. In all the other groups there are two aortae, leading out of the anterior and posterior ends of the ventricle in Pelecypoda and Dibranchiate Cephalopoda, while a single aorta leads out of the posterior end alone, and subsequently bifurcates, in most of the Gasteropoda. One aorta, the cephalic, supplies the front part of the body, the oesophagus, stomach, mantle, etc.; the other, the visceral aorta, supplies the posterior part, the liver and sexual organs.

The general circulatory system in the Mollusca has not yet been thoroughly investigated. As a general rule, the blood driven from the ventricle through the aorta into the arteries, passes, on reaching the alimentary canal and other adjacent organs, into a number of irregular spaces called lacunae. These in their turn branch into sinuses, or narrow tubes covered with muscular tissue, which penetrate the body in every direction. In the Dibranchiate Cephalopoda true capillaries are said to occur, which in some cases form a direct communication between the arteries and veins. According to some authorities[277] capillaries and veins exist in certain Pelecypoda in connexion with the intestinal lacunae, but this again is regarded by others as not established. A similar difference of opinion occurs with regard to the precise function of the foot-pore which occurs in many Mollusca, some holding that it serves as a means for the introduction of water into the blood-vascular system, while others regard it as a form of secretion gland, the original purpose of which has perhaps become lost.

=Blood.=--As a rule, the blood of the Mollusca--i.e. not the corpuscles but the liquor sanguinis--is colourless, or slightly tinged with blue on exposure to the air. This is due to the presence of a pigment termed haemocyanin, in which are found traces of copper and iron, the former predominating. Haemoglobin, the colouring matter of the blood in Vertebrates, is, according to Lankester,[278] of very restricted occurrence. It is found--(1) in special corpuscles in the blood of Solen legumen (and Arca Noae); (2) in the general blood system of Planorbis; (3) in the muscles of the pharynx and jaws of certain Gasteropoda, e.g. Limnaea, Paludina, Littorina, Chiton, Aplysia. This distribution of haemoglobin is explained by Lankester in reference to its chemical activity; whenever increased facilities for oxidisation are required, then it may be present to do the work. The Mollusca, being as a rule otiose, do not possess it generally diffused in the blood, as do the Vertebrata. The actively burrowing Solen possesses it, and perhaps its presence in Planorbis is to be explained from its respiring the air of stagnant marshes. Its occurrence in the pharyngeal muscles and jaws of other genera may be due to the constant state of activity in which these organs are kept.[279]

According to Tenison-Woods[280] a species of Arca (trapezia Desh.) and two species of Solen, all Australian, have red blood. It is suggested that in these cases the habits of the animal (the Solen burrowing deeply in sand, the Arca in mud) require some highly oxidising element, surrounded as the creature is by ooze. In Arca pexata (N. America) the blood is red, the animal being familiarly known as the ‘bloody clam.’ Burrowing species, however, are not all distinguished by this peculiarity. Tenison-Woods finds red fluids in the buccal mass of many Gasteropoda, e.g. in species of Patella, Acmaea, Littorina, Trochus, Turbo, giving the parts the appearance of raw meat.

=The Mantle=

On the dorsal side of the typical molluscan body, between the visceral sac and the shell, lies a duplicature of the integument, generally known as the mantle. The depending sides of the mantle, which are usually somewhat thickened, enclose between themselves and the body mass a chamber of varying size and shape, called the mantle cavity, which communicates freely with the external air or water, and encloses and furnishes a protection for the organ or organs of respiration. On its upper or dorsal surface the mantle is closely applied to the shell throughout its whole extent, the cells with which it is furnished secreting the materials from which the shell is formed (see p. 255). The whole mantle is capable, to some degree, of secreting shelly matter, but the most active agent in its production is the mantle edge or margin.

In the Prosobranchiata the mantle cavity, for reasons which have already been explained, is found on the left side of the animal, its front portion being in many cases produced into a tubular siphon. Within the mantle cavity are found, besides the branchia, the anus, the apertures of the kidneys, and the osphradium. In the pulmonata the mantle fold encloses a so-called lung-cavity. The front edge of the mantle coalesces with the integument of the neck in such a way as to enclose the cavity very completely, the only communication with the outer air being by means of the contractile breathing or pulmonary aperture on the right side. In the Tectibranchiate Opisthobranchs the mantle fold is inconsiderable, and is usually not of sufficient extent to cover the branchia, while in the Nudibranchs, which have no true branchiae, it disappears altogether.

In the Pelecypoda the mantle cavity is equally developed on each side, enclosing the two sets of branchiae. The mantle may thus be regarded as consisting of two equal portions, which form a sort of lining to the two valves. The lower or ventral portion of the mantle edges may be simple, or provided with ocelli (Pecten, Arca), tentacles, cilia (Lima, Lepton), or doubled folds. The two portions of the mantle touch one another along the whole line of the edge of the two valves, and, although thus in contact, may remain completely separate from one another, or else become permanently united at one or more points. This fusion of the mantle edges corresponds to important changes in the organisation of the animal as a whole. The anal and branchial siphons are no more than prolongations of the mantle edges on the posterior side into a tubular form. These ‘siphons’ exhibit the siphonal form more distinctly according as the adjacent portions of the mantle become more definitely fused together.

This progressive fusion of the mantle edges may be taken as indicating definite stages in the development of the Pelecypoda. A perfectly free mantle edge, joined at no point with the edge of the adjacent mantle, occurs in Nucula, Arca, Anomia, and Trigonia (see Fig. 80, A, B). Here there is nothing in the nature of a siphon, either anal or branchial; in other words, no contrivance exists to prevent the spent water which has passed over the branchiae from becoming mixed with the fresh water which is to reach them. When the mantle edges are fused at one point only, this is invariably on the middle part of the posterior side, thus separating off an anal opening which may become prolonged into a tube-like form. At the same time the adjacent underlying portions of the mantle edges draw together, without actually coalescing, to form an opening for the incurrent stream of water, the rudiments of the ‘branchial siphon’ (Fig. 80, C). This is the case with most Mytilidae (see Fig. 75) with Cardita, Astarte, and Pisidium. In the next stage the branchial opening is separated off by the concrescence of the mantle edges beneath it, and we have the mantle united in two places, thus forming three openings, the ventral of which is the opening for the protrusion of the foot (Fig. 80, D). This is the case in Yoldia, Leda, the majority of the Eulamellibranchiata (e.g. Lucina, Cyrena, Donax, Psammobia, Tellina, Venus, Cardium, Mactra), and all Septibranchiata. In Chama and Tridacna the fused portions of the mantle become more extended, and in Pholas, Xylophaga, Teredo, Pandora, and Lyonsia this concrescence takes place over the greater length of the whole mantle edge, so that the mantle may be regarded as closed, with the exception of the three apertures for the foot and the two siphons (Fig. 80, E).

In certain genera there occurs, besides these three apertures, a fourth, in the line of junction between the pedal and branchial orifices. It appears probable that this fourth orifice (which has been regarded by some as an inlet for water when the siphons are retracted), stands in relation to the byssal apparatus (Fig. 80, F). In Lyonsia, for instance, a thick byssus protrudes through the orifice, which is large and open. In Solen, Lutraria, Glycimeris, Cochlodesma, Thracia, Aspergillum, and a few more genera, which have no byssus, the orifice is very small and narrow. It is possible that in these latter cases, the byssal apparatus having become atrophied, the orifice has been correspondingly reduced in size.[281]

=Mantle Reflected over the Shell.=--It is sometimes the case that the mantle edges tend to double back over the external surface of the shell, and to enclose it to a greater or less extent. When this process is carried to an extreme, the edges of the reflected mantle unite, and the shell becomes completely internal. We see an incipient stage of this process in Cypraea and Marginella, where the bright polish on the surface of the shell is due to the protection afforded by the lobes of the mantle. A considerable portion of the shell of Scutus is concealed in a similar way, while in Cryptochiton, Lamellaria, and Aplysia the shell is more or less completely enclosed. Among Pulmonata, it is possible that in forms like Vitrina, Parmacella, Limax, and Arion, we have successive stages in a process which starts with a shell completely external, as in Helix, and ends, not merely by enveloping the shell in the mantle, but by effecting its disappearance altogether. In Vitrina and some allied genera we have a type in which the mantle lobes are partly reflected over the shell, which at the same time exhibits rather less of a spiral form than in Helix. In the stage represented by Parmacella, the mantle edges have coalesced over the whole of the shell, except for a small aperture immediately over the spire; the nucleus alone of the shell is spiral, the rest considerably flattened. In Limax the shell has become completely internal, and is simply a flat and very thin plate, the spiral form being entirely lost, and the nucleus represented by a simple thickening at one end of the plate. In Arion, the final stage, we find that the shell, being no longer needed as a protection to the vital organs, has either become resolved into a number of independent granules, or else has entirely disappeared.

Some indications of a similar series of changes occur in the Pelecypoda. The mantle edge of Lepton is prolonged beyond the area of the valves, terminating in some cases in a number of filaments. In Galeomma and Scintilla the valves are partially concealed by the reflected mantle lobes, and in a remarkable form recently discovered by Dall[282] (Chlamydoconcha) the shell is completely imbedded in the mantle, which is perforated at the anterior end by an orifice for the mouth, and at the posterior end by a similar orifice for the anus. In all these cases, except Lepton, it is interesting to notice that the hinge teeth have completely disappeared, the additional closing power gained by the external mantle rendering the work done by a hinge unnecessary. It is quite possible, on the analogy of the Gasteropoda mentioned above, and also, it may be added, of the Cephalopoda and other groups, that we have here indicated the eventual occurrence of a type of Pelecypoda altogether deprived of valves, a greatly thickened mantle performing the part of a shell.[283]

* * * * *

The following works will be found useful for further study of this portion of the subject:--

=F. Bernard=, Recherches sur les organes palléaux des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7) ix. (1890), pp. 89–404.

=G. Cuvier=, Le Régne animal (ed. V. Masson); Mollusca, Text and Atlas.

=C. Grobben=, Beiträge zur Kenntniss des Baues von Cuspidaria (Neaera) cuspidata Olivi, nebst Betrachtungen über das System der Lamellibranchiaten: Arb. Zool. Inst. Wien, x. (1893), pp. 101–146.

=E. Ray Lankester=, Encyclopaedia Britannica, 9th ed., vol. xvi. (1883), Art. ‘Mollusca.’

=A. Ménégaux=, Recherches sur la circulation des Lamellibranches marins: Besançon, 1890.

=K. Mitsukuri=, On the structure and significance of some aberrant forms of Lamellibranchiate gills: Q. Journ. Micr. Sc., N.S. xxi. (1881), pp. 595–608.

=H. L. Osborn=, On the gill in some forms of Prosobranchiate Mollusca: Stud. Biol. Lab. Johns Hopk. Univ. iii. (1884), pp. 37–48.

=R. Holman Peck=, The structure of the Lamellibranchiate gill: Q. Journ. Micr. Sc., N.S. xvii. (1877), pp. 43–66.

=P. Pelseneer=, Contributions à l’étude des lamellibranches: Arch. Biol. xi. (1891), pp. 147–312.

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Chapter VII

ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE FOOT--THE NERVOUS SYSTEM

Organs of Sense: I. Touch

Tactile organs, although occurring in some of the Mollusca, do not appear to attain special or marked development, except in a few cases. The whole surface of the skin, and particularly of the foot, is very sensitive to the slightest impression. Nearly all Gasteropoda are furnished with at least two cephalic tentacles, projecting like horns from each side of the fore part of the head. At or near the base of these are generally situated the eyes. In the Helicidae the eyes are situated, not at the base, but at the apex of the tentacles, and in that case--except in Vertigo--a second pair of shorter tentacles appears beneath the longer pair. It frequently happens that several senses are centred in a single organ; thus the upper tentacles of snails not only carry the eyes and serve to a certain extent as tactile organs, but they also carry the organs of smell.

The edges of the mantle, which are sometimes specialised into lobes, appear to be keenly sensitive to touch in all Gasteropoda.

In Cypraea (Fig. 81) these lobes, or tentaculae, are a prominent feature of the animal, and also in certain genera of the Trochidae (Fig. 82). In most of the carnivorous land Pulmonata--e.g. Testacella, Rhytida, Ennea--there are developed, under the lower pair of tentacles, and close to the mouth, large labial palps or feelers. These are connected with the cerebral ganglion by a very large nerve, and may therefore be supposed to be of extreme sensitiveness. In some of the large carnivorous forms (Glandina, Aerope, compare Fig. 21, p. 54) these palpae are of great size, and curl upwards like an enormous pair of moustaches. When a Glandina seizes its prey, the palpae (see Fig. 83) appear to enfold it and draw it in towards the mouth.

[Illustration: FIG. 81.--Cypraea moneta L., showing tentaculae at edge of mantle, which partly envelops the shell: =Si=, siphon; =M=, =M=, mantle; =F=, foot; =T´=, tentaculae at edge of mantle. (After Quoy and Gaimard.) × 3/2.

It is in the Opisthobranchiata that the organs of touch attain their maximum development. Many of this group are shell-less or possess a small internal shell, and accordingly, in the absence of this special form of defence, a multiplied sense of touch is probably of great service. Thus we find, besides the ordinary cephalic tentacles, clusters or crowns of the same above the head of many Nudibranchiata, with lobe-like prolongations of the integument, and tentacular processes in the neighbourhood of, or surrounding the branchiae (see Figs. 58 and 84), or even projecting from the whole upper surface of the body (Fig. 5, C).

In the Pelecypoda, the chief organs of touch are the foot, which is always remarkably sensitive, especially towards its point, the labial palps on each side of the mouth, and the siphons. In certain cases the mantle border is prolonged into a series of threads or filaments. These are particularly noticeable in Pecten, Lepton, and Lima (Fig. 85), the mantle lobes of the common L. hians of our own coasts being very numerous, and of a bright orange colour. In many genera--e.g. Unio, Mactra--this sensibility to touch appears to be shared by the whole mantle border, although it is not furnished with any special fringing. The ‘arms’ of the Cephalopoda appear to be keenly sensitive to touch, and this is particularly the case with the front or tentacular pair of arms, which seem to be employed in an especial degree for exploration and investigation of strange objects.

=Taste.=--The sense of taste is no doubt present, to a greater or less extent, in all the head-bearing Mollusca. In many of these a special nerve or nerves has been discovered in the pharynx, connecting with the cerebral ganglion; this no doubt indicates the seat of the faculty of taste. The Mollusca vary greatly in their likings for different kinds of food. Some seem to prefer decaying and highly odoriferous animal matter (Buccinum, Nassa), others apparently confine themselves to fresh meat (Purpura, Natica, Testacella), others again, although naturally vegetarian, will not refuse flesh on occasion (Limax, Helix).

Mr. W. A. Gain[284] has made some interesting experiments on the taste of British land Mollusca, as evidenced by the acceptance or rejection of various kinds of food. He kept twelve species of Arion and Limax, and eight species of Helix in captivity for many months, and tried them with no less than 197 different kinds of food, cannibalism included. Some curious points came out in his table of results. Amalia gagates appears to be surprisingly omnivorous, for out of 197 kinds of food it ate all but 25; Arion ater came next, eating all but 40. Limax arborum, on the other hand, was dainty to a fault, eating only seven kinds of food, and actually refusing Swedes, which every other species took with some avidity. Certain food was rejected by all alike, e.g. London Pride, Dog Rose, Beech and Chestnut leaves, Spruce Fir, Common Rush, Liverwort, and Lichens; while all, or nearly all, ate greedily of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries, Boletus edulis, and common grasses. Few of our common weeds or hedgerow flowers were altogether rejected. Arion and Limax were decidedly less particular in their food than Helix, nearly all of them eating earthworms and puff-balls, which no Helix would touch. Arion ater and Limax maximus ate the slime off one another, and portions of skin. Cyclostoma elegans and Hyalinia nitida preferred moist dead leaves to anything else.

II. Sight

=Position of Eyes.=--In the majority of the head-bearing Mollusca the eyes are two in number, and are placed on, or in the immediate neighbourhood of the head. Sometimes they are carried on projecting tentacles or ‘ommatophores,’ which are either simple (as in Prosobranchiata) or capable of retraction like the fingers of a glove (Helix, etc.). Sometimes, as in a large number of the marine Gasteropoda, the eyes are at the outer base of the cephalic tentacles, or are mounted on the tentacles themselves, but never at the tip (compare Fig. 60, p. 153 and Fig. 98, p. 199). In other cases they are placed somewhat farther back, at the sides of the neck. The Pulmonata are usually subdivided into two great groups, Stylommatophora and Basommatophora (Fig. 86), according as the eyes are carried on the tip of the large tentacles (Helix, and all non-operculate land shells), or placed at the inner side of their base (Limnaea, Physa, etc.). In land and fresh-water operculates, the eyes are situated at the outer base of the tentacles.

In the Helicidae, careful observation will show that the eyes are not placed exactly in the centre of the end of the tentacle, but on its upper side, inclining slightly outwards. The eye is probably pushed on one side, as it were, by the development of the neighbouring olfactory bulb. The sense of smell being far more important to these animals than the sense of sight, the former sense develops at the expense of the latter.

=Organisation of the Molluscan Eye.=--The eye in Mollusca exhibits almost every imaginable form, from the extremely simple to the elaborately complex. It may be, as in certain bivalves, no more than a pigmented spot on the mantle, or it may consist, as in some of the Cephalopoda, of a cornea, a sclerotic, a choroid, an iris, a lens, an aqueous and vitreous humour, a retina, and an optic nerve, or of some of these parts only.

In most land and fresh-water Mollusca the eye may be regarded, roughly speaking, as a ball connected by an exceedingly fine thread (the optic nerve) with a nerve centre (the cerebral ganglion). In Paludina this ball is elliptic, in Planorbis and Neritina it is drawn out at the back into a conical or pear shape. In Helix (Fig. 87) there is a structureless membrane, surrounding the whole eye, a lens, and a retina, the latter consisting of a nervous layer, a cellular layer, and a layer of rods containing pigment, this innermost layer (that nearest the lens) being of the thickness of half the whole retina.

Comparing the eyes of different Gasteropoda together, we find that they represent stages in a general course of development. Thus in Patella the eye is scarcely more than an invagination or depression in the integument, which is lined with pigmented and retinal cells. The next upward stage occurs in Trochus, where the depression becomes deeper and bladder-shaped, and is filled with a gelatinous or crystalline mass, but still is open at the top, and therefore permits the eye to be bathed in water. Then, as in Turbo, the bladder becomes closed by a thin epithelial layer, which finally, as in some Murex, becomes much thicker, while the ‘eyeball’ encloses a lens (Fig. 88), which probably corresponds with the ‘vitreous humour’ of other types.

In Nautilus the eye is of a very simple type. It consists of a cup-shaped depression, with a small opening which is not quite closed by the integument. The retina consists of cells which line the interior of the depression, and which communicate directly with the branches of the optic nerve, there being no iris or lens. This type of eye, it will be observed, corresponds exactly with that which occurs in Patella. It appears also to correspond to a stage in the development of eyes in the Dibranchiata (e.g. Octopus, Sepia, Loligo). Lankester has shown[285] that in Loligo the eye first appears as a ridge, enclosing an oval area in the integument. By degrees the walls of this area close in, and eventually join, enclosing the retinal cells within the chamber in which the lens is afterwards developed (Fig. 89). It thus appears that in some cases the development of the eye is arrested at a point which in other cases only forms a temporary stage towards a higher type of organisation.

The developed eye in the dibranchiate Cephalopods consists of a transparent cornea, which may or may not be closed over the front of the lens. Behind the cornea is a narrow chamber (the anterior optic chamber) which is continued for three parts round the whole circle of the eye, and into which project the front portion of the lens and the folds of the iris. Throughout its whole extent, the anterior optic chamber is lined by the integument, the portion of which on the inner side is the choroid. The lens is divided into an outer and inner segment by a thin membrane, and is supported by the ciliary body which forms a continuation of the retina. The main portion of the lens lies within the posterior optic chamber, at the back and sides of which is found the retina (Grenacher).

There can be no doubt that the Cephalopoda use their eyes to observe, but there is nothing to show that any other Mollusca use their eyes for this purpose, the sense of smell in their case largely taking the place of visual observation. Madame Jeannette Power once saw[286] the Octopus in her aquarium holding a fragment of rock in one of its arms, and watching a Pinna which was opening its valves. As soon as they were perfectly open, the Poulpe, with incredible address and promptitude, placed the stone between the valves, preventing the Pinna from closing again, upon which it set about devouring its victim. The next day the Poulpe was seen, after crushing some Tellina, to stretch himself down close by a Triton nodiferus, and watch it attentively. After four hours the Triton emerged from its shell, when the Octopus sprang upon it, and surrounded it with its arms.

=Powers of Vision in Land Mollusca.=--The Helicidae are undoubtedly very short-sighted. Seldom emerging from their retreats except in twilight and darkness, they are naturally myopic, and see better in a subdued than in a bright light. Experiment has shown that a Helix can perceive an object better at 6 centimetres distance in a weak light than at 4 or 5 millimetres in a strong one. Cyclostoma elegans and Paludina vivipara are comparatively long-sighted, perceiving objects at a distance of 20 to 30 centimetres.[287] The increased power of vision is due, in these two cases, to increased elaboration in the construction of the eye, Paludina possessing a large and almost spherical lens, to which the vitreous humour closely adheres, while in Cyclostoma the lens is remarkably hard, and the aqueous humour very abundant. According to V. Willem,[288] the Pulmonata are very sensitive to the slightest movement of the air or jarring of the surface on which they crawl, but are so short-sighted as only to perceive a confused image of a large object at about 1 cm., and to distinguish the form of objects at not more than 1 or 2 mm. The senses of touch and smell are far more active than that of sight. A bean-pod enclosed in a narrow glass case and placed before a hungry snail was not noticed, but when taken out of the case and placed 8 cm. behind the snail, the latter at once turned towards it to devour it.

Some interesting experiments were conducted by the same author with the view of ascertaining whether snails avoid or court the light. He placed a number of species in different wooden boxes, which were divided into a light and a dark compartment, having previously well soaked the boxes in water to secure a humid atmosphere and surface, and so induce the snails to move about. The result showed that nearly all species have a marked predilection one way or the other, but not all in the same way. Helix aspersa, Arion empiricorum, six species of Limax, and three of Planorbis, are lovers of darkness, while H. nemoralis, Succinea putris, and two species of Limnaea are lovers of light. Physa fontinalis stands alone in being quite indifferent.

M. Willem endeavoured further to discover whether any of the Mollusca possessed ‘dermatoptic perception,’ or the faculty of perceiving variation of light by means of the skin alone. He accordingly repeated the above-mentioned experiments, having previously extirpated the eyes in all cases. The result was remarkable. In a few instances the experiment was not conclusive, but H. aspersa, A. empiricorum, several species of Limax, and one Limnaea shunned or sought the light just as they had done when their eyes were present. A few marine Mollusca (Littorina littorea, Trochus cinerarius, T. umbilicatus, Patella vulgata) were also shown to be exceedingly sensitive to the impact of a shadow, whether with or without their eyes.

=Blind and Eyeless Mollusca.=--In a large number of marine Mollusca which habitually creep about half buried in wet sand (Bullia, Sigaretus, Scaphander, Philine), eyes are altogether absent. In some species of Natica and Sigaretus, and in Doris, eyes are developed, but are enclosed in a thick layer of skin, through which they can probably do little more than faintly appreciate different degrees of light and darkness. Chiton has cephalic eyes in the embryo, but loses them in the adult stage. The two great Auricula, A. auris Judae and A. auris Midae, which habitually creep about in the liquid mud of mangrove swamps, have entirely lost their eyes. Certain pelagic Mollusca seem to have a tendency, which is not easily explained, to lose their eyes or the power of seeing with them. Thus Ianthina has no eyes at all. Pteropoda as a rule have no eyes, and the few that have (Creseis, Cavolinia) possess only certain pigmented spots placed near to the nervous centres. In the Heteropoda, however, and the Cephalopoda, many of which are pelagic, the eyes are unusually large.

=Eyes in Deep-sea and Underground Mollusca.=--Deep-sea Mollusca, as a rule, possess no visual organs, or possess them only in a rudimentary state, but this rule has its exceptions. Dr. Pelseneer found[289] no trace of eyes in two species of Pleurotoma from 1850 and 1950 fath., none in a Fossarus from 1400 f., none in a Puncturella from 1340 f. A remarkable form of Voluta (Guivillea) from 1600 f. possessed eyes which could hardly be functional, as they were destitute of pigment, and exhibited other changes of structure. On the other hand, it is remarkable to notice that in three different species of Trochus from 450 f., 565 f., and 1375 f., the eyes were pigmented and well developed.

In land Mollusca which live beneath the surface of the ground or in absolute darkness, the eyes are generally more or less modified. Thus in Testacella, which usually burrows deeply in the soil, but occasionally emerges into the open air, the eyes are very small, but distinct and pigmented. Our little Caecilianella acicula, which is never seen above the surface, is altogether destitute of eyes. A species of Zospeum, a Helix, and a Bithynella from dark caves in Carniola have suffered a similar loss. On the other hand, a small Hyalinia from a dark cave in Utah (probably a recent addition to the cave fauna) has the eyes normally developed.

=Eyes of Onchidium.=--Many species of Onchidium, a naked land pulmonate which creeps on rocks near high-water mark, are provided with dorsal eyes of various degrees of organisation, and in numbers varying up to nearly one hundred. The tropical Onchidium are the prey of a fish (Periophthalmus) which skips along the beach by the aid of its large ventral fins, and feeds principally on insects and Onchidium. Karl Semper suggests[290] that the eyes are of service to Onchidium as enabling it to apprehend the shadow of the approaching Periophthalmus, and defend itself by suddenly contracting certain glands on the skin and expressing a liquid secretion which flies into the air like shot and frightens the Periophthalmus away. This theory for it is no more than theory--may or may not be true, but it is remarkable that Onchidium with dorsal eyes have precisely the same geographical distribution as Periophthalmus, and that where no Periophthalmus exists, e.g. on our own S.W. coasts, the Onchidium are entirely destitute of dorsal eyes. In those species of Onchidium which have no dorsal eyes, the latter are on the tips of the tentacles, as in Helix. The eyes are developed on the head, and afterwards ascend with the growth of the ommatophores, while in Helix the ommatophores are formed first, and the eyes developed upon them.[291]

=Dorsal Eyes in the Chitonidae.=--The remarkable discoveries of Moseley with regard to the dorsal eyes of Chiton were first published in 1884.[292] He happened to notice, while examining a specimen of Schizochiton incisus, a number of minute black dots on the outer surface of the shell, which appeared to refract light as if composed of glass or crystal. These ‘eyes,’ in all the species of Chiton yet examined, are restricted to the outer surface of the exposed area of the shell, never being on the laminae of insertion or on the girdle. In certain sub-genera of Chiton the eyes are scattered irregularly over the surface, in others they are arranged symmetrically in rows diverging from the apex of each plate, but in old specimens the eyes towards the apices are generally rubbed off by erosion or abrasion. Moseley regarded the occurrence of scattered eyes as indicating an original stage of development, when the eyes were at first disposed irregularly all over the surface of the shell; the gathering into regular rows showing a later stage.

The eyes appear to be invariably more numerous on the anterior plate. Thus in Corephium aculeatum there are about 12,000 in all, of which more than 3000 are on the anterior plate. In Schizochiton they are arranged in very symmetrical rows, six of which are situated on the anterior, and only two, sometimes only one, on the central plates. In Tonicia marmorata the eyes are sunk in little cup-shaped depressions of the shell, possibly to escape abrasion. As regards shape and size, in Ch. incisus they are circular, and about 1/35 inch in diameter, this being the largest size known; in Ch. spiniger and Ch. aculeatus they are oval, measuring about 1/400 x 1/600 inch. There are no eyes in Chiton proper, nor in Mopalia, Maugeria, Lorica, and Ischnochiton.[293] None of our English species appear to possess them.[294]

=Eyes in Bivalve Mollusca.=--Some, possibly most, of the Pelecypoda possess, in the larval state, true paired eyes at the oral end of the body. These become aborted as the animal develops, since that part of the body becomes entirely screened from the light by the growth of the shell. To compensate for their loss, numerous ocelli, or pigmented spots sensitive to the action of light, are in many cases developed on different parts of the mantle, functionally corresponding to the ‘eyes’ of Chiton described above. As in Chiton, too, we have here an interesting series of instances in which true eyes have suffered total obliteration, through disuse, and, as if to restore to the animal in some measure its lost sense, visual organs of a low power have subsequently been developed and are now observed in various stages of specialisation.

=Concentration of Eyes in Special Parts of the Mantle.=--Sharp has shown[295] that in several species of Ostrea, Cardium, Anomia, Lima, Avicula, Arca, and Tellina pigmented cells, with a highly refractive cuticle, are scattered over a considerable portion of the mantle. Experiment has proved the powers of ‘vision,’ i.e. of sensitiveness to different degrees of light, possessed by these organs. In Dreissena polymorpha, Tapes decussatus, and two species of Venus these cells are concentrated on that particular part of the mantle which is not always covered by the shell, i.e. the siphon, but since the siphon can be completely retracted within the shell, there is no special provision for their protection. A further step is shown in the case of Mya arenaria, where the siphon is scarcely capable of complete retraction. Here, while some of the pigment cells are scattered about over the surface of the siphon, the majority are placed in grooves at the base of the siphonal tentacles, forming an intensely black band round them. A higher stage still is shown in Solen vagina, S. ensis, and Mactra solidissima, where the cells are situated only in the siphonal grooves, which are more or less specialised in numbers and complexity.

Arca Noae, according to Patten, is very sensitive to any sudden change in the amount of light falling upon its mantle-edge. A faint shadow cast upon it by the hand is sufficient to cause it to close its valves quickly, but always one or two seconds afterwards, the promptitude in all cases depending upon the depth of the shadow. Sensitiveness in this direction was found to depend greatly upon the vitality of the animals themselves, since it always became less in those specimens which had been kept for long in confinement. A shadow was not always necessary to make them close. An ordinary black pencil, if approached within two or three inches with extreme caution, produced the same result, while a glass rod brought within the same distance, and even moved rapidly to and fro, appeared to cause no alarm. Sensitiveness to change in intensity of light was experimentally noticed by the same author in the case of Ostrea, Mactra, Avicula (to a special extent), and Cardium. It is very remarkable to find that increased elaboration in the structure of the eyes does not necessarily carry with it increased sensitiveness, i.e. higher visual powers. Avicula, which is only provided with a few scattered ommatidia, which would entirely escape the notice of any one who had not seen them better developed elsewhere, was considerably more sensitive to light and shade than Arca, with its eyes of conspicuous size and much more perfect organisation, instantly contracting the mantle upon the impact of a shadow so faint as to be invisible to the experimenter.[296]

=Visual Faculties of Solen and Ostrea.=--The visual power of Solen may be exemplified by any one who is walking along almost any of our sandy bays at extreme low-water mark. If the day be warm and sunny, numbers of Solen will be seen raising themselves an inch or two out of their holes; but if you wish to catch them you must approach very cautiously, and on no account allow your shadow to fall upon them, or they will pop down into their burrows in an instant, and it is vain to attempt to dig them out. ‘How sensitive,’ remarks Mr. W. Anderson Smith, with reference to oysters,[297] ‘the creatures are to the light above them; the shadow [of the boat] as it passes overhead is instantaneously noted, and, snap! the lips are firmly closed.’

=Ocelli of Pecten.=--In Pecten and Spondylus the ocelli are remarkably large and prominent, shining like precious stones, and are placed along the two edges of the mantle so as to receive the light when the shell gapes (Fig. 93). In Pecten opercularis, jacobaeus, and maximus their number varies from 80 to 120. In Spondylus gaederopus, a very inequivalve shell, 60 have been counted on the right or fixed valve, and 90 on the left or upper valve. Each ocellus is connected, by means of its optic nerve, with the large circumpalleal nerve, and so with the branchial ganglion. They possess a cornea, lens, choroidea, and optic nerve, and, according to Hickson,[298] bear a considerable resemblance to the vertebrate type of eye. In spite of this, the power of vision in these genera does not appear at all superior to that of other Pelecypoda.

According to the elaborate investigations of Patten, the ‘eyes’ in Arca occur upon the middle or ‘ophthalmic’ fold of the mantle-edge, which is thickened at the end to admit of their reception. Along this is ranged a row of dark brown spots of various sizes, which are larger at the anterior and posterior ends of the mantle-edge, but smaller and more numerous towards the middle. These brown spots, or ‘eyes,’ are many of them compound, being made up of the fusion of a number of ommatidia (from 10 to 80) into one large round eye, which is generally elevated above the surface of the surrounding epithelium. Sometimes these eyes themselves tend to fuse together. In one specimen of Arca Noae, 133 of these faceted eyes were counted in one mantle border, and 102 in the other.

There can be little doubt that the development of these functional eyes, or sensitive spots, in bivalve Mollusca, is due to special needs. They appear to be entirely absent in fresh-water bivalves (with the exception of Dreissensia, which is obviously a marine genus recently become fresh-water), while they are most abundant in genera living between tide marks (Solen, Mya, Mactra), and most highly specialised in a genus that is, for a bivalve, of singularly active habits (Pecten). Now genera living in sand between tide marks, as the three above-mentioned genera are in the habit of doing, and also protruding their siphons, and occasionally a considerable portion of their shells, out of their burrow, are manifestly very much at the mercy of their watchful enemies the gulls, and anything which would enable them to apprehend the approach of their enemies would be greatly to their advantage. Here, perhaps, lies the explanation of the greater elaboration of these pigmented spots in littoral genera, as compared with those inhabiting deeper water. Pecten, again, a genus distinguished by great activity, which can ‘fly’ for considerable distances in the water by flapping its valves together and expelling the water from the apertures at either side of the hinge, may be greatly assisted by its ocelli in directing its flight so as to escape its enemies.

III. Smell

The sense of smell--touch at a distance, as Moquin-Tandon has called it--is probably the most important sense which the Mollusca possess, and is unquestionably far more valuable to them than that of sight. Any one who has ever enjoyed the fun of hauling up lobster pots will recollect that part of the contents was generally a plentiful sprinkling of Buccinum, Nassa, and Natica, attracted by the smell of the stinking piece of fish with which the trap was baited. According to Mr. J. S. Gibbons,[299] Bullia rhodostoma congregates in hundreds on gigantic medusae which are stranded on the sandy bays near the Cape of Good Hope. Dr. J. G. Jeffreys says[300] that quantities of the common Neptunea antiqua “are procured on the Cheshire coast by the fishermen placing a dead dog on the sands at low-water mark during spring tides. The bait is then completely covered with stones, which are piled up like a cairn. On the next turn of the tide the heap of stones is visited, and the whelks are found on the surface in great numbers, having been apparently attracted by the smell of the bait, but unable to get at it.” Mr. W. A. Lloyd kept specimens of Nassa reticulata in a tank in the sand, at the bottom of which they usually remained buried. If a piece of meat of any kind were drawn over the sand, the Nassa would appear above the surface in a few minutes. Half-picked beef or mutton bones, if placed in the tank, were covered in a few minutes. In fact, no animal matter, whether living or dead, could be introduced without the Nassa smelling it, and coming up to see what they could get.[301]

Any one can experiment for themselves on the olfactory powers of our common snails or slugs. Moquin-Tandon records[302] two interesting cases, one communicated to him by letter, the other occurring to himself. His correspondent, a M. Parenteau, was one day walking along a dusty high-road, when he noticed, near the middle of the road, an empty bean-pod and two Arions eating it. Attributing the meeting of feeders and food to mere chance, he was walking on, when he noticed a second bean-pod, and, about two yards away from it, a third Arion, hurrying straight towards it. When the Arion had yet more than a yard to traverse, M. Parenteau picked up the bean and put it in his pocket. The Arion stopped, raised its head, and turned in every direction, waving its tentacles, but without advancing. M. Parenteau then carried the bean to the other side of the road, and put it in a small hole behind a piece of stone. The Arion, after a moment’s indecision, started off straight for the bean. Again the position of the precious morsel was changed, and again the Arion made for it, this time without being further tantalised. M. Moquin-Tandon noticed, one rainy day in the botanical gardens at Toulouse, two Limax maximus approaching a rotten apple from different directions. He changed the position of the apple several times, placing it at a sufficient distance, to be sure they could not see it, but they always hit it off correctly, after raising their heads and moving their long tentacles in every direction. It then occurred to him to hold the apple in the air, some centimetres above the head of the Limax. They perceived where it was, raised their heads and lengthened their necks, endeavouring to find some solid body on which to climb to their food.

Several of the land Mollusca have the power of exhaling a disagreeable smell, Hyalinia alliaria smelling strongly of garlic, and Stenogyra decollata of laudanum; but this need not be any argument for the sense of smell in the creatures themselves.

=Position of Olfactory Organs in Pulmonata.=--Most authorities are of opinion that the olfactory organs are situated in the tentacles. Moquin-Tandon considered that in the Helicidae and Limacidae the sense of smell is confined to the little knob or elevation at the end of the longer tentacles, close to the eye. He found that when he cut off these tentacles both in Limax and Arion, the creatures were quite unable to discover the whereabouts even of strongly-scented food. The same author believed that in the Basommatophora the sense of smell was present in the whole of the tentacle, which is covered with an exceedingly sensitive ciliated epithelium. Lacaze-Duthiers, however, places the olfactory sense in this group at the outer side of the base of the tentacles, near to the eyes. Some authorities[303] deny that the Helicidae have the olfactory organ at the tip of the tentacles, and locate it in a pedal gland near the mouth, which contains conspicuous sensitive cells. A Helix whose tentacles had been removed manifested its repulsion to the smell of spirits of turpentine, while another Helix, which was unmutilated, did not object to the turpentine being held between its tentacles. Altogether, then, the exact position of the smell-organ in the Helicidae must be considered as not yet thoroughly determined. Simroth holds that the sense of smell is distributed over the whole soft integument, and is especially concentrated in the feelers, and in the neighbourhood of the respiratory orifice.[304]

In nearly all marine Mollusca yet examined, the organ of smell or osphradium is in situation intimately connected with the breathing organs, being generally placed near their base, with the object, apparently, of testing the quality of the water before it passes over the branchiae. It consists of a patch of the epithelium, modified in a special manner, and connected by its own nerve with one of the visceral ganglia.

An osphradium does not necessarily occur in all genera; for instance, it has not been detected in Fissurella. It is most highly specialised in the Conidae, and in the carnivorous Gasteropoda generally. In Buccinum undatum, for instance, it is very large indeed, and, from its plumed form, has sometimes been mistaken for an accessory branchia (Fig. 95). In Haliotis it is paired, one lying in close proximity to each of the two branchiae, but in Turbo it is single, corresponding to the single branchia. In Chiton there is an osphradium at the base of each separate gill filament, making a total of twenty or more on each side. Its position in Physa and in Cyclostoma will be seen by reference to Figs. 103 and 104 (p. 205). In the Pelecypoda the osphradia are paired, and lie adjacent to the posterior adductor muscle, close to the hinder end of the axis of the branchiae. In the Tetrabranchiate Cephalopoda there are two osphradia, placed between the bases of the two pairs of gills. In the Dibranchiates on the other hand, a groove above the eyes has been regarded as the seat of the organ of smell. This groove contains sensory and ciliated cells, and appears to be connected with a special nerve centre of its own, which ultimately is derived from the cerebral ganglion.

Scarcely any instances of the exercise of the sense of smell on the part of bivalve Mollusca have been recorded. Something of the sort, however, seems to have been present in a case related by Mr. R. L. King.[305] A skull of a fox had been placed in a small ditch in order to soak, and after a few days, when taken out, was found to be covered with Pisidium pusillum to the number of at least two hundred, which had been probably attracted from the water in the immediate neighbourhood by the smell of the decaying flesh.

IV. Hearing

Experiments made with a view to ascertain whether the Mollusca are sensitive to noises have usually led to the conclusion that they are deaf to very loud sounds. This is the more curious, because an undoubted auditory apparatus has been discovered in a large number of genera. In the case of an experiment, it is not easy to be sure that the animal is not affected, at least in part, by the shock or jar, rather than by the actual sound. In some experiments, however, conducted at the Plymouth Marine Biological Laboratory, Mr. Bateson found[306] that Anomia could be made to shut its shell by smearing the glass of the tank with the finger in such a way as to make a creaking sound. It was evident that the cause of alarm was not the jarring of the solid framework of the tank, for the same result occurred when the object on which the Anomia were fixed was suspended in the water by a thread. It was found that the sound had to be of a particular pitch to excite the attention of the mollusc.

As a rule the organ of hearing is nothing more than a small vesicle or sac (the otocyst), filled with a fluid secretion, in which are suspended one or usually more calcareous concretions known as otoliths. By means of cilia, which connect with sense-cells, these otoliths are given a peculiar movement or oscillation in the medium in which they are suspended. The number of the otoliths varies in different genera and species; there are several hundreds in Arion and Limax, about a hundred in Helix pomatia, nemoralis, hispida, arbustorum, rotundata, Succinea putris, and Limnaea stagnalis; about fifty in Planorbis contortus and Physa fontinalis, only one in Cyclostoma elegans. The number increases with age. In young specimens of Limn. stagnalis as few as ten, nine, and seven have been noticed.[307]

The otocysts are always paired, and, in Gasteropoda, are placed close to the pedal ganglia. The acoustic nerve, however, has been shown by Lacaze-Duthiers to connect with the cerebral ganglia in certain cases. The otocysts are never on the surface of the body and are rarely connected with it by any passage or tube; it is probable therefore that sound reaches them simply through the medium of the tissues.

In the Pelecypoda the otocyst is similarly situated near the pedal ganglion, and is probably (though this has not yet been proved) similarly connected with the cerebral. There is only a single otolith. Pelseneer finds[308] in Nuculidae alone a free communication between the otocyst and the exterior. Anodonta has been observed[309] to withdraw its foot into the shell at the noise of an opening door, a loud voice, or a shrill whistle, whether in a basin of water or lying on a study table.

Delage extirpated the otocysts in certain Octopoda, and obtained some unexpected results. He found that remarkable effects were produced upon the animal’s powers of locomotion, so that it was unable to preserve its proper balance in the water when in rapid motion, but its body was forced to undergo a form of rotation more or less pronounced. He concluded that the otocysts must possess, besides their auditory functions, a power which stands in some relation to the proper orientation of the body in locomotion, a power which is not wholly supplied by sight and touch alone. The otocysts may thus regulate locomotion by stimulating muscular acts which tend to keep the body in the straight line during the process of movement.[310]

=The Foot=

One of the most characteristic organs of the Mollusca is the foot, which, under one form or another, occurs throughout the whole phylum. The foot is a thickening, on the ventral side, of a portion of the integument of the animal, modified to serve different forms of motion. It attains its maximum relative area in the Chitonidae, many Nudibranchs, and the slugs generally, in nearly all of which there is no portion of the body which is not subtended by the foot. Here too it presents the form of a regular disc or ellipse, which is more or less produced. In many cases, however, the foot becomes modified in such a way that we are enabled to recognise well-marked anterior and posterior portions, which have received the name of propodium and metapodium respectively, while the intervening central portion is termed the mesopodium.

The propodium is most strongly developed in genera which crawl about in wet sand, e.g. Natica, Sigaretus, Oliva, Harpa, Scaphander (Figs. 97 and 98, and compare Fig. 91). In such cases it seems to serve as a sort of fender or snow-plough, to push the sand away on both sides of the path the animal is traversing. In some species of Sigaretus the propodium becomes as it were banked up against the head and proboscis, which are thus unnaturally elevated, or tend to disappear altogether. Bullia (Fig. 62), which crawls about rapidly on wet sand, appears to attain its object by a wide extension of the foot on all sides, and so slides over the sand instead of ploughing through it; the little lappets at the end of the ‘tail’ probably serve as rudders.

In Melampus and Pedipes the propodium is marked off by a groove across the ventral surface. When the animal is in motion it first advances the propodium and then pulls the rest of the foot after it with the looping gait of certain caterpillars. In many Cyclostomatidae this groove, instead of being transverse, is longitudinal, and the animal advances first the right and then the left segment of the foot, which gives it a swaying motion from side to side.

Upon the metapodium lies the operculum, when it occurs. As a rule the metapodium is not sharply marked off from the rest of the foot. In Strombus (Fig. 99) it becomes erected into a sort of hump or column, on the top of which the operculum is situated.

The epipodium is a prominent fold or border, which occurs upon the upper edge of the foot in most Diotocardia. In Haliotis it is of considerable breadth, and is covered by a number of lobes which spring from a moss-like prolongation of the skin. From the epipodium are developed the lateral tentaculae of Monodonta (Fig. 82, p. 178), and of other sub-genera of the Trochidae.[311]

In the Opisthobranchiata the lateral edges of the foot (the parapodia) are frequently produced into broad folds or wing-like extensions, which in many cases tend to fold over the shell, and, in conjunction with the mantle, eventually imbed it altogether. By the wavy motion of the parapodia the animal is enabled to progress through the water. The paired natatory lobes of the Pteropoda are simply the parapodia of the Tectibranchs modified for swimming purposes.

It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda, groups which have, for the most part, exchanged a crawling for a swimming life, that the modifications of the foot are most considerable. In Oxygyrus and Atlanta, for instance, the propodium and metapodium are sharply distinguished from the mesopodium, and no doubt have acquired, as a means of propulsion, the power of separate movement, the animal swimming with these portions of the foot uppermost. In Carinaria and Pterotrachea the metapodium has probably become continuous with the long axis of the body, while the so-called ‘foot’ with its sucker represents only the original propodium. In the Cephalopoda the arms and funnel represent the modified foot, the sides of which are prolonged into a number of very long specialised tentaculae. In the adult Cephalopod some of the arms have assumed a position in advance of the mouth, the latter being in fact surrounded by a circle of arms. But in the Cephalopod embryo the mouth opens as in the Gasteropoda, i.e. in advance of the arms, and it is only gradually that it becomes encircled by them. Arms and funnel alike are found to be innerved from the pedal ganglion.[312]

The pointed axe-shaped foot, which is characteristic of the majority of Pelecypoda, is doubtless derived from a form more akin to the flattened ‘sole’ of the Gasteropoda. A foot with something of this disc-shaped base actually occurs in some of the Nuculidae, the parapodia being furnished with pleats which recall similar formations in other Orders (Fig. 100). The principal modifications of the foot are due to its employment as a burrowing organ. In genera which burrow but slightly it is small and feebly developed, while in genera which habitually excavate, it becomes the largest and strongest organ of the body. At the same time it has a tendency to shift its position from the ventral to the anterior margin, accompanied by a corresponding narrowing of the shell, until it arrives at the position seen in Mollusca of the shape of Mya, Pholas, and Solen. In sedentary or attached genera, e.g. Pecten, Chama, Ostrea, the foot tends to become aborted.

The byssus gland, secreting a byssus of horny threads, is characteristic of many Pelecypoda, and may be observed by any one in the common mussel. It occurs in the larvae of many species which do not possess a byssus in the adult stage. The pedal gland of many Prosobranchiates, which secretes a tough and almost thread-like slime, is possibly homologous with the byssus gland of bivalves.

=Nervous System=

The Mollusca possess a nervous system, which usually consists of a number of nerve centres or ganglia, linked together by bands (the commissures) and sending out thread-like nerves which ramify into the various organs. The character of the nervous system varies greatly in different groups, ranging as it does from a condition of extreme complexity, in which the ganglia are numerous and the commissures equally so, to that of considerable simplicity, in which ganglia are almost entirely absent.

The most important ganglia are (1) the cerebral,[313] which are always placed above or on either side of the mouth, and from which proceed the nerves of the eyes and tentacles; (2) the pedal, which in Gasteropoda are situated below the oesophagus, in Pelecypoda at the base of the foot, and from which the nerves of the foot and sometimes the acoustic nerve arise; (3) the pleural,[314] whose position varies considerably, but is always below the oesophagus and slightly above the pedal ganglia; these innervate the mantle, branchiae, heart, and viscera generally.

=Gasteropoda.=--The simplest form of nerve system as thus understood occurs in the Amphineura, and more particularly in the Chitons. Here we find four longitudinal nerve-cords, parallel to one another for nearly the whole length of the mollusc. The two exterior cords probably represent the pleural, the two interior the pedal nervous system. There being no head or tentacles, but simply a mouth at the anterior end, the cerebral ganglia do not exist, but they are represented by the curved ring formed by the massing together of the two nerve-cords on each side. The only distinct ganglia are a pair of buccal ganglia (which are developed on a pair of commissures which pass forward from the cerebral mass and innervate the lips and buccal region), and a much smaller group, the sublingual. The two pedal cords are united by a number of transverse parallel connectives, which recall similar modes of connexion in the Chaetopod worms and in Arthropoda.

This quadruple set of nerve-cords is characteristic of all the Amphineura, but the absence of ganglia is most marked in the Chitons. In Proneomenia and Neomenia there is a distinct cerebral ganglion, formed by the massing of the two ganglia into one, while in Proneomenia the lateral cords are joined to the pedal, as well as the pedal to one another, by connectives. In Chaetoderma the cerebral ganglia, though adjacent, are distinct, and both the pedal and lateral cords connect directly with them, while there are no transverse connectives.

The remaining three great divisions of Gasteropoda, namely, the Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as comprising two distinct types of nervous condition, according as the loop formed by the two visceral nerve-cords is twisted over itself, forming a figure of 8, or continues straight and uncrossed. In the former case, we get the condition known as streptoneurous, in the latter that as euthyneurous.[315] The Euthyneura include the whole of the Opisthobranchiata[316] and Pulmonata, the Streptoneura all the Prosobranchiata.

The simplest form of nervous system in the euthyneurous Gasteropoda occurs in the Opisthobranchiata. The cerebral, pleural, and pedal ganglia tend to become concentrated in a ring-like form, united by short commissures at the posterior end of the pharynx. The visceral loop is in some cases long, and the two or three visceral ganglia are then situated at its posterior extremity. The nervous system of the Pulmonata is of a similar type, the visceral loop being often much shorter, and tending to draw in towards the central group of ganglia. The tentacular and optic nerves are, as usual, derived from the cerebral ganglion, with which also the octocysts are probably connected by rather long nerves. A pair of buccal ganglia innervate the buccal mass, and are united by commissures with the right and left cerebral ganglia. The osphradial nerve springs from one of the ganglia on the visceral loop, the osphradium itself being situated (in Limnaea) immediately above the pulmonary orifice and adjacent to the anus (Fig. 102). This massing of the ganglia is still better illustrated by the accompanying figure of Physa (Fig. 103), in which the animal is represented as if transparent, so that the ganglia and nerves are seen through the tissues.

Of the streptoneurous Gasteropoda, the nervous system of Fissurella and Haliotis shows distinct points of similarity to that of the Amphineura. The pedal nerves are united by transverse commissures throughout their entire length, while a double commissure unites the cerebral ganglia to the mass from which the pedal nerves proceed. In the great majority of the Streptoneura the ganglia (except the visceral) are more concentrated and the commissures are consequently much shorter. The accompanying figure of Cyclostoma, in which the animal is represented as in that of Physa just described, illustrates this grouping of the ganglia, the twist of the visceral loop, and the position of the visceral ganglia at its posterior end.

=Scaphopoda.=--In the Scaphopoda the nervous system resembles that of the Pelecypoda. The cerebral and pleural ganglia lie close together, while the pedal ganglia are placed in the anterior part of the foot, connected with the cerebral ganglia by long commissures; the visceral loop is rather long, and the two visceral ganglia are adjacent to the anus.

=Pelecypoda.=--The nervous system in the Pelecypoda is the simplest type in which well-marked ganglionic centres occur. The ganglia are few, symmetrically placed, and are usually at a considerable distance apart. There are, as a rule, three distinct pairs of ganglia, the cerebral (cerebro-pleural), pedal, and visceral. The cerebral are formed by the fusion of the cerebral and pleural ganglia, which, however, in some cases (Protobranchiata) continue distinct.[317] They lie above or on each side of the mouth, united by a commissure of varying length. Another pair of commissures unites them with the pedal ganglia, which are placed at the base of the foot, and are usually very close together, sometimes (as in Anodonta) becoming partially fused. The length of these commissures depends upon the distance between mouth and foot; thus they are very long in Mya and Modiola, and very short in Pecten. In cases where the foot is rudimentary or becomes aborted through disuse (e.g. Ostrea), the pedal ganglia may dwindle or disappear altogether. The visceral ganglia are on the ventral side of the posterior adductor muscle, beneath the rectum, and innervate the branchiae, osphradia, and the whole of the visceral sac. A pair of cerebro-visceral commissures traverses the base of the foot, surrounding it with a comparatively short loop (compare Fig. 106, c.v.c´), while a long commissure, which runs round the entire edge of the mantle, and supplies branching nerves to the mantle border and siphons (Fig. 106, c.v.c), may also connect the visceral and cerebral ganglia.

=Cephalopoda.=--In the Cephalopoda the concentration of ganglia attains its maximum, and may perhaps be regarded as approaching the point at which a definite brain may be said to exist. Another point of distinction is the formation of special small ganglia upon the nerve-cords in different parts of the body. In the Tetrabranchiata (Nautilus) the cerebral and pedal ganglia form a broad ring which surrounds the oesophagus, the former giving out the optic nerves, with their special optic ganglion, and a pair each of buccal and pharyngeal ganglia, the latter the nerves of the arms and funnel. The visceral loop is still present in the form of a separate band, which innervates the branchiae, osphradia, and viscera generally, forming a special genital ganglion in connexion with the reproductive organs. The principal ganglia of the Dibranchiata are still more concentrated, even the visceral loop being possibly united with the rest in forming an unbroken mass in which scarcely any trace of commissures can be detected. The pedal ganglion becomes separated into two portions, one of which innervates the arms, the other the funnel. Two peculiar ganglia (the stellate ganglia) supply a number of branching nerves to the mantle.

* * * * *

=E. L. Bouvier=, Système nerveux, morphologie générale et classification des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), iii. 1887, pp. 1–510.

=J. Brock=, Zur Neurologie der Prosobranchier: Zeit. wiss. Zool. xlviii. 1889, pp. 67–83.

=O. Bütschli=, Bemerkungen über die wahrscheinliche Herleitung der Asymmetrie der Gasteropoda, etc.: Morph. Jahrb. xii. 1886, pp. 202–222.

=B. Haller=, Zur Kenntniss der Muriciden. I. Anatomie des Nervensystems: Denksch. Math. Nat. Kl. Ak. Wien, xlv. 1882, pp. 87–106.

„ Untersuchungen über marine Rhipidoglossen. II. Textur des„ Centralnervensystems und seiner Hüllen: Morph. Jahrb. xi. 1885, pp. 319–436.

=H. Grenadier=, Abhandlungen zur vergleichenden Anatomie des Auges: Abh. Naturf. Gesell. Halle, xvi. 1884, pp. 207–256; xvii. 1886, pp. 1–64.

=A. P. Henchman=, The Origin and Development of the Central Nervous System in Limax maximus: Bull. Mus. C. Z. Harv. xx. 1890, pp. 169–208.

=V. Hensen=, Ueber das Auge einiger Cephalophoren: Zeit. wiss. Zool. xv. 1865, pp. 157–242.

=C. Hilger=, Beiträge zur Kenntniss des Gasteropodenauges: Morph. Jahrb. x. 1885, pp. 352–371.

=Lacaze-Duthiers=, Otocystes ou Capsules auditives des Mollusques (Gastéropodes): Arch. Zool. Exp. Gén. i. 1872, pp. 97–166.

„ „ Du système nerveux des Mollusques gastéropodes pulmonés aquatiques: ibid. pp. 437–500.

=P. Pelseneer=, Recherches sur le système nerveux des Ptéropodes: Arch. Biol. vii. 1887, pp. 93–130.

„ Sur la valeur morphologique des bras et la composition du système nerveux central des Cephalopodes: Arch. Biol. viii. 1888, pp. 723–756.

=H. Simroth=, Ueber die Sinneswerkzeuge unserer einheimische Weichthiere: Zeit. wiss. Zool. xxvi. 1876, pp. 227–348.

=J. W. Spengel=, Die Geruchsorgane und das Nervensystem der Mollusken: Zeit. wiss. Zool. xxxv. 1881, pp. 333–383.

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