II. The Sub-kingdom Protozoa
The consideration of the whole special group of organisms forming the subject matter of this chapter, under the heading of Protozoa, were formerly included among Infusoria, which also embraced every kind of microscopical aquatic body, whether belonging to the vegetable or animal series. A more critical survey of the organisation and affinities of Infusoria and the members which constituted the group led to a re-arrangement, which has been very generally accepted as forming a sub-kingdom, Protozoa. This may be defined as embracing all those forms of life, referable to the lowest grade of the animal kingdom, whose members for the most part are represented by organisms possessing a single cell or aggregation of cells (and also included under the general term of unicellular organisms) the whole of which are engaged in feeding, moving, respiring, and reproducing by segmentation or fission much in the same way as that of the unicellular plants described in a previous chapter. Following out this sub-division of the entire series of Protozoa, the several groups range themselves into four readily distinguishable sections. In the first, the most lowly organised and most abundant have no oral orifice in the literal meaning of the word, food being intercepted at any point of the surface of the body. This most simple elementary type of structure of the Protozoa is represented in the Amœba and Actinophrys, the various representatives of the Foraminifera, and certain Flagellata, as Spumella and Anthrophysa. Next in the ascending scale is a group of Protozoa, in which, though differentiation has not proceeded so far as to arrive at the constitution of a distinct oral aperture, the inception of food substance is limited to a discoidal area occupying the anterior extremity of the body and is associated with the special food-arresting apparatus. To this section of the Protozoa are relegated the minuter flagellate, “collar-bearing” animals, and also the entire group of sponges or Porifera.
In the third section the highest degree of organisation is arrived at. Here is represented a single, simple, often highly-differentiated oral aperture or true mouth. Associated with this section are found the majority of those organisms that collectively constitute the class Infusoria in the proper acceptation of the term, and it embraces the majority of the Ciliata, the Cilio-flagellata, as Euglena, Chilomonas, &c., in which the presence of a distinct and circumscribed oral aperture is clearly seen. With the fourth and remaining section of Protozoa, the oral or inceptive apparatus exhibits a highly characteristic structural modification. This is not restricted to a definite area, nor is it associated with the entire surface of the body, but it consists of a number of flexible, retractile, tentacle-like organs radiating from diverse and definite regions of the periphery, each of which subserves as a tubular sucking-mouth, or for the purpose of grasping food. These may be literally described as many-mouthed, and have been appropriately designated Polystomata. The true zoological position of the Spongida or Porifera is not finally settled, the members of this important section having been formerly regarded as a subordinate group of the Rhizopoda or an independent class of the Protozoa; consequently a tendency has been shown to assign to them a position more nearly approximating to that of the Cœlenterata, or zoophytes and corals, or place them among the more highly organised tissue-constructed animals, the Metazoa, these being characterised by groups of cells set apart to perform certain functions for the whole animal. A division of labour is seen to be marked in these lower animals as the organism becomes more specialised, and the number of functions a cell performs becomes more and more limited as the body becomes more complex.
It has been found convenient to adopt the following definition of the Infusoria as one more generally acceptable. The Protozoa in their adult condition are furnished with prehensile or locomotive organs, that take the form of cilia, flagella, or of adhesive or suctorial tentacula, but not of simple pseudopodia; their zooids are essentially unicellular, free swimming or sedentary; they are either naked, loricate, or inhabit a simple, mucilaginous matrix; single or united in aggregations, in which the individual units are distinctly recognisable; not united and forming a single gelatinous plasmodium, as in Mycetozoa, nor immersed within and lining the interior cavities of a complex protoplasmic and mostly spiculiferous skeleton, as in the Spongida, their food substances being intercepted by a single distinct oral aperture, or by several apertures through a limited terminal region or through the entire area of the general surface of the body. They increase by simple longitudinal or transverse fission, by external or internal gemmation or division, preceded mostly by a quiescent or encysted state, into a greater or less number of sporular bodies. Sexual elements, as represented by true ova or spermatozoa, are entirely absent, but two or more zooids frequently coalesce as an antecedent process to the phenomena of open formation.[63]
The infusorial body in its simplest type of development, as in Amœba, exhibits a structural composition substantially corresponding with that of the lowest organised tissue cell. There is no distinct bounding membrane, or cell-wall, and it is throughout, and apart from the nucleus or endopart, one continuous mass of granular matter, but otherwise homogeneous and undifferentiated protoplasm. Professor Greef, who has made a study of the Amœba, describes motor fibrils in the exoplasm which are active and large in A. terricola. These are readily seen by staining with osmic acid, and, after washing this out with water, immersing in a weak alcoholic solution. In Amœba so prepared and examined with a high power, the whole body will be seen to be surrounded by a distinct double integumentary layer. Highly refractive bodies may also be seen in the interior, connected together by extremely fine filaments. Professor Greef concludes that here we have to do with muscular fibrillæ, which traverse the contractile outer zone in a radial direction and there terminate for the time being. By a similar method, axial filaments can be demonstrated in Heliozoa; these, it is believed, are the true motors of their pseudopodia, and also the axial structures of the Acineta, a marine animal related to ciliate infusoria.
In the Amœba, at one time well known as the Proteus animalcule, Fig. 325, the marvellous body creeps onward in a flowing manner, occasionally and languidly emitting a single pseudopod first on one side, then on the other. More commonly it puts on a dendroid or palmate form; then again it assumes more or less grotesque shapes in which almost any conceivable image may be imagined. The body, as will be seen in this highly-magnified figure, is full of granules (with the exception of a thin clear outer hyaline zone), and near the centre is a globular or discoid body known as the nucleus, composed of slightly denser material than that which surrounds it. The division of the body into two is preceded by a division of this nucleus. Near the latter is a clear spherical space--the contractile vacuole--which gradually expands, and then rather suddenly collapses and reappears at the same spot, the systole and diastole being slow and continuous. The contractile vacuole contains a clear liquid which is expelled on the collapse of the vacuole. This organ probably serves the double function of respiration and excretion. The Amœba is omnivorous, chiefly a vegetarian, and, therefore, found on the ooze of ponds or on the under surface of the leaves of aquatic plants, especially among Confervæ. It can be readily produced by placing a few fibres of fresh meat in an infusion of hay.
The Gregarinæ consist of a remarkable group of organisms, but these, although unicellular, are, for the most part, confined to the intestinal tract of worms and of the higher animals, and will therefore be described among internal parasites.
Tho fungus-animals, Mycetozoa, have already been referred to in a previous chapter. The best known species, however, is found in tan yards in the form of creeping masses of naked protoplasm, termed Plasmodia. Cakes of protoplasm become segregated from the main mass, and break up into Amœba-like spores, which unite again to form Plasmodia.
The Rhizopoda, or root-footed class of animals, are among the most interesting simple organisms with which the microscope has made us acquainted. In the living state they have the power of protruding pseudopodia from the body, by which they creep about, or cling to plants when in search of food. This group, in fact, includes Amœba, Foraminifera, Sun-animalcules, and Radiolarians. In the first the pseudopodia are simple and lobose; in the second they are slender, confluent and reticulate; while in the two last they are simple, radiating and somewhat stiff, and partake of a calcareous formation.
Of the Lobosa, we may take a well-known representative of the group, the Protomyxa, found at the bottom of fresh-water pools, especially those near bog-moss, where its minute orange-coloured particles of jelly-like substance are seen creeping over stones or shells. If quietly watched the pseudopodia, some of which are broad and others slender, become quiescent spheres, which break up into numerous portions, each of which becomes a new animal.
This group is divided into the shell-less (Nuda) and shell-formed (Testacea). The brown, horny covering is often finely faceted, and is either shaped like a dome, semi-circular, or flat as a box, through which they protrude their few or many pseudopodia (seen in Fig. 326).
In the Difflugia the lorica or shell is strengthened by the addition of silicious particles; in Euglypta it is sac-shaped, with a jagged free margin, the surface being covered by overlapping scales; while Arcella are capable of secreting vesicles of air in their interior, whereby they are enabled to rise to the surface. On some parts of our coast, if the sea sand be carefully looked over with a pocket lens, there will often be found minute grains of a porcelain oval kind, belonging to the Miliolina, segmented or strung together not quite in the same plane.
The Foraminifera are rhizopods, whose simple protoplasmic bodies send forth, through perforations in the membrane or outer covering of calcium carbonate and silica, branching rays of pseudopodia. The order is divided into two groups, the Imperforata and the Perforata; in the former the shell or harder structure possesses only one or more apertures, whereas in the latter, in addition to the main opening, the shell has its walls perforated throughout, which admits of minute pseudopodia or fine threads being protruded (Fig. 328). (See also Plate III., Nos. 75-85.) The vast majority of Perforata form their shells, or rather skeletons, of calcium carbonate and silica, which renders them almost indestructible. Consequently the form is preserved through ages, and they present objects of the greatest interest to the microscopist.
A curious and interesting feature of the Foraminifera--often an element of difficulty to the student--is the tendency of modifications of types comprising the larger groups to run into parallel isomorphous series. Thus, if the entire class be roughly divided, as it sometimes has been, into three orders, comprising respectively the forms characterised by porcellaneous, arenaceous, and hyaline “tests,” the same general conformation and arrangement of chambers will be found in each of the three series. The most remarkable example, even among the smaller groups, is the Rotaliidæ, of which three or four genera may be arranged in parallel lines, and in more or less closely isomorphous series. In the report appended to the “Challenger” scheme of classification many examples are enumerated. In Arenacea we have a small family of Foraminifera, the external surfaces of which present a ridge and furrow arrangement, and the incrustations are entirely of a sandy nature held together by a cement secreted by the animal. (Plate XV., No. 1, Astrorhiza limicola.)
Gromia.--Among the more remarkable of the Perforata group the Gromia have a foremost place. They are very minute globular or oval-shaped bodies, about one-twenty-fourth of an inch in length, found in fresh, brackish, and salt water. The forms brought up in Dr. Wallich’s deep sea soundings of 1860 were taken attached to pieces of corallines, or found loose among Globigerina ooze. At first there appears to be nothing peculiar about these tiny specks of matter resembling the ova of a zoophyte, but presently, at the smaller end, a very fine thread is protruded, and then another, dividing into finer branches, and, ultimately, a complete network of filaments extends on all sides, and become attached to the side of the glass jar that contains them. Now, on employing magnifying power, every thread exhibits a circulatory motion, an up and down stream or cyclosis of granules suspended in a fluid mass. It is by means of these pseudopodia, as the threads are termed, that the Gromia moves its body along and clings to the glass. We may surmise, then, that these pseudopodia are either gelatinous, glutinous, or terminate in sucker-like processes. Increase in the “test,” integument, is brought about, as in Difflugia, by the secretion of calcareous matter or by cementing fine silicious particles to the outer wall, as the protoplasm is seen to flow over the test, so that when it comes in contact with a diatom it is thereby drawn towards the oral opening and slowly digested.
Some considerable time elapsed between the discovery of Gromia by Mr. W. Archer, F.R.S., and the demonstration of a nucleus and contractile vesicle by Dr. Wallich. It was thought that in the whole of the Monozoa the nucleus was absent, but it is now known that this important body is embedded in the protoplasmic substance, and the reproduction of these curious animals is thereby secured. Among the better known species of Gromia is G. Dujardinii, chiefly distinguishable by the darker colour of the “test,” by the greater quantity of silica that enters into the formation of its pseudopodia, and by the formation of isogamous zoospores, two of which are seen in conjugation in Plate XV., No. 2. An excess of protoplasm must also be secreted to admit of so large a protrusion outside the testa.
G. Lieberkühnia (of Claparède and Lachman), No. 5, differs in formation. Its shape is pyriform, and the opening whence the pseudopodia streams out is situated in a lateral depression about midway in the testa, c, o. Hence a trunk branch is seen to issue forth, and from this a ramification of threads, psdp, extends to a considerable distance in all directions.
The Micro-gromia of Hertwig, No. 4, is the minutest form of the genus yet discovered, and differs from those already described in the mode of reproduction. The individual takes the shape of a water bottle with a short neck, whence issue forth a limited number of very slender threads. The test is quite transparent, and it was in this species that the nucleus and contractile vesicle, which lie embedded near the mouth, were first clearly made out.
The zoospores of Micro-gromia have a curious habit of uniting with their neighbours to form a colony, No. 4. Their colonisation is apparently intended to facilitate multiplication. Reproduction is carried on somewhat after the manner of Volvox. The globular bodies formed sink to the bottom of the glass vessel, and there remain for a time in a quiescent state. In the course of a day or two the mass assumes a motive appearance, increases in bulk, becomes more ovoid in shape, and ultimately the nucleus shows the first sign of division. Vertical segmentation takes place, as at A, into two equal parts; each half is seen to possess its fair share of the nucleus and contractile vesicle. It then turns in the horizontal direction, and now there appears to be an upper and a lower division, the uppermost having a neck-like attachment, and this is making its way to the narrow oral opening in the parent testa, as at B. Here it is seen pressing forward, and at C the neck is protruding some distance, and the second half assumes a bottle shape; at D the greater part of the animal is nearly set free, and after a short rest it fully launches forth. It finally pulls itself together, as at E, and either develops a pair of flagella and swims off, or assumes the form of an Actinophrys. In either case, and in a very short space of time, the separated young animal is quite ready to re-unite, as at F, and assist in forming a new colony of the species.
The Polymorphina belong to a low genus of the Foraminifera. They consist of a number of forms and exhibit a rather extensive series of variations, although consisting of a few simple types, and showing transitions between forms which at first seem to be distinct. The majority of species keep to the sea bottom; some few are pelagic, and occur in abundance on the surface of the ocean. Among the latter are the Globigerina: its shell is about one-fortieth of an inch in diameter, and usually composed of seven globular chambers arranged spirally in such a manner that all are visible from above, each chamber opening by a crescentic-shaped orifice into a depression in the middle of the next. Perfect specimens bristle with long slender spines, the pores affording passage to pseudopodia, which stream out along the spines. The more carefully-conducted deep-sea investigations have brought to light the fact that the floor of the ocean, at great depths, and over a vast area, is formed of these white or pinkish coloured bodies, all containing on an average about 60 per cent. of calcium carbonate. It is a question whether the Globigerinidæ which make up the bulk of the ooze actually live at the bottom as well as the surface of the sea. This question has given rise to much discussion. Dr. Murray came to the conclusion that pelagic species do not live near the ocean floor. This opinion is partly based on the fact that the area of the Globigerina ooze coincides with the area of surface of temperature at which these bodies are found to exist. When the surface water is too cold for them, they are not to be found, neither are they found below. Major S. R. J. Owen, while dredging the surface of mid-ocean--the Indian, and the warmer portion of the Atlantic--found attached to his nets a number of these interesting bodies, and which always made their appearance just about sunset. In Plate III., Nos. 43-52, a number of these interesting and variously-formed bodies are given, and an attempt is also made to show the richly-tinted colour appearances presented by the sarcode or protoplasm of the Globigerina.
“Many of the forms,” writes Major Owen,[64] “have hitherto been claimed by the geologist, but I have found them enjoying life in this their true home, the silicious shells filled with coloured sarcode, and sometimes this sarcode in a state of distension somewhat similar to that found projecting from the Foraminifera, but not in such slender threads. There are no objects in nature more brilliant in their colouring or more exquisitely delicate in their forms and structure. Some are of but one colour, crimson, yellow, or blue; sometimes two colours are found on the same individual, but always separate, and rarely if ever mixed to form green or purple. In a globular species, whose shell is made up of the most delicate fretwork, the brilliant colours of the sarcode shine through the little perforations very prettily. In specimens of the triangular and square forms (Plate III., Nos. 43, 44, 45 and 46), the respective tints of yellow and crimson are vivid and delicately shaded; in one the pink lines are concentric; while another is of a stellate form, the points and uncoloured parts being bright clear crystal, while a beautiful crimson ring surrounds the central portion. A globular form resembles a specimen of the Chinese ball-cutting--one sphere within another; this, however, appears to belong to a distinct species.
“The shells of some of the globular forms of these Polycystina, whose conjugation I believe I have witnessed, are composed of a fine fretwork, with one or more large circular holes; and I suspect the junction to take place by the union of two such apertures. That the figures of these shells become elongated, lose their globular form after death, and present a disturbed surface is seen in some of the figures represented in Plate III., Nos. 82-85.” Those without internal chambers have been described as Orbulina universa, Plate III., Fig. 78, while Nos. 75 and 76, although members of the same family, have been separated, but all should certainly be united under Globigerina.
“The minute silicious shells of Polycystina present wonderful beauty and variety of form; all are more or less perforated, and often prolonged into spines or other projections, through which the sarcode body extends itself into pseudopodial prolongations resembling those of Actinophrys. When seen disporting themselves in all their living splendour, their brilliancy of colouring renders them objects of unusual attraction. It will appear that they wish to avoid the light, as they are rarely found on the surface of the sea in the daytime; it is after sunset and during the twilight that they make their appearance.”
Many forms of Globigerina and Foraminifera are represented in Figs. 329 and 330. These varied and beautiful forms were dredged up with soundings made in 1856 for the purpose of ascertaining the depth of the Atlantic, prior to the laying down of the electric telegraph wire from England to America, and taken at a depth of 2,070 fathoms.
Heliozoa.--Actinophrys-Sol, “sun-animalcules,” belong to this group; most of them inhabit fresh water (Plate III., No. 66). The chief characteristic, and the one to which they owe their name, is the possession of long, slender, somewhat stiff pseudopodia; these radiate from all parts of the body. The living animal usually contains green-coloured particles within a minute translucent spherical globule of about 1/250th of an inch in diameter. It is, therefore, variously designated the green sun-animalcule, Acanthocystis, or Actinophrys-Sol. It is commonly found amongst the weeds in clear pools of water, where desmids abound. The pseudopodia appear to be stiff; they are, however, quite flexible, and the body contains more than one clear vesicle with a nucleus; reproduction is secured by the simple division commencing in the nucleus. The little animal can move over a hard surface by the alternate relaxation and stiffening of its pseudopodia; when one of these touches a small organism, it is believed to paralyse it, then envelop, and deliberately digest it. In another species, the lattice-animalcule (Cathrulina), the pseudopodia or silicious threads are arranged tangentially. It grows on a long flexible stalk, attached to an aquatic plant, the total length of which is about 1/200th an inch. The globular body is perforated in all directions, through which the fine stiff pseudopodia are thrust out; it is often known to form colonies.
In this order may well be placed the Radiolaria; they are, however, usually separated. But Radiolarians, whether seen alive or in their skeleton form, are surpassingly beautiful. By the favour of Messrs. Warne, I am enabled to append a frontispiece plate to this volume taken from their “Royal Natural History.” These bodies are all marine, and live in zones of several thousand fathoms, and like their congeners, the Globigerina, they avoid a strong light, and only appear after sunset. Their bodies are supposed to emit a phosphorescent glow, but more is known of their silicious skeletons than of their living forms; yet it is not this feature that separates them from other orders of rhizopods, but the possession of a membranous central capsule enclosing the nucleus. The body substance outside this capsule is highly vacuolated in some species, especially in surface forms. A few are without a skeleton, and these consist of oval masses of protoplasm, with slender pseudopodia. In a few species the skeleton is formed of a glassy horny substance, termed acanthin, arranged in the form of radiating spines.
Radiolarians secrete a silicious skeleton, which assumes a variety of forms, as trellis-work, boxes joined by radiating spines, helmets, baskets, bee-hives, discs, rings, and numerous other forms. Haeckel has described upwards of four thousand species, and possibly as many more could be added to this number. Radiolaria are divided into two groups. In the one there is either no skeleton or one of silex; in the other the skeleton is formed of radiating spines of a horny nature. These are again subdivided according to the characters of the central capsule. In those forms with a silicious skeleton the geometrical pattern conforms more or less to the shape of the central capsule, being either spherical or conical. The central capsule is regarded as being homologous with the calcareous shell of Globigerina. Reproduction takes place by simple division into two, or by the body breaking up into spores, each provided with a flagellum, or two spores may fuse together, and the result will be an adult Radiolarian. Certain yellow corpuscles present in the outer part of their body-surface change into unicellular parasitic algals; these can be separated and cultivated independently of their host. The Radiolarians live floating at all depths from 1,000 to 2,500 fathoms, and are distributed over areas in the central Pacific and the south-eastern part of the Indian Ocean, the ooze forming the ocean bed being made up of their skeletons to an extent of 80 per cent. of the deposit; hence it has become known as Radiolarian ooze. The chalky-looking Barbadoes earth, a Tertiary formation, is composed almost entirely of their skeletons. Somewhat similar deposits exist in the Nicobar Islands, in Greece, and in Sicily.
It will have been noticed that by far the greater number of Foraminifera are of marine origin, and these occur in such widespread profusion that the finest calcareous particles which constitute the seashore in some places consist almost wholly of their microscopic remains. At former periods of the earth’s history they appear to have existed even in greater profusion than at the present time. This is evidenced by their remains forming the principal constituent of our largest geological formations.
Moreover, during the Canadian Geological Survey large masses of what appeared to be a fossil organism were discovered in rocks situated near the base of the Laurentian series of North America. Sir William Dawson, of Montreal, referred these remains to an animal of the foraminiferal type; and specimens were sent by Sir W. Logan to the late Dr. Carpenter, requesting him to subject them to a careful examination. As far back as 1858 Sir W. Logan had suspected the existence of organic remains in specimens from the Grand Calumet limestone, on the Ottawa River, but a casual examination of the specimens was insufficient to determine the point. Similar forms being seen by Sir W. Logan in blocks from the Grenville bed of the Laurentian limestone were in their turn tried, and ultimately revealed their true structure to Sir William Dawson and Dr. Sterry Hunt, who named the structure Eozoon Canadense.
The masses of which these fossils consist are composed of layers of serpentine alternating with calc spar. It was found by these observers that the calcareous layers represented the original shell, and the silicious layers the softer parts of the once living Foraminifera. The results were arrived at through comparison of the appearance presented by the Eozoon with the microscopic structure which Dr. Carpenter had previously shown to characterise certain members of the Foraminifera. The Eozoon not only exceeded other known Foraminifera in size to an extent that might have easily led observers astray, but, from its apparently very irregular mode of growth and general external form, no help was derived in its identification, and it was only by microscopical examination of its minute structure that its true character was ascertained. Dr. Carpenter wrote:--“The minute structure of Eozoon may be determined by the microscopic examination either of thin transparent sections, or of portions which have been subjected to the action of dilute acids, so as to remove the calcareous portion, leaving only the internal casts, or models, in silex, of the chambers and other cavities originally occupied by the substance of one animal.” Subsequently he found portions of minute structure so perfect that he was able to say that “delicate pseudopodial threads were originally put forth through openings in the shell wall of less than 1/10000th of an inch in diameter” (Plate III., Nos. 64, 65). In a paper read at the meeting of the Geological Society he stated that he had since detected Eozoon in a specimen of ophicalcite from Bohemia, in a specimen of gneiss from near Moldau, and in specimens of serpentine limestone sent to Sir C. Lyell by Dr. Gümbel, of Bavaria. These also were found to be parts of the great formation of the “fundamental” gneiss, considered by Sir Roderick Murchison as the equivalent of the Laurentian rocks of Canada.[65]
If the remains of Foraminifera be dissolved in dilute hydrochloric acid, an organic basis is left, after the removal of the calcareous matter, accurately retaining the form of the shell with all its openings and pores. The earthy constituent is mainly calcium carbonate; but there is also a small amount of phosphate of lime in the shells of many of them.
Infusoria.
We are now brought face to face with animals which possess considerable variation of structure, Infusorial animalcules, as they are termed. It was Ehrenberg who attributed to them a highly complex organisation, but later observations negatived these views and showed them to be animals formed of one or more cells, or colonies of so-called individuals. It is true that this cell or united protoplasm may show a wonderful amount of differentiation, what with its nucleus and vacuole, mouth and gullet, its variously-arranged cilia or flagella, its contractile fibres, its separation into an outer denser and a more fluid inner protoplasm, and its horny cup and stalks.
In these few lines we have a condensed summary of the special qualities of minute forms of life that afford much interesting work for the microscope.
Among those widespread, and in some respects heterogeneous, forms of life associated under the comprehensive title of Infusoria, we encounter types that not only differ very widely from one another, but which occupy a different rank or position, so to speak, with regard to the relation they bear to each other, and also to the outlying representatives of the series--differences that permeate throughout the ranks of this extensive group. Furthermore, a considerable number of Infusorial animalcules foreshadow or typify, in a corresponding degree, the separate or associated cell elements out of which higher tissue structures--metazoic organisms--are built up. We may take the well-known example Euglena viridis (Plate III., No. 67), or Paramecium (No. 74), and their allies; these would appear to be the prototypes of Turbellaria. Another more lowly organised group of the Ciliata exhibits a distinct and highly-interesting affinity to the Opalinidæ. There are many other species (Acineta, Plate III., No. 68, for instance), which at first sight would seem to stand by themselves and present no marked agreement with any metazoic type. Indeed, the function of these and other polypites consists simply in seizing food and conveying it through perforations at the extremity of each separate tentaculum to its interior. In Acineta certain of the tentacles only are suctorial, and these, being the inner ones, fulfil the ingestive function, while the peripheral series are prehensile. This stalked club-shaped body (Fig. 332), which fixes itself to seaweeds or Bryozoa, is seen to have a nucleus, and also clear vesicles in the body-substance; its embryos are ciliated. It is an object of considerable interest even among curious marine animalcules; one or two species inhabit fresh water. The spiral-mouthed Spirostomum are among the largest of the class, and in sunlight are visible to the naked eye as slender golden threads of about 1/10th of an inch in length. The mouth slit, extending half the length of the body, is bordered on one side by cilia. The body is cylindrical and the surface covered with rows of cilia. Its multiplication takes place by transverse fission through the middle.
Flagellate Infusoria.--The characteristic of this group, as its name implies, is the possession of one or more flagella or whip-like appendages, at the base of which is an opening in the denser surface layer of protoplasm, and in the interior a nucleus and one or more contractile vacuoles, and not infrequently a brilliant red spot of pigment known to microscopists as the eye-spot. The Monads, which constitute the simplest members of the group, are commonly found in fresh-water pools and vegetable infusions. The typical form consists simply of a spherical or oval cell provided with a flagellum. The Volvox was formerly placed in this group, but as it contains chlorophyll it is properly claimed by the botanist. The collared group possesses cup-like collars, and these frequently secrete horny receptacles or cups, and form elegant tree-like colonies.
The mail-coated group are of very varied form, the body being often prolonged into spiny processes. They have two long flagella which fit into grooves purposely provided. But the most interesting and remarkable are the phosphorescent animalcules (Noctiluca), whose beautiful bluish-green luminosity on the surface of the sea has attracted attention from very early periods. It was, however, not until the first half of the present century that the luminosity was discovered to be due to the presence of multitudes of these minute jelly-like spheres.
The body of the Noctiluca (Fig. 333) is a nearly globular-shaped cyst, enclosed in a tough membranous wall, from a grooved opening in which a striated muscular flagellum or proboscis is projected forth, and it is by means of this the animal swims away even in rough seas. A fine whip-like flagellum is also located in the same groove. At the apex of the funnel there is a mass of protoplasm which extends itself as a widely-meshed, highly-vacuolated network to the inner wall of the cyst, whence it is believed the phosphorescent light emanates. It multiplies by self-division, first becoming encysted after withdrawing its flagellum, and then breaking up into numerous ciliated helmet-shaped swarm spores. Frequently two organisms fuse into one and then divide into spores.
Noctiluca mainly confines itself to the shallower seas, but there are related forms met with in the warmer open seas; these belong to the genus Pyrocystis (Fig. 334). In one variety the body is perfectly spherical and without the big flagellum or proboscis. Professor Butschli, however, regards this species as an encysted or resting phase of the commoner and better-known form.
The late Mr. Philip Gosse, F.R.S., was the first microscopist to describe the Noctiluca. After careful observation, he wrote in his “Naturalist’s Rambles” as follows:--“I had an opportunity of becoming acquainted with the minute animals to which a great portion of the luminousness of the sea is attributed. One of my large glass vases of sea-water I had observed to become suddenly at night, when tapped with the finger, studded with minute but brilliant sparks at various points on the surface of the water. I set the jar in the window, and was not long in discovering, without the aid of a lens, a goodly number of the tiny jelly-like globules of Noctiluca miliaris swimming about in various directions. They swam with an even gliding motion, much resembling that of the Volvox globator of our fresh-water pools. They congregated in little groups, and a shake of the vessel sent them darting down from the surface. It was not easy to keep them in view when seen, owing rather to their extreme delicacy and colourless transparency than to their minuteness. They were, in fact, distinctly appreciable by the naked eye, measuring from 1/50th to 1/30th of an inch in diameter.”
Among the numerous fresh-water members of the flagellate infusoria, there is one which especially calls for notice, Codosiga, discovered by the late Professor H. J. Clark. This minute body bears a delicate funnel-shaped protoplasmic expansion or collar, common to the several members of this organic series. The flagellum is placed at the base of the oral opening, and within the circumscribed area of the collar, which is of such extreme tenuity that its true form and nature can only be determined by a very careful adjustment of the achromatic condenser and accessory apparatus employed, together with a wide-angled objective. It is seen to greater advantage by supplying the animal with very fine particles of colouring matter. In this way it is found that the infundibuliform cup consists of protoplasm, through which the flagellum is protruded and withdrawn into the general substance of the Monad’s body (Fig. 335). As many as twenty or more zooids are attached to the extremity of a slender footstalk. The length of the body, exclusive of the collar, is 1/2500th to the 1/1200th of an inch. The habitat of these bodies is fresh water. Mr. Saville Kent in 1869 discovered some of these interesting infusoria in the London Docks.
“The more exact significance of the special organ, the collar, is manifest by the circulatory currents or cyclosis induced, and there can be no room for doubt that this structure finds its precise homologue in the pseudopodia of the foraminiferous group of the Rhizopoda, in which a similar circulation or cyclosis of the constituent sarcode is exhibited. The whole of this highly-interesting flagellate order, a comparatively small one as yet, are remarkable for their pale glaucous green or florescent hue, such colour assisting materially in their recognition, even when the magnifying power employed is insufficient for the detection of the very characteristic collar with its enclosed flagellum.”[66]
Ciliata.--Types of Ciliata obtained from hay infusions are very numerous. Ehrenberg’s animalcules were mainly of a large size, and of those belonging to the higher order of the Ciliata, pertaining to such genera as Paramecium, Colpoda, Cyclidium, Oxytricha, and Vorticella. These, however, represent but an insignificant minority of the hosts of flagellate forms which abound in our humid climate, and in hay infusions in particular. In such infusions, watched from day to day and produced from hay obtained from different localities, the number of types developed in regular sequence is found to be perfectly marvellous, commencing with the Monas proper, Amphimonas and Heteromita; while Bacteria, in their motile and quiescent forms, are invariably present and furnish an abundant supply of material for the microscope.[67]
Vorticellidæ constitute one of the most numerous families of the ciliate infusoria. All its members are at once recognised by their normal stationary condition, and by the structure of their oral system. In but few of the genera is there any marked divergence from this formula, and when any exists it is made manifest by an increase in development of some one of its elements at the expense of another. For instance, in the genus Spirochona, the external edge of the encircling border or peristome is suppressed, while the inner portion is abnormally developed into a transparent and highly elevated spiral membrane. The bell-animalcules usually possess stalks, and are either solitary or form branching colonies. Conichilus vorticella (Plate III., No. 80) is a well-known member of the colony stock, all the zooids of which are united on a slender branching pedicle, which consists of a central contractile cord enclosed within a tubular hyaline sheath. There are many other shrub-like colonies all variously modified in form and character. The Epistylis opercularia, or nodding-bell animalcule, is an interesting member of a numerous host of solitary short-stalked forms (Fig. 337). When the animal is disturbed, the heads drop down towards the stalk. This animalcule has been found to form a colony; and another, Carchesium, whose tiny branched tree-like colonies resemble little white globular masses of moulds, are seen at once to drop down towards the base of the colony with a jerky movement if the cell be touched. By a process of encysting, all the Vorticellæ and many of the more highly-organised ciliata have the means of what may be termed self-preservation. Should the water dry up in which they have been living, the little animal encases itself in mud at the bottom of the pool. Should this be baked by the sun not the least injury arises, for at this stage it crumbles into dust, and is carried by the wind to long distances, but the first shower of rain calls it back to active life, and soon after it is seen to issue forth as a free swimming bud.
Thuricola valvata (Plate III., No. 72) possesses a hinge-like process which closes up like a door when the animal contracts itself into its case. This very effectually protects it from assault. Both portions of the valve are capable of extension. Another group of ciliate infusoria also possess a limited number of cilia, but these, although restricted to the under surface of their bodies, have an unrestricted range of motion. The group are all free swimmers, belonging to the genus Oxytricha. They possess two separate alimentary orifices, neither of which are situated at the extremities or encased by a dense integument. Their locomotive organs consist either of setæ, vibratile cilia, or non-vibratile styles or uncini, variously situated, and all serving to make these infusorial animals very active (Plate III., Nos. 73 and 77). A typical species is the mussel-animalcule (Stylonychia, Fig. 338), common in all infusions and pools of water. Its body is oval and flattened, and about 1/100th of an inch in length. At one end a funnel-shaped depression or mouth, with a ciliated margin, leads to the inner part of the body, in which are two oval bodies, a nucleus and a contractile vacuole, which is seen to contract rhythmically. The creature can also stalk along by means of its cilia or setæ, and set up currents to the mouth. Plate III., Nos. 70, 71, 72, 73, and 74, are types of these interesting bodies.
Dr. Balbini believes a true sexual generation occurs among these organisms, but, with the exception of the Paramecium, this has not been seen to take place; even Gruber’s more recent investigations appear to be inconclusive on this point. Conjugation, however, it is said takes place among some attached forms, as in the Stentors. These have been seen to put forth a bud from the body base, and soon after become free swimming bodies. The trumpet-animalcule (Stentor), a conspicuous member of the ciliata, is comparatively large, being about the 1/25th of an inch in length when extended to the full size. It is usually found attached to the under sides of duckweed, and is continually changing its form from that of a small knob when contracted, to the trumpet shape seen in Fig. 339, No. 6, when fully extended, and from which it derives its name. The long cilia projected from the upper part form a spiral within the margin of the open mouth leading to the digestive sac. A contractile vacuole lies to the right of the oral opening. New individuals are produced by the process of budding, and in the form of ciliated embryos from the nucleus. Stentors are commonly met with in fresh water, and are usually of a brilliant green colour. These little bodies will bear cutting up: if only a fragment of the nucleus be included in the section, the injury is soon repaired.
Rotifera, or Wheel-animalcules (Fig. 339).--In this group we have a higher type of animal, with a more complex organisation than those previously noticed. The great majority inhabit fresh water, and are readily developed in hay infusions, in bog-moss, in house-top gutters, everywhere if looked for after a shower of rain. The rotating organs from which these fascinating animalcula derive their name consist of two disc-like bodies whose margins are fringed with rows of cilia, which create currents toward the oral aperture, and which have given rise to the optical delusion of rotating wheels. The disposition of the cilia is so arranged as to bring food to the rotifer and conduct it to the mastax or digesting apparatus--a muscular bulb moved by a series of muscles--the gastric glands and stomach. The great transparency of the whole structure permits of the animal economy being easily studied. The body is covered with a horny envelope of two layers, and is divided into segmental divisions, which slide into each other telescopic fashion. Consequently, as the water dries up, the animal is for a long time rendered indestructible and capable of resisting varying temperatures and the action of caustic reagents.
Rotifers are oviparous, and their eggs are conspicuous and of three kinds. The common soft-shelled eggs produce females, the smaller and more spherical produce males. The ephippial, or summer eggs, are often beset with spines or bosses; these have only a membranous covering, and are hatched soon after they are laid, or before leaving the ova sac. The male rotifer is but a third of the length of the female, often without cilia, and appears to have no alimentary tract; indeed, the only internal organ is a large sperm sac. Rotifers have been divided by Dr. Hudson and the late Mr. Gosse in their charming work on these very interesting “Wheel-animalcules” into four orders, according to their powers of locomotion, as follows:--(1) Rhizota, the rooted; (2) Bdelloida, the leech-like, that swim and creep like a leech; (3) Ploïma, the sea-worthy, that only swim with their ciliary wreath; (4) Scirtopoda, the skippers, that swim with their cilia and skip with arthropodous limbs. These, again, are subdivided into families. With such hardy creatures as Philodina, Adineta, Brachionus, &c., creatures to whom extremes of cold, heat, and drought are the ordinary conditions of life, nothing can be easier to keep going throughout the year. Mr. C. F. Rousselet, who has so thoroughly succeeded in mounting Rotifers with their cilia fully extended, recently exhibited at one of the evening meetings of the Royal Microscopical Society, London, no less than four hundred specimens in a natural and perfect condition, the nervous system being seen more clearly from its successful staining throughout the body than in the living rotifer.
There is also a family of Rotatoria with a single rotatory organ, disposed around the margin of the case. This comprises at present a very small group. The Œcistes is a member of the family (Plate III., No. 69). A single ciliary wreath leads to the alimentary canal, and a pharyngeal bulb or mastax comprises the apparatus of nutrition. The visual organs are red, as in other rotifers, and the ovarium contains several ova, shown in No. 69. The envelope is a gelatinous transparent sheath, into which the animalcule can withdraw itself, its attachment to the bottom being by the end of the foot-like tail. The most interesting among this genus are the Floscularians. These creatures may undoubtedly be described as among the most beautiful and interesting of infusorial animals.
The Stephanoceros, “crowned animalcule,” as it is termed, is about 1/36th of an inch in length, and enclosed in a transparent cylindrical flexible case, beyond which it protrudes five long arms in a graceful manner. These, touching at their points, give a form from which it derives its name. These arms are furnished with several rows of short cilia, which seize the food brought within their grasp until it can be swallowed. In addition to the rotatory organs, they have short flexible processes, or cornu, attached to the outside of one or more of their lobes. The water vascular system consists of two canals arising from a small pyriform contractile vesicle, situated below the stomach. The ova, after leaving the ova sac, remain quiescent until their cilia are developed. Floscularians, like Melicertans, have a certain affinity in form with Vorticellians and Stentors, and also with Campanulariæ, among polypes. Their cilia are less regular when in action than in other Rotatoria. When they retreat into their transparent cells they appear to fold themselves up. Their internal structure can be seen through the external case, and ova are observed enclosed in an ova sac; when thrown off they remain quiescent until the formation of their cilia. The whole family furnish interesting objects for microscopic investigation.
Melicerta ringens (“beaded Melicerta”).--Of all the Melicerta, or “horny floscularia,” this is the most beautiful. Its crystalline body is enclosed in a pellucid covering, wider at the top than the bottom, of a dark yellow or reddish-brown colour, which gradually becomes encrusted by zones of a variety of shapes, cemented together with a peculiar secretion that hardens in water. It derives its name from these pellets, which have the appearance of rows of beads. Mr. Gosse furnished an excellent account of the architectural instincts of Melicerta ringens: “An animalcule so minute as to be with difficulty appreciable by the naked eye, inhabiting a tube composed of pellets, which it forms and lays one by one. It is a mason who not only builds up his mansion brick by brick, but makes his bricks as he goes on, from substances which he collects around him, shaping them in a mould which he carries on his body.
“The pellets composing the case are very regularly placed in position; in a fine specimen, about the 1/30th of an inch in length, when fully expanded, as many as fifteen longitudinal rows of pellets were counted, which gave about thirty-two rows in all. As it exposes itself more and more, suddenly two large rounded discs are expanded, around which, at the same instant, a wreath of cilia is seen performing surprising motions.
“On mixing carmine with the water, the course of the ciliary current is readily traced, and forms a fine spectacle. The particles are hurled round the margin of the disc, until they pass off in front through the great sinus, between the larger petals. If the pigment be abundant, the cloudy torrent for the most part rushes off, and prevents our seeing what takes place; but if the atoms be few, we see them swiftly glide along the facial surface, following the irregularities of outline with beautiful precision, dash round the projecting chin like a fleet of boats doubling a bold headland, and lodge themselves, one after another, in the little cup-like receptacle beneath. Mr. Gosse, believing that the pellets of the case might be prepared in the cup-like receptacle, watched the animal, and presently had the satisfaction of seeing it bend its head forward, as anticipated, and after a second or two raise it again; the little cup having in the meantime lost its contents. It immediately began to fill again; and when it was full, and the contents were consolidated by rotation, aided probably by the admixture of a salivary secretion, it was again bent down to the margin of the case, and emptied of its pellet. This process he saw repeated many times in succession, until a goodly array of dark-red pellets were laid upon the yellowish-brown ones, but very irregularly. After a certain number were deposited in one part, the animal would suddenly turn itself round in its case, and deposit some in another part. It took from two-and-a-half to three-and-a-half minutes to make and deposit a pellet.”
Melicerta may be found in clear pools, mill-ponds, and other places through which a current of water gently flows. If a portion of water-weed be brought home and placed in a small glass zoophyte-trough, and carefully examined with a magnifying power of about fifty diameters, a few delicate-looking projections of a reddish-brown colour will probably be seen adhering to the plant; these are the tubular cases of Melicerta, which, after a short period of rest, will be seen to be animals of 1/12th of an inch or more in length.
Porifera. Spongiadæ.
Sponges.--The term Porifera, or “canal-bearing zoophytes,” was applied by the late Dr. Grant to designate the remarkable class of organisms known as sponges, met with in every sea, and numbering about two thousand species, varying in size from a pin’s head to masses several feet in height; and weighing from a few grains to over a hundred pounds. Sponges assume an endless variety of shapes, as cups, vases, spheres, tubes, baskets, branched-like trees, but often as shapeless masses. When living they are all colours and all consistences, soft and gelatinous, fleshy, leathery or stony. A fuller knowledge of sponges was gained in 1825, when Dr. Robert Grant examined a fragment of living sponge under the microscope. On bringing it to the side of the glass cell in which he had preserved it, he beheld this living fountain pouring forth a torrent of liquid matter in rapid succession, and he was at once convinced that a current flowed out of the larger orifices. He introduced a small portion of fine chalk, and saw particles driven into the interior, and pass out again by different ways. To determine the cause of the currents, it was necessary to make a closer examination of the anatomy of the sponge. For this purpose he cut or peeled off thin sections, and saw that the whole substance was divided into flagellated chambers, enclosing spherical and other bodies, and perforated by pores. Each chamber proved to be about 1/500th of an inch in diameter, groups of them opening by a wider orifice into a common space, or canaliculus, and joining others to form canals terminating in larger oscular canals. The walls throughout are lined with flat cells, but in the flagellated chambers the living cells are more or less cylindrical, and each is provided at the free end with a whip-like appendage, or flagellum. Furthermore the upper margin was seen to be expanded into a thin hyaline collar, so that the whip appeared to have its origin in the centre of a basin or funnel. The currents of water traversing the body of the sponge are kept up by the movements of the flagella of the collar-cells. These beat the water in the flagellated chambers into the rootlets of the canals leading to the oscules. To replace this, water flows into the flagellated chambers from the rootlets of the canals passing down from the groups of pores in the skin. The currents entering the sponge bring in oxygenated sea-water and minute food particles, such as diatoms and infusorial organisms; the currents from the oscules contain an excess of carbonic acid of waste products, resulting from vital activity and indigestible remains. The cells lining the canals effect the exchange of gases, and take up food particles.
Professor Grant’s careful and instructive researches were begun on the smaller kind of British sponges hanging down from rocks (Spongia coalita), and on which he gazed for “twenty-five minutes, until obliged to withdraw his eyes from fatigue.” This sponge fixes itself by a root; and the currents enter through the stem and body, and leave principally by oscules placed on the branches.
At present too little is known as to the physiology of digestion in sponges to permit of a definite statement on the subject. In specimens fed upon carmine the collar-cells have been found loaded with granules; in others, again, the flat cells lining the subdermal cavities have been found gorged with colour granules. From Bowerbank’s monograph on the British Spongiadæ (1864 and 1874) nothing of importance can be gained on the subject; in fact, it relates almost entirely to the structure and organisation of sponges in their dried or preserved condition, and therefore is only of value for purposes of specific identification. One of the simplest of living sponges, the microscopic structure of which it is possible to trace, Ascetta primordialis, is found on seaweeds in the Mediterranean. In its simple unbranched condition it forms a minute white sac about one twenty-fifth of an inch in height, opening above by a wide round oscule and narrowing below to a stalk (Fig. 342). The walls are very thin and perforated by pores, through which the water passes into the interior. The walls of the sac are composed of two layers, an inner lining of collar-cells, and an outer layer consisting of a gelatinous matrix containing amœboid bodies and transparent three-rayed spicules. These serve to support the walls and as a frame-work for the pores, as in all the sponges. By eliminating the spicular skeleton, and by supposing the tube to be more globular, the “olynthus form” will be obtained, which has been regarded as the hypothetical ancestor of all sponges. A canal system arises when the walls grow thick or form folds, or give off pouches or tubes. From these channels arise incipient in-current canals, between the inside or lumen of the folds and that forming the out-current canal system.
There is a common ciliated Sycon found on seaweed round the British coast; it has the appearance of a white sac about an inch in height, with a crown of glassy spicules around the orifice. The vertical cavity of the sac is surrounded by a wall of closely-packed horizontal tubes, opening at their inner ends into the central cavity, but externally ending blindly. The central cavity of the sac is surrounded or lined with flat-cells, and the radial tubes with collar-cells, and the walls of the tubes are perforated. Here the spaces between and outside the densely-packed tubes are the in-current canals. In an equally common British sponge, Grantia, which forms small flat white bags, a rudimentary cortex covers the outer ends of the tubes. In Grantiopois, the cortex becomes quite thick; as the radial tubes in this species become more branched and the mesoderm thicker, so the passages or in-current canals become more complicated. Common silicious, sponges develop in a different manner from the calcareous ones, namely, from a hollow conical sac open at the top and with a flat base; the spherical flagellated chambers at a very early stage forming a mammillated layer in the walls. Plakina, one of the simplest silicious sponges, encrusts stones with a fleshy crust, consisting of a sac with a flat base attached to the stone in sucker-like fashion, and with the rest of the walls forming simple folds. The spaces between and outside the folds form the in-current, and those in the lumen of the folds the out-current, channels. Each of the flagellated chambers in the walls of the folds communicates with the in-current spaces through several pores, and opens into the out-current spaces by one large pore, the currents of water passing out by the central oscule. Here we have a general idea of the formation of all the commoner forms of sponges. In the more delicate species, as that of Venus’ flowerbasket, the cells are formed by a trellis work of large spicules of silica. Groups of cells congregate in the ground substance and secrete a network of cylindrical fibres and spicules, which, although they remain to a certain extent separate, are always beautifully adapted for purposes of support. In addition to the support these afford, the skeleton spicules afford a means of defence against the attacks of small animals.[68]
A fairly good idea will be gained of the internal structure of sponges from the section made of a Geodia Barretti, Fig. 343.
Reproduction.--As regards the modes of reproduction, both male and female cells are found in the mesoderm. The male cells generally give rise by division of the nucleus to masses of spermatozoa, each of which possesses a conical head and a long vibratile filament. The ova appear as large round cells, and when conglomerated in masses, resemble those of Micro-gromia, which, after fertilisation, undergo segmentation or division, first into two cells, and again dividing and sub-dividing, until a cluster or mass of cells results (as seen in Fig. 343). The outer layer of the egg-shaped embryo becomes more cylindrical in shape, and is now provided with cilia, and soon appears as an independent minute oval body. If a bread-crumb sponge be cut open in the autumn, the embryos will be seen as bright yellow spots within the body-substance. By keeping specimens in a vessel of water, the embryos will be seen to escape from the oscules, and swim freely about with the broad end forwards. After twenty-four hours of independent existence, the embryo remains stationary, and fixes itself by its broad end, which becomes flattened out. By a remarkable transformation, the larger granular cells of the interior burst out and grow over the outer flagellate layer of cells, and the latter become the collar-cells of the adult sponge. A minute sponge with one oscule results from the development of the fertilised ovum. An extensive crust with numerous oscules may be regarded either as a colony in which each oscule represents an individual, or simply as one individual in which the growth of the body necessitates the formation of new channels for the conveyance of food materials. The embryos of some of the fresh-water sponges (Spongillidæ) living in ponds, canals, lakes and rivers all over the world, as soon as they become fertilised undergo segmentation, and form oval ciliated bodies, in appearance somewhat resembling the gastrula of Monoxenia, one of the simplest kinds of corals. Fresh-water sponges are green in colour, due to the granular bodies which crowd the cells near the surface of the sponge; that this colour is not due to the formation of chlorophyll is seen on keeping them in a shady place, when they become pale grey or yellowish-brown, and if kept quite in the dark they entirely lose all colour.
A few sponges possess no skeleton whatever, excepting the gelatinous ground substance; in some specimens the skeleton is mainly or entirely composed of foreign particles of sand or the remains of Foraminifera. Others are composed of calcium carbonate, and form the class Calcarea, the spicules of which are white, and opaque in mass; but on placing portions in hydrochloric acid, the skeleton is dissolved away with effervescence, and the spicules are left behind transparent and glassy. A great variety is seen in the different species, as will be gathered from the few typical forms shown in Plate XVI., and which even in their fossilised state remain unaltered, the silica which enters so largely into their composition being indestructible, the calcareous matter alone becoming separated in exposure to the action of air, or by boiling in hydrochloric acid. The only perceptible difference noticed is an increase in transparency, and this, on mounting them in Canada balsam, adds to their beauty when examined by polarised light.
Hyalonema, the “glass-rope” sponge of Japan, consists of a bundle of from 200 to 300 threads of transparent silica, glistening with a satiny lustre like the most brilliant spun glass; each thread is about eighteen inches long, in the middle the thickness of a knitting-needle, and gradually tapering towards either end to a fine point; the whole bundle coiled like a strand of rope into a lengthened spiral, the threads of the middle and lower portions remaining compactly coiled by a permanent twist of the individual threads; the upper portions of the coil frayed out, so that the glassy threads stand separate from each other. The spicules on the outside of the coil stretch its entire length, each taking about two and a half turns of the spiral. One of these long needles is about one-third of a line in diameter in the centre, gradually tapering towards either end. The spirally-twisted portion of the needle occupies rather more than the middle half of its entire length. In the lower portion of the coil, which is embedded in the sponge, the spicule becomes straight, and tapers down to an extreme tenuity, ultimately becoming so fine that it is scarcely possible to trace it to its termination.
Within the mesoderm, and in oscule, was noticed a deep brownish-orange coloured shrunken membrane; this was traced to a parasitic polyp. Since this was first observed on an early specimen of the Japanese glass-sponge, the same parasite has always been found growing on and in all these curious sponges. The surface of the stalk above the portion embedded in the mud is seen to be covered with a warty crust of parasitic polyps. All the specimens of Hyalonema in the European museums in 1860 had their stalks overgrown with Palythoa, while many had their bodies also covered with another parasite, and which, fortunately for the sponge, did not form a sandy crust. The polyps, having no skeleton, dry up entirely, and leave behind no trace except the stain first referred to. Unlike a parasite, however, the polyps do not feed upon the juices and soft parts of the sponge, nor indeed do they share its food, but simply settle upon the sponge and feed upon any food that may chance to come within their reach.
The dredgings of the Challenger brought to the surface many entirely new forms of glass-sponges and from great depths. One of the most beautiful, known as Carpenter’s glass-sponge (Pheronema), is composed of concentric laminæ of silica deposited around a fine central axial canal. These form a gauze-like network throughout, but with no regularity of structure.
Clionæ.--Not the least wonderful circumstance connected with the history of sponges is the power possessed by certain species of boring into substances, the hardness of which might be considered as a sufficient protection against such apparently contemptible foes. Shells (both living and dead), coral, and even solid rocks are attacked by these humble destroyers, gradually broken up, and, no doubt, finally reduced to such a state as to render substances which would otherwise remain dead and useless in the economy of nature available for the supply of the necessities of other living creatures.
These boring sponges constitute the genus Cliona of Dr. Grant. They are branched in form, or consist of lobes united by delicate stems, and after having buried themselves in shells or other calcareous objects, preserve their communication with the water by means of perforations in the outer wall of the shell. The mechanism by which a creature of so low a type of organisation contrives to produce effects so remarkable is still doubtful, from the great difficulties which lie in the way of coming to any satisfactory conclusions upon the habits of an animal that works so completely in the dark as the Cliona celata. Mr. Hancock, in his valuable memoir upon the boring sponges, attributes their excavating power to the presence of the multitude of minute silicious crystalline particles adhering to the surface of the sponge; these he supposes are set in motion by ciliary action. In whatever way this action may be produced, however, there can be no doubt that these sponges are constantly and silently effecting the disintegration of submarine calcareous bodies--the shelly coverings, it may be, of animals far higher in organisation, and in many instances they prove themselves formidable enemies even to living molluscs, by boring completely through the shell. In this case the animal whose domicile it so unceremoniously invades has no alternative but to raise a wall of new shelly matter between himself and his unwelcome guest, and in this manner generally succeeds in barring him out.
From a close examination of the structural and developmental characters of the Spongideæ, it must be conceded that they belong rather to the flagellata Protozoa than to any other order. This was the view held by the late Professor Clark, and Mr. Saville Kent quite concurs in it.[69] Summing up the entire evidence adduced, scarcely a shadow of doubt is admissible concerning the intimate relationship that subsists between the Choano-flagellata and other flagellate Protozoa and that of sponges. The primary and essential element of the apparently complex sponge stock is the assemblage of collared flagellate zooids that inhabit its interstitial cavities under various plans of distribution. Individually these collared zooids correspond structurally and functionally in every detail with the collared units of such genera as Codosiga, Salpingœca, and Proto-spongia. The collar in either case presents the same structure and functions, exhibits the same circulatory currents or cyclosis, and acts in the same way for the capture of food. The body contains an identical centrally located spheroidal nucleus or endoplast, and a corresponding series of rhythmically pulsating contractile vesicles. The developmental reproductive phenomena are also strictly parallel. Both originate as simple Amœba or simple flagellate Monads, exhibiting no trace in their earliest stage of the subsequently acquired characteristic collar. Both again after a time withdraw their collar and flagellum, and assume the amœboid state; then, coalescing, enter upon a quiescent or encysted condition, and break up into a number of sporular bodies, and thus provide for the further existence and distribution of the species. The whole process again is much akin to that which obtains in the protophytic type, Volvox globator, which liberates from its interior free swimming gemmules that take the form of spherical aggregation of biflagellate daughter-cells. In their isolated state, on the other hand, the swarm gemmules of the sponge stock are directly comparable with the free swimming subspheroidal colony stock of the flagellate infusoria Synura, Syncrypta, and Uroglena, or with the attached subspheroidal clusters of Codosiga and Anthophysa.
III. Zoophytes, Cœlenterata, Medusæ, Corals, Hydrozoa
A study of the earliest growth of the Cœlenterata has shown that their internal cavities are nothing more than regular radiate out-growths of the internal structures. The result of this development is a condition which does not occur again in the whole of the animal kingdom. There is a system of cavities all in open communication one with another, no closed blood vascular system, and no specialised respiratory apparatus. Again, all the animals that constitute this large group are radiate in structure, that is, when viewed from above they are typically star-shaped, and if cut across, every horizontal section shows a symmetrical arrangement of the several parts around a centre. There are other radiate animals, as the Echinoderms, but while in these five is the fundamental number of rays, in the Cœlenterata the rays are a multiple of four, six and upwards. The skeleton or framework of each differs, and when the Cœlenterata form calcareous structures, these are quite different from the tests of the sea-urchins; and in all cases the anterior portion of the body is crowned with one or more circles of tentacles, which remain perfectly flexible and flower-like. The most highly-developed of the free forms are the sea-anemones and the jelly-fish. These have no hard or calcareous skeleton whatever, but withal they are, in the opinion of naturalists and microscopists, the most beautiful objects among Zoophytes.
In spite of their variety of forms, the Cœlenterata seem to be as incapable of higher development as do the Echinoderms, and they have failed to make headway in fresh water, but it is not improbable that some of the simplest forms of the whole group may have given rise to higher animal forms, while the sea anemones, corals, &c., being those descendants of the primitive simple form, have retained the original type of organization almost unchanged.
The type of the group is the Hydra, a fresh-water polyp, commonly found attached to the leaves and stems of many aquatic plants, or floating pieces of stick. Two species are well known to microscopists, the H. viridis, or green polyp, and the H. vulgaris, somewhat darker in colour, probably dependent upon the nature of its food. The third, less common species, the H. fasca, is distinguished from both by the length of its tentacles, which, when fully extended, greatly exceed those of either of the before-mentioned. The fresh-water group measures from one-eighth to the one-third of an inch in length, and form simple stocks of one, two or more branches. They almost exactly resemble in form the polyps of the Hydractinia, which are provided with a circle of tentacles. When placed in a vessel of water and left undisturbed they often attach themselves to the side, where they may be examined with a moderate power at leisure. They are then seen to spread out their tentacles like fine threads, and seize upon any small creature that may come in their way, and by the same means convey it to a mouth capable of great extension. All Hydra possess stinging-cells, by means of which they paralyse their prey. Many Hydra attain to a large size, and shoot out long poisonous filaments; they also possess smaller kinds of smooth cells, which appear to be employed for an entirely different purpose, but for what is not positively known. Hydra usually multiply by means of buds, an out-growth from the body, and these remain attached to the mother stalk for some time, often long enough to give rise to one or two smaller buds. Single eggs are also developed in the ectoderm beneath capsules, or wart-like prominences. The adult animal can be cut to pieces, and from each piece a new individual will be developed. This method of reproduction was first tried by the naturalist Trembley in 1739, whose experiments in this direction excited the greatest interest among the naturalists of the middle of the last century. Hydra fusca in various stages of development is given in outline in Fig. 346.
In the polyps belonging to this family the body-structure for the most part consists of a homogeneous aggregation of vesicular granules, held together by an intercellular sarcode, and capable of great extension and contraction, so that these animals can assume a variety of forms and extend their body and tentacles until the latter become almost invisible. It was the resemblance in this respect to the fabled Hydra that originated the name. Its organ of prehension is termed the hasta; this consists of a sac or opening at the terminal end of the tentacle, within which is seen a saucer-shaped vesicle, supporting a minute ovate body, which carries a sharp calcareous piece termed a sagitta or arrow. Although the fresh-water Hydra may be regarded as typical of this group of animals, marine fauna furnish a far more extensive group in the corals, jelly-fish, and sea-anemones.
A smaller group, the Ctenophora, although members of this sub-kingdom, have not yet found their true position; nevertheless they are interesting glassy, transparent creatures, either shaped like apples, melons, or Phrygian caps, or else forming bands of some considerable length; all are wonderfully transparent, with the single exception of the Beroë. These inhabit the open sea, and are only seen inshore when driven in by currents or strong winds. Their position in the water is usually more or less vertical, the mouth being turned downwards. The portion from which this group derives its name is the ribs, which are symmetrically arranged, and consist of rows of short transverse combs, each forming rows of cilia, which, as they wave to and fro, constitute a swimming or rowing plate, their activity in the water depending upon the will of the animal. They are also provided with an oral umbrella, and capturing filaments or tentacles with hair-like branches. These tentacles, attached to the sides of the animal, are capable of erection or withdrawal into pockets. Great variety is seen in these accessory organs of locomotion; for instance, the Cydippidæ (Plate XVII.) have only arms, but these are remarkable for their length, and serve for the purpose of capturing food as well as for steering. The most interesting, if not the most beautiful of the Ctenophora, are the Beroidæ; it is this family that bear a resemblance to the Phrygian cap (Plate XVII., e). The mouth is wide, but it appears to have no capturing tentacles, and yet their habits are carnivorous; they will even devour their own relations. Many of the genus are phosphorescent, and in place of stinging-cells have small spherical knobs beset with sticky globules, in which their food becomes entangled, and these are apparently in constant use.
The Stinging Series, Cnidaria, comprise sea-anemones, corals, jelly-fish among marine animals, and Hydra among the fresh-water Cœlenterata; and derive their name from a remarkably curious feature, the so-called stinging capsules. These are not only offensive, but also defensive weapons with all the animals belonging to this group; the possession of which has converted the bell-like jelly-fish into a simple Cnidarian. The principal change is in the gelatinous layer between the outer wall and the inner digesting layer of the ectoderm. But without entering further into their structure and relations, the stinging-cells and batteries claim especial attention. These cells vary considerably in size without their characteristics being essentially changed. The protoplasm of the cell is modified into a tolerably firm substance, enclosing an oval or cylindrical vesicle. Closely associated with this is a pointed process, standing up far above the level of the outer covering, known as the cnidocil. Within the vesicle is found, either spirally rolled up or in an irregular tangle, a long filament or hollow tube, a prolongation of the vesicle, but turned outside in. This tube is more than twenty times as long as the cell, is pointed at the tip, and beset with two rows of fine spirally-arranged barbed hooks. When the cnidocil is touched or irritated, this filament is violently shot out, being then turned inside-out, like the fingers of a glove. So long as the thread remains rolled up within the vesicle the barbed hooks remain in their tube, but when shot out, they change to the outside. The rolled-up filament appears to be filled with some poisonous material, which is ejected when the tube is shot out, and where the point strikes a wound is inflicted, so that unless the prey is stronger than the attacker it cannot escape. The greater the struggle, the larger the number of capsules discharged in order to kill.
Polypomedusæ.--Among the higher development of the stinging group is the jelly-fish. The Siphonophora, as represented by the Portuguese man-of-war, are in their turn the highest development of swimming-bells, and exhibit many modifications and combinations of individuals. The tentacles of the Physalia, the best known, are stiff with batteries of stinging-capsules, the sting of which is more like the shock of the electric current. The Challenger soundings brought to light some remarkably interesting forms, and these have furnished much work for the microscope, as all their larval forms are extremely curious. Among the Hydromedusæ there are many different life histories. Take the jelly-fish, the eggs of which have given up forming stocks, and are hatched out at once as Medusæ. There are others, the eggs of which form stocks; others, again, in which the sexual individuals do not swim away as jelly-fish. The last were at one time described under a new name, because of one or two curious forms being taken creeping on the ground. This creeping Medusa (Clavatella prolifera) has six arms, the tips of which are provided with true suckers, and on these it walks as on stilts, while from each arm a short stalk arises, the swollen end of which is beset with stinging capsules. It has an extensile mouth-tube, and feeds upon small crustaceans found on seaweeds.
Among the forms that swim away as jelly-fish a very curious example is presented in Corymorpha mutans. These swim about for a time, and then firmly attach themselves by numerous thread-like appendages, forced into the sand, and where the young prepare for their next metamorphosis. As an example of the stocks of those representatives which do not swim away as jelly-fish, take the beautifully-feathered, plant-like creatures found erect along the seashore, the Sertularia (Fig. 358, No. 12) and Plumularia. Plumularia primata, Fig. 348. Other members of these groups will be found in Plate IV., Nos. 95-99.
In addition to the nutritive individuals, there are the egg-bearing; these do not become free-swimming individuals. One small family is neither branched nor feathered--the Hydractinia echinata, found in the North Sea and on the Norwegian coasts, where it attaches itself to the shells of gastropods, selecting those inhabited by hermit crabs. The part of the stock common to all the individuals is the skin-like portion which adheres to the surface of the shell. In some spiny processes are produced, and the nutritive canals running down the stems of the polyps are continued into the membrane belonging to the stock, as seen in Fig. 349.
The nutritive individuals are distinguished by long tentacles, mouths, and digestive canals. The females have no mouths, and are supplied with food through the system of canals running to them from the nutritive males. These reproductive members are furnished with stinging threads instead of tentacles for the protection of their ova. The ciliated larvæ, in a very short time, swim off to found new colonies.
The free-swimming jelly-fish (Fig. 350, and Plate XVII., c and d) belong to the order Scyphomedusæ. These are characterised by their delicate colouring, and from the arrangement of their nervous system, which can only be made out by staining. Some new and curious forms were dredged from a depth of more than 6,000 feet off the coast of New Zealand, varying in size from an inch to twenty inches; many having from four to eight or ten eyes arranged along the margin.
Anthozoa.--From the free-swimming we turn to a group of permanently fixed polyp forms, the sea-anemones and corals. The development of Monoxenia commences with the egg, repeatedly dividing into many parts (Fig. 351, C, D, and E), by a process common to the animal kingdom, termed egg-segmentation, in this particular instance proceeding from an apparently hollow sphere, A, enclosing a single layer of cells, G. Each cell sends out a long cilia, or whip-like process, F, by means of which the larva turns about and swims in the body fluid of the parent polyp. One half of the sphere now becomes enfolded into the other half, H, and forms what is termed a gastrula, I, K. The gastrula stage of Monoxenia is of the simplest kind, the larva forming a sac, with walls consisting of two layers, an outer, or ectoderm, and an inner, or endoderm. The transition from the flat dish shape, H, to the sac with a narrow mouth is at once clear, and the knowledge that all the Cœlenterates proceed from similar larvæ, and that all the complications of their various systems are developed from a simple gastrula, throws much light on their anatomy. During these transitions the endoderm, whose cells multiply, continues as an uninterrupted lining to the stomach and its appendages, while the ectoderm yields the cuticular elements.
A third intermediate gelatinous layer, the mesoglæa, arises between the two layers in which muscles and connective interstitial tissue appear. In the mesoglæa of one species of coral calcification takes place; this internal calcification has but a small share in the work of the great rock-making corals, their most important calcification being external. In Monoxenia, although the transition from the gastrula larva to the adult animal has not been seen, there can be no doubt as to how this is carried out, the transformations having been watched throughout in other species. The larva attaches itself with the end opposite the mouth, the cilia disappear, and after the mouth-tube has been formed by the folding in of the anterior end along the longitudinal axis of the body, and has thus become marked off from the stomach, eight hollow tentacles rise round the mouth as outgrowths of the body cavity, or as direct continuations of the stomach.
Like all other corals, Monoxenia periodically multiply by means of eggs, which are formed either in the walls of the radiating partitions or septa, or along the free edges. These are ejected through the oral opening. As a rule, the polyps are either male or female; but in stock-forming species individuals of the two sexes are often mixed. Monoxenia may be taken as the simplest type of the regularly radiate polyps; in all the different organs being repeated in regular rings round a central axis; the mouth also is circular. From this interesting account, drawn by Haeckel, of a simple polyp, it will be at once seen what kind of radiate animal it is that builds up the coral reefs. “No garden on earth can match the gardens of the sea that circle the northern part of Australia. As the tide ebbs in azure sunset, coral-reefs peer out symmetrically arranged in beds and intersected by emerald pathways coursing through corals of all hues and tints fathoms deep in the channels.”
In a growing polyp-stock the individuals usually remain in organic connection; that is to say, each first provides for itself and then shares its superfluity with others, sometimes by means of a continuous reticulated system of canals perforating the calcareous substance which often separates the members of one stock from another. The whole colony may thus be physiologically one creature with many mouths. There are others that remain single, as the inverted pyramidal-looking bodies, Fungidæ, commonly called “Sea-mushrooms,” found in great variety. The colour of the polypidom is white, of a flattened round shape, made up of thin plates or scales, imbedded in a translucent jelly-like substance, and within is concealed a polyp; the footstalk, by means of which the animal is attached to the rock, is of a calcareous nature (Fig. 352, No. 1).
Hexactinia (six-rayed polyps) are not limited to six rays, as the name given them may seem to imply; they are, in fact, very numerous in some of the largest and most gorgeous of the sea-anemones. All are distinguished by their solitary manner of life, their size, and their vivid and variedly beautiful colouring. The endoderm is firm, and when the animal withdraws its tentacles and shuts its body substance in, there is some difficulty in penetrating to the interior. It does not, however, secrete a calcareous skeleton inside or out, as do the true coral polyps. Among the Hexactinia the sea-anemone (Fig. 352) takes the first place.
These beautifully coloured creatures are, for the most part, found attached to the spot selected by the larvæ; a few species bore into the sand with the posterior part of the body, or build a sheath, which they inhabit. They are voracious feeders, and devour large pieces of flesh, and even mussel and oysters, sucking them in by means of their long grasping tentacles. Well-fed anemones change their skin frequently, during which process they remain closely retracted; the shed skin forms a loose girdle around the base. Actinia bellis not infrequently attach themselves to the shells of crabs and whelks, and are thus carried to pastures new.
On account of the ease with which anemones are kept in captivity, their mode of reproduction can be closely observed. With but few exceptions they develop from eggs, and in the course of a few weeks are hatched into ciliated infusorial larvæ, presenting most curious and exquisite representations of jugs and jars, with cover lids (as seen in Fig. 353, Actinia effœta). These evince the handiwork of a master hand in the ceramic art. They are, however, of so translucent a nature as to permit of the internal structure being seen to consist of nerves and vessels, and which are rendered more apparent by staining. These settle down in a week or ten days, and then shed their cilia, the first tentacle appearing during the process of attachment.
In some species the young Actiniæ are seen to pass through their whole development within the body cavity of the parent. Most anemones are provided with several circles of more or less cylindrical tentacles, and there are a few specially beautiful species which, besides tentacles of the usual form, have, either within or without the ordinary circle of tentacles, lobed or leaf-like tactile and seizing organs. These belong to the family of the beautiful Crambactis of the Red Sea. Below these grasping tentacles comes a circle of thicker arms unlike the former, being spindle shaped. All the tentacles of the sea-anemones are hollow with a fine aperture at the tip, through which, on closing rapidly, it is seen to expel a jet of water.
True Corals.--It will have been noticed in the foregoing remarks that in the soft body-division of the Hexactinia there are both single individuals and colonies joined together to form stocks. The same diversity in this respect will be found among corals proper, with this difference, that the skeleton-forming polyps, by combining, build up substantial structures in the most secure and advantageous positions. Now it so happens that all the corals found about our coasts are generally small and solitary dwellers, one of the best known of which is the scarlet crisp coral, Flabellum, and is characterised by the slit-like form of the mouth. Viewed sideways it resembles a small fan fastened along the edges, and just inside a row of fully developed tentacles is seen protruding. An interesting form of budding occurs in these corals: the buds fall off, and in this budding condition the coral might pass, and indeed has been described as a different species of Flabellum. The colour of the coral is a beautifully transparent red. Remarkable as the solitary corals are, they are surpassed both in number and in form by those which form compound stocks, that is to say, in which the buds do not fall off, but go on building up coral islands and barrier reefs in the warmer seas. Some very few typical forms only are given in the group accompanying, shown in Fig. 358.
A different kind of stock is developed in a number of forms, some producing many buds, as in the Madrepores, in which selected polyps spring up above the rest, their sides also becoming covered with small buds, each one of which is a living, feeding, coral animal surrounded by a crown of tentacles. These Madrepores play a very important part in the building up of coral reefs.
Another massive coral, the Astroides calycularis, has a different mode of growth, the tubes not being fused together. When seen standing out these yellowish-red polyps have been mistaken for small anemones. The larvæ of this coral leave the egg while still in the large chambered body cavity of the parent, where they swim about for a time, till they escape through the mouth. They are worm-like in form, and swim by means of cilia, which are thicker at the foremost end. The mouth first appears after leaving the parent, but as they soon become exhausted by the effort they assume a contracted form, and attach themselves, as do anemones, by pressing the thicker end of the body against a rock, the whole contracting into a thick round disc, while longitudinal furrows become visible at the upper part where the mouth sinks in. At the end of these furrows twelve tentacles appear. The accompanying illustration shows the various stages through which the larvæ pass in rapid succession (Fig. 354); at the same time it has already commenced to secrete its calcareous skeleton. This is not formed as a connected whole but from a number of separate centres of secretion formed between the polyp and the substance to which it has attached itself, and which become gradually fused into a perfect skeleton. A section of the polyp at this stage forms an interesting microscopical object.
The so-called eight-rayed corals consist of the one genus Tubipora, the members of which are few in number and not varied in form (Fig. 358, No. 10). In the structure, however, of skeletons they are unique among extant corals. Each individual secretes a smooth-walled tube without calcification of the vertical septa. These tubes, like the pipes of an organ, stand almost parallel, and are united to form a stock by means of transverse platforms. The formation of buds does not appear to take place in this family.
Another of the eight-rayed corals is Gorgoniidæ. These are permanently fixed to the spot on which they are found, and form a bush-like growth, giving no idea of the living coral, as it rises in graceful branching colonies, in deep water, and represents a portion of Gorgonia nobilis with polyps expanded (Figs. 344 and 358, No. 9).
Other corals present numerous other departures from the types we have been considering, but so far modified in form as that of the Sea-pen, Veretillum (Fig. 355), the stock part of which is surrounded by polyps continued down a portion of the cylindrical stalk. The best known of the species is Pennatula phosphorea of the Mediterranean.
Pennatulidæ.--This family derives its name from penna, a quill. Their spicula also resemble a penholder in appearance, shown in Fig. 358, No. 3. The polyps are without colour, provided with eight rather long retractile tentacula, beautifully ciliated on the inner aspect with two series of short processes, and strengthened by these crystalline spicula, a row being carried up the stalk, together with a series of ciliated processes. The mouth, occupying the centre of the tentacula, is somewhat angular. The ova lie between the membranous part of the pinnæ; these are globular, of a yellowish colour, and by pressure can be made to pass through the mouth. Dr. Grant wrote:--“A more singular and beautiful spectacle could scarcely be conceived than that of a deep purple Pennatula phosphorea, with all its delicate transparent polyps expanded and emitting their usual brilliant phosphorescent light, sailing through the still and dark abyss, by the regular and synchronous pulsations of the minute fringed arms of the polyps.”
The spicula are seen to be a continuous series of cones fitting into each other.
Bryozoa, Moss-animals.
The exact position in which the Bryozoa, or moss-animals, should be placed in the animal kingdom has not been finally determined. They were at one time associated with corals; then with sponges; but, on further acquaintance, it became evident that they did not belong to either. Naturalists also claimed them as Rotifers and Ciliata, but this claim met with no better reception. Since they appear to have no settled classification, there can be no objection to linking them once more to corals, as they apparently resemble these animals by always living in colonies, the individual members of which are joined in a number of different ways to form stocks, the individuals themselves, however, being very much smaller than those of corals proper. The advantage is that the structure of the Bryozoans can be more readily studied, as many of them live in transparent chambers or cells, the walls of which, although somewhat firmly agglutinated together, are flexible enough to fold up, as the animals instantly withdraw their bodies and close up the top on the slightest alarm (Fig. 356).
The general structure of the Bryozoan individual, figured attached by its footstalk to a stem of wood, consists of a mouth at the anterior part of the body opening into a muscular pharynx in the alimentary canal, together occupying a considerable amount of space. The terminal portion turns upon itself towards the oral opening, its chief attachment being a short strand of tissue termed the funiculus (shown in Fig. 358, No. 11). In all adults two masses of cells are found attached to the wall of the chamber; the upper yields the eggs, within the lower the male elements are developed. Moss-animals are hermaphrodite, fertilisation being effected by the two elements mingling together in the body fluid. These are the essential points in the structure of the whole seventeen hundred species. Among the larger colonies a number of fresh-water genera are found attached to the roots and branches of aquatic plants, most of which, however, are inconspicuous. The beauty of these minute bodies can only be seen under the microscope. Many consist of delicate branching growths, the Sea-mats (Flustra), for instance; others again appear as attractive lace corals, between the open meshes of which multitudes of minute apertures crowned with tentacles are displayed. The several individuals of the genus Lepralia are arranged in rows, and further distinguished by the animals being developed only on one side of the stock. The marvellous variety of forms presented by these small animals is in a measure determined by the particular manner of their buddings. The greater number of fresh-water moss-animals belong to the order Phylactolæmata, so called because the mouth is provided with a tongue-shaped lid. The crown of tentacles is furnished with rows of cilia, and is horseshoe-shaped, the whole being surrounded at its base by an integument forming a kind of cup, which is either soft or horny. Those belonging to the wandering types (Cristatella, Plate IV., Nos. 95-98) form flattened elliptical colonies, some of which creep or move about on a kind of foot. A nervous system pervades the mass of polyps, while in each separate polyp a nerve ganglion is seen to be situated between the œsophagus and the posterior part of the alimentary canal. The colony nerve system regulates the movements of the stock.
There are many beautifully formed freshwater polyps deserving of more than a passing notice, as the slender Coryne (Coryne stauridia), found adhering to the footstalk of a Rhodymenia (Fig. 359), about which it creeps in the form of a white thread. On placing both under the microscope, the thread-like body of the little animal appears cylindrical and tubular, perfectly transparent, and permeated by a central core, apparently cellular in texture, hollow, and within which a rather slow circulation of globules is perceived. The parent Coryne sends off numerous branches, the terminal head of which is oblong, cylindrical, and at the extreme end there are arranged four tentacles, long and slender, each being furnished with a nodular head. A magnified view of one detached is shown erect (Fig. 359, No. 2). This polyp is much infested by parasites, vorticella growing on it in immense numbers, forming aggregated clusters here and there, individuals of the parasitic colony adhering to each other, and projecting outwards in every direction.
Alcyonella, another fresh-water polyp, is found in the autumn of the year in all the London Docks adhering to pieces of floating timber. A. stagnorum partakes of the character of a sponge rather than that of a polyp. It is usually found in gelatinous colonies, and when stood aside for a short time these put forth a number of ciliated tentacles (shown in Fig. 360, magnified 100 diameters).
The ova contained within the sac, and viewed by transmitted light, appear as opaque spheres surrounded by a thin transparent margin; these increase in thickness as the ova is developed, and such of the ova as lie in contact seem to unite and form a statoblast. A rapid current in the water around each animal, drawing with it loose particles and floating animalcules, is seen moving with some velocity as in other ciliated bodies; and a zone of very minute vibrating cilia surrounds the transparent margin of each tentacle.
Dr. Percival Wright discovered on the western coast of Ireland a new genus of Alcyonidæ, which he named after the well-known naturalist Harte, Hartea elegans (Plate IV., No. 86). This polyp is solitary, the body cylindrical, and fixed by its base to the rock; it has eight ciliated tentacles, which are knobbed at their base and most freely displayed. It is a very beautiful polyzoon of a clear white colour, and when fully expanded stands three-quarters of an inch high.
Lophopus crystallinus (Plate IV., No. 98) displays beautiful plumes of tentacles arranged in a double horseshoe-shaped series. When first observed these polyps resemble in many respects masses of the water snail ova, for which they are often mistaken. On placing these jelly-like masses into a glass trough with some of the clear water taken from the stream in which they are found, delicate tubes are seen to cautiously protrude, and the beautiful fringes of cilia are quickly brought into play. The organisation of L. crystallinus is simple, although it is provided with organs of digestion, circulation, respiration, and generation. The nervous[70] and muscular systems are well developed. This polyp increases both by budding and by ova, both of which conditions are shown in Plate IV., No. 98. The ova are enclosed in the transparent case of the parent. In Lophopus and some other fresh-water genera, Cristatella, Plumatella, and Alcyonella, the neural margin of the Lophopore is extended into two triangular arms, giving it the appearance of a deep crescent.
Another family presents a contrast: there is no lid to the mouth, and the tentacles are arranged in a circle on a disc. An important rise in organisation is found in the Gymnolæmata, especially in the lip-mouthed forms; the individuals belonging to this order vary in structure and fulfil different physiological functions. There are structures known as zoæcia, stolons, avicularia, vibracula, and ovicells, some of which are merely modified individuals. The zoæcia are the normal individuals of the colony, fully developed for most of the functions of life; the stolons have a much humbler function, but are indispensable--they are the root-like outgrowths of the stock, and serve for attaching the colony to foreign objects. The most remarkable are those known as avicularia, so called because they resemble the head of a bird. This process acts as a pair of forceps, the large upper blade of which is very like the skull and upper jaw of a bird, and the smaller lower blade (like the lower jaw) constantly opens and shuts by means of a complicated arrangement of muscles (shown in Fig. 361). These avicularia are movably attached by short muscles to the neck, and are found near the entrance to a zoæcium. They turn from side to side, snapping in all directions, catching at every particle of food that may come near; at length the morsel is drawn into the mouth by the cilia on the tentacles. From this very peculiar structure the Chilostomata were originally named bird’s-head corallines, then specifically shepherd’s-purse corallines, Notamia bursaria. Equally interesting, again, are the vibracula, long thread-like structures, attached by short footstalks. These keep up a constant whip-like motion, the object of which is not quite clear. The ovicells, or egg receptacles, are found at the lower ends of the zoæcia in the form of shields, helmets, or vesicles. In Plate IV., Nos. 95 and 96, a front and edge view of the statoblast is shown highly magnified.
Another sub-order consists of the Cyclostomata, or round-mouthed Bryozoans, of which the Tubulipora is the typical form. The stocks are cup-shaped incrustations, the individuals radiating outwards, as in Plate IV., No. 92. Tubularia dumortierii is a very interesting form, the germinal bodies, statoblasts, being formed as cell masses on the strand, or funiculus, which also maintains the stomach in its place. They are round or oval in shape, and brown or yellow in colour, and consist of two valves fitted one upon the other like watch glasses, as shown in No. 96. A number of other statoblasts are shown, Nos. 97, 98, and 99. The edge running round No. 95 is seen to have barbed tips; the ring itself contains small air chambers, and is termed the swimming belt. It is, in fact, a perfect hydrostatic apparatus, giving support to the winter buds or statoblasts on the surface of the water. The barbed hooks apparently act as anchors, and by their means they catch on at points suitable for their development during the coming spring. As soon as the time comes, the two halves split apart and the germinal mass emerges forth. Out of these winter buds and statoblasts asexually produced individuals arise, which reproduce themselves sexually, their descendants again yielding winter germs. In short, an alternation of generations is a continually recurring process.
Brachiopoda.--Here again we have to do with an enigmatical class of arm-footed animals, of which the Lamp-shells may be regarded as typical. These have remained unaltered from the earliest geological epochs. Brachiopods are divided into two orders: those having shells without hinges, and those with shells hinged together. On the whole they possess less interest for the microscopists than many other animals, except in their earliest developmental stages of existence.
One of the most interesting of the hinge-class group, living chiefly near the shores of the warmer seas, is the Lingulidæ. The valves are almost exactly similar, but are not hinged together, and have no processes for the support of the thick fleshy spiral arms of the animals. In L. pyramidata, found around the Philippine Islands (Fig. 362), the stalk is nine times longer than the body. The animal does not attach itself by this, but moves about like a worm, making tubes out of sand, into which it can withdraw itself and disappear. The cilia at the mantle edge form a fine sieve, thus preventing foreign particles from entering the gills. Its internal structure possesses points of interest, and the parasitic growths covering the cartilaginous structure, miscalled a shell, are curious, and excite the attention of the naturalist.
Another bivalve so unlike a crustacean, among which it has been placed, I may venture to describe among Lamp-shells. I refer to the barnacle (Lepas) generally met with covering the bottoms of ships. These, as in the former genus, are more interesting to the microscopist in the early stage of existence, and also for the curious parasites known to infest them. The barnacle protrudes through its two valves six pairs of slender, bristly, two-branched filamentous limbs, which keep up a constant sweeping motion, and whereby it secures its supply of food (Fig. 363). When first hatched the young are in the Nauplius stage, being furnished with a median eye and three pairs of flagellated appendages. After enjoying a free life the larva moults and passes into a second stage, in which with its two eyes and compressed carapace (shown in Fig. 364) it so nearly resembles a Daphnia. Before these thoracic appendages entirely disappear they first change places, and then each is seen to be provided with a sucker; by this means the larva fixes itself to its permanent resting-place, while a cement gland pours out a secretion that glues it firmly to the point of attachment chosen. These Cirripedes are not true parasites, inasmuch as they do not extract nourishment from the body to which they are attached.
One species, the Proteolepas, is in the adult stage a maggot-like, limbless, shell-less animal found living within the mantle chamber of other members of the same order, while the root-headed Cirripedes (Peltogaster curvatus, as Fig. 364, No. 1) live parasitically upon higher crustaceans.
Echinodermata.--This sub-kingdom includes the star-fishes, stone-lilies, sea-urchins, feather-stars, and sea-cucumbers, some of which have been already alluded to, and are so well known that they need no lengthy description, while of the fossil sea-urchins of our chalk formations, the Pentremites and Crinoids, whose silicious remains are so abundant and so familiar to naturalists and geologists, but little remains to be said. They are chiefly interesting to the microscopist from their calcareous and silicious appendages, known as spicula. In the sea-urchin, brittle-star, or feather-star, the outer body surface consists almost wholly of a deposit of calcium carbonate, combined in the form of little plates built up into a rigid “test,” whereas in the star-fish it usually forms a kind of scaffolding, between the layers of which there stretches a firm leathery skin. Among the sea-cucumbers, the living specimens of which present extraordinary variations both in form and character, the deposit consists chiefly of small spicules which grate when the skin is cut with a knife. If a thin section of the skin is examined under the microscope, the spicules are seen to be profusely distributed in the middle layer. The same deposit takes place in the stalked column of a crinoid and in sea-urchins (Echinodermata), which has tended to preserve them in the fossilised state. Fig. 365 is selected as exhibiting to perfection the Medusa-headed Pentacrinoid. This echinoderm differs in two characters: first, its microscopic structure is that of a meshwork deposited in the spaces of a network of soft tissue; secondly, that each element, whether a spicule or a plate, is, despite its trellised structure, deposited around regular lines of crystallisation (shown in Plate IV., Nos. 89 and 90). Owing to these characteristics the minutest portion of an echinoderm skeleton is readily recognised, even when fossilised, under the microscope. Even the species of the sea-cucumber can be determined by the shape of their spicules.
Another noticeable feature in the radiate structure is that in many cases it gives to the animal a star-shape, to which the names of star-fish and brittle-star are given (see Plate IV., No. 91, and Plate XVII., f and n). The ordinary five-rayed star-fish is found everywhere around the English coasts. This constant arrangement of organs holds good in the majority of the echinoderms; it can be detected in the Holothurians, where, beside the feathery tentacles of the head, rows of shorter sucker-like processes will be found, which in some instances extend the whole length of the body, the fixed number of rows being also five in their internal organs. Hence these animals were formerly grouped under Radiata. But if a sea-cucumber or sea-urchin be dissected, a marked distinction will be found between them, in one portion of the organism in particular: the intestine is shut off from the rest of the body-cavity, often coiling round inside. Examine a star-fish or sea-urchin on the under-surface of the rays, and, passing in five bands from top to bottom, a number of small cylindrical processes are seen gently waving about; these lie in two rows with a clear space between them, and are termed in consequence ambulacrum. They end in sucker-like discs, which enable the animal to attach itself, or pull itself against strong currents.
Just one other special feature should be noticed: radial canals pass along under the ambulacra, and join a ring-canal around the mouth, well supplied by nerve cells.
Crinoids (stone-lilies), on the other hand, are formed of a series of flat rings, pierced through by a narrow canal. The ossicles, as they are termed, are joined by ligaments passing through their solid substance and endowed with muscular power; the central part serves for the passage of blood-vessels, and is surrounded by a sheath of nervous tissue that controls the movements of the stem, the latter being encrusted by a number of fine rootlets. The stems possess a limited power of bending. In the words of Professor Agassiz, “The stem itself passes slowly from a rigid vertical attitude to a curved or even a drooping position; the cirri move more rapidly than the arms, and the animal uses them as hooks to catch hold of objects, and on account of their sharp extremities they are well adapted to retain their hold of prey.” The rosy-feather star-fish is often found clinging to a tube of the Sabella worm; the food of crinoids consists of foraminifera, diatoms, and the larvæ of crustaceans. There are so many curious features in connection with the Echinodermata that my readers may with advantage consult “The Challenger Reports” and Warne’s “Natural History” on other points of interest.
Holothuroidea (sea-cucumbers) are elongated slug-like creatures, the skin being in structure similar to that of the slug, with a comparatively small amount of calcareous matter. Usually this occurs in small spicules, which assume very definite shapes, as the anchors of Synapta (Plate IV., No. 87, and in Fig. 355). There are also rings of calcareous plates around the gullet, five of which have the same relation to the radial water-vessels as the auricles round the jaws of a sea-urchin, and which likewise serve for the attachment of muscles. These plates are seen in Plate VIII., Nos. 171 and 172, as they appear coloured by selenite films under polarised light. Around the mouth in Cucumaria is a fringe of branched tentacles connected with the water-vascular ring; these appear to be used as a net to intercept floating organisms.
Correlated with the star-fishes is a small family based on the character of their pincer-like organs, called pedicellariæ, on the surface of the test (shown in Plate IV., Nos. 93 and 94, magnified × 25). Movable spines cover the surface of these echinoderms, varying in size from minute bristle-like structures to long rods. The pedicellariæ are, it is believed, derived from the smaller spines, and two of them are united at the base by muscles, slightly curved, and made to approach each other at their extremities. There is a gradual modification of this type through the whole series. Many uses have been assigned to them, as the holding of food, as they have been seen to hold to the fronds of seaweed and keep them steady until the spines and tube feet can be brought into action. The inner surface of the pedicellariæ are known to be the most sensitive, and the blades close on the minutest object touching the inner surface. Beside these peculiar bodies the surface of the skin has small tubular processes, and tubular feet with suckers at the end. At the extremity of each arm is a single tube-foot with an impaired tentacle, and above this again is a small eye coloured by red pigment.
Passing by many other points of interest in the Echinoidæ, the spines are seen to be attached to the test or shell by a ball and socket joint and well-arranged muscles, whereby the spines can be moved in any direction. The tubercles, however, do not cover the whole test, but are disposed chiefly in five broad zones extending from one pole to another. When a transverse section of a spine is examined by a medium power it is seen to be made up of a series of concentric and radiating layers (shown in Plate XVIII., Nos. 1 and 2), the centre being occupied by reticulated structure and structureless spots arranged at equal distances; these may be termed ribs or pillars. Passing towards the margin are other rows conveying the impression of a beautiful indented reticulated tissue. Many of the spines present no structure, while others exhibit a series of concentric rings of successive growth, which strongly remind one of the medullary rays of plants. When a vertical section of a spine is submitted to examination, it is seen to be composed of cones placed one above the other, the outer margin of each cone being formed by the series of pillars. In certain species of Echinus the number of cones is very considerable, while in others there are seldom more than one or two to be found; from these, transverse sections may when made show no concentric rings, only the external row of pillars.
The skeleton of echinoderms contains but a small amount of organic matter, as will be seen on dissolving out the calcareous portion in dilute nitric or hydrochloric acids. The residuum structure will appear to be meshes or areolæ, bounded by a substance having a fibrous appearance, intermingled with granulous matter; in fact, it bears a close resemblance to the areolar tissue of higher animals, and the test may be considered as formed, not by the consolidation of the cells of the ectoderm, as in the mollusc, but by the calcification of the fibro-areolar tissue of the endoderm. This calcification of a simple fibrous tissue by the deposit of a mineral substance, not in the meshes of areolæ but in intimate union with the organic basis, is a condition of much interest to the physiologist; it presents an example of a process which seems to have an important share in the formation and growth of bone, namely, in the progressive calcification of the fibrous tissue of the periosteum membrane covering of the bone.
The development of the sea-urchin from the fertilised egg first divides and then sub-divides, and in a short time the embryo issues forth with a small tuft of cilia, by means of which it swims off freely. The larvæ, in its full development, measures about one millimetre in diameter, and is a curious and remarkable creature.
=The sub-kingdom Mollusca= comprises some fifty thousand species, and fresh forms are being constantly discovered, the number of the aquatic genera being more than double that of the terrestrial species, for it matters not to what depth of ocean the dredge is let down, some new form is certain to be gathered. The Challenger expedition has enriched our knowledge of the deep-sea fauna to an enormous extent; so much so, that fifty volumes have already been published descriptive of animals brought to the surface. Nevertheless, we are told that the great coast lines of South America, Africa, Asia, and parts of Australia have been but imperfectly explored for smaller kinds of Mollusca.
Molluscs are soft-bodied, cold-blooded animals, without any internal skeleton, but this is compensated for by the external hardened shell, which at once serves the purpose of bones, and is a means of defence. These bodies are not divided into segments like those of worms and insects, but are enveloped in a muscular covering or skin, termed the mantle, the special function of which in most species is the formation and secretion of the shell. The foot, which serves the double purpose of locomotion and burrowing in the sand or rock, is an organ particularly characteristic of most molluscs. There are many departures from this rule, as, for instance, in the group Chitonidæ, where the shell takes the form of a series of eight adjacent plates; and in another, the Pholadidæ, there are one or more accessory pieces in addition to the two principal valves. Some are bivalved, others univalved, and concealed beneath the skin. All shells are mainly composed of carbonate of lime, with a small admixture of animal matter. Their microscopic examination reveals a great diversity of structure, as we shall presently see, and they are accordingly termed porcellaneous, nacreous, glassy, horny, and fibrous. Most molluscs have the power of repairing injuries to their shells; many exhibit an outer coat of animal matter, termed the peristracum, the special function of which is to preserve the shell from atmospheric and chemical action of the carbonic acid in the water in which they dwell.
The shells of gastropods are enlarged with the growth of the mollusc by the addition of fresh layers to the margin. In some species the periodic formation of spines occurs; a typical case will be found among Muricidæ. The varied colours of shells are due to glands situated on the margin of the mantle, and beneath the peristracum; occasionally the inner layer of porcellaneous shells is of a different colour to the outer, as, for example, in the helmet-shells (Cassis), much used by carvers of shell cameos. Light and warmth, as in the vegetable kingdom, are the great factors in the production of brilliant colours. In cold climates land snails bury themselves in winter time in the ground or beneath decaying vegetable matter, and in hot seasons they close up the aperture of the shells with a temporary lid, called an epiphragm. These exhibit great tenacity of life, as, for instance, in the Egyptian desert-snail, Helix desertorum. The reproductive system is in all cases effected by means of eggs. The ova are usually enclosed in capsules, and deposited in masses, and the number of eggs contained in the squid and the whelk have been stated to be thirty or forty thousand. The ova of molluscs may be gradually developed into the adult, or there may be a free-swimming ciliated larval stage, or a special larval form, as in the fresh-water mussel. Most are provided with a more or less distinct head; both cephalopods and gastropods are furnished with eyes. In land snails these are found placed on projecting stalks. In most cases the utility of molluscs far outweighs the injury occasioned by a few species, as, for instance, the Teredo, and the burrowing habits of the Pholas and Saxicava, compact marble having been found bored through by them.
Mr. J. Robertson wrote me in 1866:--“Having, while residing here (Brighton), opportunities of studying the Pholas dactylus, I have endeavoured during the last six months to discover how this mollusc makes its hole or crypt in the chalk--by a chemical solvent? by absorption? by ciliary currents? or by rotatory motions? My observations, dissections, and experiments set at rest controversy on this point. Between twenty and thirty of these creatures have been at work in lumps of chalk in sea water in a finger glass and a pan, at my window for the last three months. The Pholas dactylus makes its hole by grating the chalk with its rasp-like valves, licking it up when pulverised with its foot, forcing it up through its principal or branchial siphon, and squirting it out in oblong nodules. The crypt protects the Pholas from Conferveæ, often found growing parasitically not only outside the shell but even within the lips of the valves, thus preventing the action of the siphons. In the foot there is a spring, or style, which when removed is found to possess great elasticity, and this seems to be the mainspring of the motion of the Pholas.”
I must pass by many groups and orders to more aberrant types, represented by the naked-gilled orders, Opisthobranchiata and Nudibranchiata. These gastropods constitute a large sub-order of extremely beautiful molluscs, remarkable in shape, and often brilliant in colour. The distinguishing character of these typical forms consists in the peculiar nature and situation of their breathing organs, which are exposed on the back of the animal or around the anterior part, and are not protected by the mantle. But the situation is varied, and the gills are sometimes placed on each side of the body, respiration being effected by the ciliated surface of the whole. For these and other reasons they have been placed in four groups. Nudibranchs are found in all parts of the world, and are most abundant in depths where the choicest seaweeds and corallines abound. Their fecundity is very great, as many as sixty thousand eggs being deposited by a single female at one time. They are eaten as a luxury where they most abound.
In the Opisthobranchs the branched veins as well as the auricle are placed behind the ventricle of the heart. They differ from Nudibranchs inasmuch as they are usually furnished with a pair of tentacles and labial palpi, or an expansion of the skin like the veil of the larval form. To clearly understand the character of the internal organisation of these curious animals, the longitudinal section given in Fig. 368 must be consulted: p is the foot; a the mouth, covered above with the veil-like expansion, over which are the tentacles, c; the branchial veins, v, carry the blood to the gills, from which it flows into the heart at h. This disposition is the opposite of that which characterises the Prosobranchus. Another anatomical peculiarity, which may here be referred to, is the direct communication of the system of blood vessels with the surrounding medium; a characteristic common to most other molluscs, and on which depends the changeable external appearance of the animal. In the illustration of Pleurobranchus here given, g indicates the opening of the duct which conveys water direct to the blood, and through which the blood vessels permeate the back and foot. Like the holes in the sponges, it can be filled or emptied at the will of the animal.
Although this, in the main, is the principle of the circulation in most of this order, one branch possesses no special breathing organs, respiration being carried on throughout the naked skin of the body.
With regard to the Nudibranchiata, the group having the most symmetrical form is the extensive family Dorididæ, characterised by differences in the branchiæ, the relative proportion of the mantle to the foot, and variations in the radula and jaws. The general aspect of the genus Doris, although drawn on a small scale, is represented in Plate XVII., Fig. b. The whole sub-order of Nudibranchs has become more generally known and admired since the publication of Alder and Hancock’s monograph with its many attractive coloured illustrations.
These gastropods can be kept alive for some time in a small aquarium if the precaution is observed of often changing the water and adding a little fresh seaweed. Numerous curious microscopic forms of life may be found adhering to them.
Tunicata.--The most remarkable group of animals belonging to this sub-order are the Ascidians. They derive their name from the test or tunic, a membranous consistence, in which they dwell, and which often includes calcareous spicules. The test has two orifices, within which is the mantle. Few microscopic spectacles are more interesting than the circulation along this network of muslin-like fabric, and that of the ciliary movement by which the fluid is kept moving. In the transparent species, as Clavelina and Perophora, the ciliary movement is seen to greater advantage. The animals are found adhering to the broad fronds of fuci near low water-mark. They thrive in tanks, and multiply both by fission and budding. Two species are figured in Plate XVII., Figs. i and k, the zooids of which were found arranged in clusters, as represented.
Aplysiidæ (sea-hares), so called on account of a slight resemblance to a crouching hare. The body form is elongated with a partially developed neck and head, oral and dorsal tentacles, and furnished beneath the mantle with a shelly plate to protect the branchiæ. The mouth is provided with horny jaws, and the gizzard is armed with spines, to prepare the food for digestion. The side lobes are thin and large, and are either folded over the back or used in swimming. Fig. 369 is a reduced drawing of A. dipilans.
The Pectinibranchs are known as violet sea-snails, Ianthinidæ and Scalariidæ. The radula consists of numerous rows of pointed teeth arranged in cross series, forming an angle in the middle. There is no central or rachidian tooth, and they have thin trochiform shells adapted for a pelagic life. They are mostly of a violet colour, from which they derive their name, the colour being more vivid on the underside, which is turned up towards the light when the animal is swimming near the surface of the sea (Fig. 370).
The most interesting feature in connection with these oceanic snails is the curious float which they construct to support their egg-capsules. It is a gelatinous raft, in fact, enclosing air-bubbles, which is attached to the foot, the egg capsules being suspended from its under-surface. They are unable to sink so long as they are in connection with their floats, and are therefore often cast on shore during storms, and furnish an endless series of microscopic specimens. The violet snails feed on various kinds of jelly-fish, and occur in shoals.
Pond Snails.--The three families, Limnœidæ, Physidæ, and Chilinidæ, form a special group of the pulminate, sessile-eyed fresh-water snails. The larger family of these belongs to the genus Limnœa, having a compressed and triangular head with two tentacles and eyes placed at their inner base. They are prolific and gregarious, and their ova are enclosed in transparent gelatinous capsules, deposited in continuous series, and firmly glued to submerged stems and leaves of aquatic plants. L. stagnalis is common in all ponds, marshes and slow-running rivers of Great Britain.
One of the species, L. trancatula, is the host of the liver-fluke so fatal to sheep. The fluke parasite passes one stage of its existence in the intestine of the pond snail.
Each ova-sac of Limnœa contains from fifty to sixty ova (represented in Fig. 371, at a). If examined with a low power soon after the eggs are deposited, they appear to consist simply of a pellucid protoplasmic substance. In about twenty-four hours a very minute yellowish spot, the nucleus, is discovered near the cell-wall. In another twenty-four hours the nucleus referred to is seen to have assumed a somewhat deeper colour and to contain within it a minute spot--a nucleolus.
On the fourth day the nucleus has changed its position, and is enlarged to double the size; a slightly magnified view is seen at b. On a closer examination a tranverse fissure is seen; this on the eighth day divides the small mass as at c, and the outer wall is thickened. The embryo becomes detached from the side of the cell, and moves with a rotatory motion around the interior; the direction of this motion is from the right to the left, and is always increased when sunlight falls upon it. The increase is gradual up to the eighteenth day, when the changes are more distinctly visible, and the ova crowd down to the mouth of the ova-sac, as at d. By employing a higher magnifying power a minute black spec, the future eye (e) and tentacles of the snail, is quite visible. Upon closely observing it, a fringe of cilia is noticed in motion near the edge of the shell. It is now apparent that the rotatory motion first observed must have been in a great measure due to this; and the current kept up in the fluid contents of the cell by the ciliary fringes. For days after the young animal has escaped from the egg, this ciliary motion is carried on, not alone by the fringe surrounding the mouth, but by cilia entirely surrounding the tentacles themselves, which whips up a supply of nourishment, and at the same time aeration of the blood is effected. From the twenty-sixth to the twenty-eighth day it appears actively engaged near the side of the egg, using force to break through the cell-wall, which at length it succeeds in accomplishing; leaving its shell in the ova-sac, and immediately attaching itself to the side of the glass its ciliary action recommences, and it appears to have advanced a stage, as at f. It is still some months before the embryo grows to the perfect form, Fig. 372; the animal is here shown with its sucker-like foot adhering closely to the glass of the aquarium. A single snail will deposit from two to three of these ova-sacs a week, producing, in the course of six weeks or two months, from 900 to 1,000 young.
The shell itself is deposited in minute cells, which take up a circular position around the axis; on its under-surface a hyaline membrane is secreted. The integument expands, and at various points an internal colouring-matter or pigment is deposited. The increase of the animal goes on until the expanded foot is formed, the outer edge of which is rounded off and turned over by condensed tissue in the form of a twisted wire; this encloses a network of small vessels filled with a fluid in constant and rapid motion. The course of the blood or fluid, as it passes from the heart, may be traced through the larger branches to the respiratory organs, consisting of branchial-fringes placed near the mouth; the blood may also be seen returning through other vessels. The heart, a strong muscular apparatus, is pear-shaped, and enclosed within a pericardium or extremely thin and pellucid enveloping membrane. The heart is seen to be furnished with muscular bands of considerable strength, the action of which appears like the alternate to-and-fro motion occasioned by drawing out a band of indiarubber, and which, although so minute, are clearly analogous to the muscular fibres of the mammal heart; it beats or contracts at the rate of about sixty times a minute, and is placed rather far back in the body, towards the axis of the shell. The nervous system is made up of ganglia, or nervous centres, and distributed throughout the various portions of the body.
The singular arrangement of the eye cannot be omitted; it appears at an early stage of life to be within the tentacle, and consequently capable of being retracted into it. In the adult animal the eye is situated at the base of the tentacle; and although it can be protruded at pleasure for a short distance, it seems to depend much upon the tentacle for protection as a coverlid--it invariably draws down the tentacle over the eye when that organ needs protection. The eye itself is pyriform, somewhat resembling the round figure of the human eye-ball, with its optic-nerve attached. In colour it is very dark, having a central pupillary-opening for the admission of light. The tentacle, which is cylindrical in the young animal, becomes flat and triangular in shape in the adult. The tentacles serve in some respect to distinguish species. In Limnœa they are, as I have said, compressed and triangular, with the eyes at their inner base. In Physa they are cylindrical and slender and without lateral mantle lobes. The development of the lingual membrane is delayed; consequently, the young animal does not early take to a vegetable sustenance: in place of teeth it has two rows of cilia, as before stated, which drop off when the teeth are fully formed. The lingual band bearing the teeth, or the “tongue,” as it is termed, consists of several rows of cutting spines, pointed with silica.
It is a fact of some interest, physiologically, to know that if the young animal is kept in fresh water alone, without vegetable matter of any kind, it retains its cilia, and arrest of development follows, and it more slowly acquires gastric teeth, and attains to perfection in form or size. If, at the same time, it is confined within a narrow cell or space, it grows only to such a size as will enable it to move about freely; thus it is made to adapt itself to the necessities of a restricted state of existence. Some young animals in a narrow glass-cell, at the end of six months, were alive and well; the cilia were seen to be retained around the tentacles in constant activity, whilst other animals of the same brood and age, placed in a situation favourable to growth, attained their full size, and produced young, which grew in three weeks to the size of their elder relations.[71]
My experimental investigations were further extended to the development of the lingual membrane, or teeth, of Gastropoda, as well as the jaw and radula. In Limnœa, the teeth when fully developed resemble those of Helix; that is to say, in the fully grown animal are found several rows or bands of similar teeth, with simple obtuse cusps and a much suppressed central tooth. In the young snail a high power of the microscope is required to make them out. The dental band, however, in most Mollusca is disposed in longitudinal series, but varies a good deal in this respect, as will be seen on reference to my several papers, with illustrations of upwards of a hundred different species, published in “Linnæan Transactions” of 1866, and in the “Microscopical Society’s Transactions” of 1868. By way of example I may say, in the Pulmonata the lingual band usually consists of a single median row, the laterals on each side being broad and similar. But in many other groups the teeth are arranged in three, five, or seven dissimilar series. Taking Nerita as a type, the broad teeth on each side of the median are termed laterals; and the numerous small teeth on the outside of the band, known as the pleuræ, are termed uncini.
Since the investigations of Lovén into the lingual dentition of the Mollusca, various observers have studied the subject, with great advantage to our knowledge of the affinities of these animals. That these investigations have proved of value is shown by the light which has been shed on the true position of many species. When once we have ascertained the homology of a genus, whose relations were otherwise somewhat doubtful, it is surprising how other characteristics, even of the shell, probably misunderstood before, concur to bear out the affinities indicated by the lingual band. These tooth-bearing membranes, armed with sharp cutting points, admirably adapted for the division of the food on which they feed, are most of them beautiful objects for the microscope.
The two ends of each longitudinal row of teeth are connected with muscles attached to the upper and lower surfaces of cartilaginous cushions; the alternate contractions and extensions of the muscles cause the bands of teeth to work backwards and forwards, after the fashion of a chain-saw, or rather of a rasp, upon any substance to which it is applied, and the resulting wear and tear of the anterior teeth are made good by a development of new teeth in the secreting sac in which the hinder end of the band is lodged. Besides the chain-saw-like motion of the band the lingual membrane has a kind of licking or scraping action as a whole. With the constant growth of the band new teeth are developed, when the teeth on the extreme portion of the band differ much in size and form from those in the median line.
As I have shown in the papers already referred to, that as each row is a repetition of the first, the arrangement of teeth admits of easy representation by a numerical formula, in which, when the uncini are very numerous, they are indicated by the sign ∞ (infinity), and the others by the proper figure. Thus, ∞ · 5 · 1 · 5 · ∞, which, in the genus Trochus, signifies that each row consists of one median, flanked on both sides by five lateral teeth, and these again by a large number of uncini. When only three areas are found, the outer ones must be considered the pleuræ, inasmuch as there is frequently a manifest division in the membrane between them and the lateral areas.
Most of the Cephalopod molluscs are provided with well-developed teeth, and they are, as we know, carnivorous. The teeth of the cuttle-fish, Sepia officinalis (Plate V., No. 111), resemble those of the Pteropoda, and have the same formula, 3 · 1 · 3. Sepia are also furnished with a retractile proboscis, and a prehensile spiny collar, apparently for the purpose of seizing and holding prey while the teeth are tearing it to pieces. In the squid Loligo (Plate V., No. 113) the median teeth are broad at the base, approach the tricuspid form with a prolonged acute central cusp, while the uncini are much prolonged and slightly curved. The lingual band increases in breadth towards the base, sometimes to twice that of the anterior portion. This band, mounted dry, forms an attractive object for black-ground illumination.
In another family, that of the rock-limpet, Patella radiata, the lingual band (Plate V., No. 116) well serves to distinguish it from the better-known common limpet. It is furnished with a remarkable long ribbon, studded by numerous rows of strong dark-brown tricuspid teeth. The lingual membrane when not in use lies folded up in the abdominal cavity. The teeth of Acmæa are somewhat differently arranged (Plate V., No. 117); their formula is 3 · 1 · 3.
Testacella maugei, belonging to Pulmonifera, is slug-like in appearance, and subterranean in its habits, chiefly feeding on earth-worms. During winter and in dry weather it forms a kind of cocoon, and thus completely encloses itself in an opaque white mantle; in this way it protects itself from frost and cold. Its lingual membrane is large, and covered with about fifty rows of divergent teeth, gradually diminishing in size towards the median row; each tooth is barbed and pointed, broader towards the base, and with an articulating nipple set in the basement membrane. A few rows are represented slightly magnified (Plate V., No. 121). Their formula is 0 0 · 1 · 0 0.
The boat-shell, Cymba olla, belonging to the Velutinidæ, formula 0 · 1 · 0, or 1 · 1 · 1. The lingual band (Plate V., No. 118) is narrow and ribbon-like in its appearance, with numerous trident-shaped teeth set on a strong muscular membrane. The end of the band and its connection with the muscles at the extremity of the cartilaginous cushion is shown in the drawing. The blueish appearance is produced by a selenite film and polarised light. In Scapander ligniarius the band (Plate V., No. 119) is also narrow, but the teeth are bold and of extraordinary size; their formula is 1 · 0 · 1. This mollusc is said to be eyeless. Pleurobranchus plumula belongs to the same family; its teeth are simple, recurved, and convex, and arranged in numerous divergent rows, the medians of which are largest. The mandible (Plate V., No. 122) presents an exceedingly pretty tesselated appearance, and the numerous divergent rows of teeth are tricuspid.
The velvety-shell, Velutina lævigata, formula 3 · 1 · 3. The teeth (Plate V., No. 108) are small and fine; medians recurved, with a series of delicate denticulations on either side of the central cusp, which is much prolonged: 1st laterals, denticulate, with outer cusp prolonged; 2nd and 3rd laterals, simple curved or hooked-shaped. The mandible (No. 109), divided in the centre, forms two plates of divergent denticulations.
The ear-shell, Haliotis tuberculatus, is a well-known beautiful shell, much used for ornamental purposes. The lingual band (Plate V., No. 114), is well developed. The medians are flattened-out, recurved obtuse teeth; 1st laterals, trapezoidal or beam-like; uncini numerous, about sixty, denticulate, the few first pairs prolonged into strong pointed cusps.
The top-shell, Turbo marmoratus. After the outer layer of shell is removed, it presents a delicate pearly appearance. Its lingual band (No. 123) closely resembles Trochus; it is long and narrow, the median teeth are broadest, with five recurved laterals, and numerous rows of uncini, slender and hooked. A single row only is represented in the plate.
Cyclotus translucidus, a family of operculate land-shells, belongs to the Cyclostomatidæ. The teeth shown in No. 110, formula 3 · 1 · 3, are arranged in slightly divergent rows on a narrow band; they are more or less subquadrate, recurved, with their central cusps prolonged. Cistula catenata, one of the family Cyclophoridæ; its band (No. 115) formula, 2 · 1 · 2. Its teeth resemble those of Littorina. The lingual band of Cyclostomatidæ points out a near alliance to the Trochidæ; but this question can only be determined by an examination of several species, when it may, perhaps, be decided to give them rank as a sub-order. They are numerous enough; the West Indian islands alone furnish 200 species.
The length of the lingual band, and number of rows of teeth borne on it, vary greatly in different species. But it is among the Pulmonifera we meet with the most astonishing instances of large numbers of teeth. Limax maximus possesses 26,800, distributed through 180 rows of 160 each, the individual teeth measuring only one 10,000th of an inch. Helix pomatia has 21,000, and its comparatively dwarfed congener, H. absoluta, no less than 15,000.
Structure of the Shell of Mollusca.--In my opening sketch of the sub-order Mollusca an idea may have been gathered of the general character of the shell covering of these animals. The simplest form of shell occurs in the rudimentary oval plate of the common slug, Limax rufus. It is embedded in the shield situated at the back, near the head of the animal. In the Chitons, a small but singular group of molluscs allied to the univalve limpets, we have an ovoid shell, made up of eight segments, or movable plates, which give them a resemblance to enormous woodlice. These have been regarded as forming a transition series--a link between one division and the other. The shell in by far the greater portion of all the molluscs is developed from cells that in process of growth have become hardened by the deposition of calcareous matter in the interior. This earthy matter consists principally of calcium carbonate deposited in a crystalline state; and in certain shells, as in that of the oyster (Plate XVIII., Fig. 8), from the animal cell not having sufficiently controlled the mode of deposition of the earth particles, they have assumed the form of perfect rhomboidal crystals.[72]
The shell of the wing-shells, Pinna ingens (Plate XVIII., No. 7), is composed of hexagonal cells, filled with partially translucent calcareous matter, the outer layer of which can be split up into prism-like columns. Figs. 3 and 6 are horizontal sections of the Haliotis splendens, with stellate pigment in a portion of the section, and wavy lines, as in the dentine of the human tooth, and of Terebratulata rubicuna, showing radiating perforations. Nos. 4 and 5, sections of the shell of a crab, show pigment granules beneath the articular layer and the general hexagonal structure of the next layer.
Some difference of opinion has been expressed with regard to the formation of pearls, but it is now generally understood to be a diseased condition. Pearls are matured on a nucleus, consisting of the same matter as that from which the new layers of shell proceed at the edge of the mussel or oyster. The finest kinds are formed in the body of the animal, or originate in the pearly-looking part of the shell. It is from the size, roundness, and brilliancy of pearls that their value is estimated.
The microscope discloses a difference in the structure of pearls: those having a prismatic cellular structure have a brown horny nucleus, surrounded by small imperfectly-formed prismatic cells; there is also a ring of horny matter, followed by other prisms, and so on, as represented in Fig. 374; and all transverse sections of pearls from oysters show the same successive rings of growth or deposit.
In a segment of a transverse section of a small purple pearl from a species of Mytilus (Fig. 375), all trace of prismatic structure has disappeared, and only a series of fine curved or radiating lines is seen. This pearl consists of a beautiful purple-coloured series of regular laminæ, many of which have a series of concentric zones, and are of a yellow tint. The most beautiful sections for microscopic examination are obtained from Scotch pearls.
Preparation of the Teeth and Shell of Mollusca for Microscopical Examination.--The method of preparing lingual membranes of Mollusca is as follows: Under a dissecting microscope, and with a large bull’s eye lens, cut open and expose to view the floor of the mouth; pin back the cut edges throughout its length, and work out the dental band with knife and forceps. The band being detached, place it in a watch-glass, and boil in caustic potash solution for a few minutes. Having by this process freed the tongue from its integuments, remove it, wash it well, and place it for a short time in a dilute acid solution, either acetic or hydrochloric. Wash it well and float it upon a slide; with a fine sable brush open it out flat, and remove whatever dirt or fibre may be adhering to it. Lastly, place it in weak spirit and water, and there let it remain for a few days before mounting in formalin. Canada balsam renders them rather too pellucid, and the finer teeth are thereby lost.
The preparation of shell structure must be proceeded with with some amount of care and caution, or the delicate reticulated network membrane will be destroyed. If any acid solvent be used to remove the calcareous structure it should be much diluted, so that the action may proceed slowly rather than hastily. In the young hermit-crab, for example, where the calcareous and membranous portions of the shell are continuous, and the calcium carbonate in a relatively small proportion, a strong acid solution would entirely destroy the specimen. In the case of nacreous shells the process of cutting and grinding must also be proceeded with with some amount of caution. The operation should be examined as the process proceeds, and under polarised light. Sections of shell structure are usually mounted in Canada balsam. Under the heading Technique much useful information on this and kindred subjects will be found in the “Journal of the Royal Microscopical Society.”
Annulosa, Worms, and Entozoa.
The Annulosa of Huxley embraces the lowest grade of articulated animals, most of which are now grouped with Metazoa, while some writers place them in a sub-kingdom Vermes. It appears to me then only possible to describe this heterogeneous group of worm-like animals among those which resemble each other in certain negative features, but not possessing any of the distinctive characters of those previously described. There are numerous species among Entozoa, every one of which is of the highest interest to mankind in general, and to animal life as a whole. To these I shall devote some attention, from the wide-spread importance attached to them. They are characterised by having a soft absorbent body with little or no colour, in consequence of being excluded from light, living within the bodies of animals and absorbing their vital juices, thereby inflicting a large amount of injury and death upon the whole vertebrate kingdom. They bear in this respect a close analogy to parasitic Fungi in the nature of their destructive action upon plant life, which I have fully discussed in a previous chapter.
The relations which obtain between parasites and their hosts are in all respects conditioned by their natural history; and without a detailed knowledge of the organisation, the development, and the mode of life of the different species, it is impossible to determine the nature and extent of the pathological conditions to which they give rise, and at the same time find means of protection against guests in every way so unwelcome.
The nutritive system of the entozoa must be regarded as in the lowest state of development, yet there are some among them of a higher grade, as will be seen as we proceed. All are remarkable alike for their vast productiveness and for their peculiar metamorphoses. For example, the greater number of the Tænia begin their lives as sexless, encysted larvæ, and on entering their final abode, segments are successively added, until the worm has finally reached the adult stage. Again, the tapeworm of the cat has its origin in the encysted larvæ found in the livers of the mouse and rat. Another species of entozoa inhabit the stomach of the stickle-back, and only attain their perfect form in the stomachs of aquatic birds that feed exclusively on fish. Another infests the mantle of pond-snails, and through their agency, the embryos pass into the stomach of sheep.
An almost endless number of similar transformations take place in other genera. The simplest form among internal parasites is the Gregarinæ, formerly grouped among Protozoa. They consist of a simple limiting membrane, with a mass of granular matter enclosed and surrounding a nucleus (Plate III., No. 53). These parasites pass through a crystoid stage in the body of one of the lower animals, usually the earthworm, Lumbricus agricola. In the more mature organism an envelope, differentiated from the protoplasm within, can be made out (No. 54); this affords an indication of greater differentiation in the subjacent layer of protoplasm. An anterior portion is in many cases separated by a constriction from the cylindrical or band-like body (No. 56). Gregarinæ multiply when encysted, and divide into a multitude of minute pseudo-navicula, so named from their resemblance in shape to a well-known form of Diatomaceæ. When a young pseudo-navicule escapes it behaves somewhat like an amœba, and if perchance it is swallowed by an appropriate host, it develops at once into the higher stage. The various forms are represented in Plate III., Nos. 53--61. Miescher, in 1843, described suchlike bodies, taken from the muscles of a mouse. A good account of specimens obtained from the muscles of a pig was published by the late Mr. Rainey in the “Philosophical Transactions,” 1857. He regarded them as cestoid entozoa. They have been described under a variety of names, as worm-nodules, egg-sacs, eggs of the fluke, young measles, &c. M. Lieberkühn carefully traced the pseudo-naviculæ after leaving the perivisceral cavity of the earth-worm; he found large numbers of small corpuscles, exhibiting amœba-like movements, as well as pseudo-naviculæ, containing granules, formed in an encysted Gregarinæ. He imagines that these latter bodies burst, and that their contained granules develop into the amœbiform bodies which subsequently become Gregarinæ.
Professor Ray Lankester made a careful examination of more than a hundred worms for the purpose of studying these questions, but he succeeded in arriving at no other conclusion than that certain forms may be the by-products of encysted Gregarinæ. The G. lumbricus is one of those forms which are unilocular. The vesicle is not always very distinct, and is sometimes altogether absent; occasionally it contains no granules, sometimes several, one of which is generally nucleated. In other of these cysts a number of nucleated cells may be seen developing from the enclosed Gregarina, which gradually become fused together and broken up, until the entire mass is converted into nucleated bodies, often seen in different stages of development, assuming the form of a double cone, as that presented by certain species of Diatomaceæ. At length the cyst contains nothing but pseudo-naviculæ, sometimes enclosing granules; these gradually disappear, and finally the cyst bursts. Encystation seems to take place much more rarely among the bilocular forms of Gregarinæ than in the unilocular species found in the earthworm and other Annelids.[73]
Dr. J. Leidy published in the “Transactions of the Philadelphia Society,” 1853, the results of his examinations of several new species of Gregarinæ. He described a double membrane “within the parietal tunic of the posterior sac, this being transparent, colourless, and marked by a most beautiful set of exceedingly regular parallel longitudinal lines.”
Professor R. Leuckart is the latest writer on the parasites of animals, and to him we are indebted for a more systematic account of the whole group, and their life-history, than to any previous investigator. I can only attempt to give a mere outline of the developmental stages of a few typical forms of parasites, commencing with the cystic tapeworm, Tænia. These worms are ribbon-like in appearance, and are divided throughout the greater part of their length into segments, and their usual habitation is the intestinal cavity of vertebrate animals. The anterior extremity of a tænoid worm is usually called the head, and bears the organ by which the animal attaches itself to the mucous membrane of the creature which it infests. These organs are either suckers, or hooks, or both conjoined. In Tænia, four suckers are combined with a circlet of hooks, disposed around a median terminal prominence. The embryo passes through certain stages of development--viz., four forms or changes: but the embryo itself is very peculiar, consisting of an oval non-ciliated mass, provided with six hooks, three upon each side of the middle line. Tænia are found enclosed in various situations besides that of the alimentary canal: the eye, the brain, the muscular tissues, the liver, &c., of animals. The following cystic worms are usually included in this genera, Cysticercus Anthocephalus, Cœnurus, and T. Echinococcus. Plate IV., No. 100, shows an adult specimen of the latter with rostellum suckers, and three successive segments, the last of which is the ova sac. The water-vascular system is represented coloured by carmine. This parasite infests the human body as frequently as many other species. My accurately-drawn figure is copied from Cobbold’s “Introduction to the Study of Entozoa.”
Cysticercus fasciolaris is developed within the liver of white mice; Cysticercus cellulosæ in the muscles of the pig; hence we have the diseased state of pork familiarly known as “measly pork.” Should a lamb become infested with Tænia the final transformation will be different; within a fortnight symptoms of a disease known as “staggers” manifest themselves, and in the course of a few weeks the Cœnurus cerebralis will be developed within the brain. Von Siebold pointed out the bearing of this fact upon the important practical problem of the prevention of “staggers.” Others belonging to the same class of parasites are quite as remarkable in their preference for the alimentary canal of fishes. The Echinorhynchus is developed in the intestinal canal of the flounder, Triænophorus nodulus in the liver of the salmon. Thus, by careful and repeated observation with the microscope, a close connection is found to exist between the cystic and cestoid entozoa.
The Echinococcus (Plate IV., No. 101) infests the human liver. These parasites are always found in cysts, and in closed cavities in the interior of the body. They are united in fours by a very short stalk or pedicle, common to the whole. By an increase of magnification the contents of a cyst present the several structures represented in Fig. 376.
Echinorhynchus, or spiny-headed threadworms, constitute a group of entozoa which undergo a metamorphosis hardly, perhaps, less remarkable than that known to take place in other Nematode worms. Leuckart instituted, in 1861, a series of experiments with the ova of Echinorhynchus proteus found parasitic upon the Gammarus pulex. The ova of E. proteus resemble in form and structure those of allied species. They are of a fusiform shape, surrounded with two membranes, an external of a more albuminous nature, and an internal chitinous one. When the eggs reach the intestine the outer of these membranes is absent, being in fact digested, while the inner remains intact until ruptured by the embryo.
The typical Threadworm belonging to the order Nematoidea infest the intestines of children, and are a source of much suffering. The egg is elliptical, and contains a mass of granular protoplasm, the external wall of which soon becomes marked out into a layer of cells. The mouth of the worm appears as a depression at the end of the blunt head. When the muscular system and alimentary canal are developed the embryo hatches out, some few of which are free living forms; most of them lead a parasitic life. Their reproduction is enormous, representing thousands of eggs and embryos.
Of the non-parasitic species of thread-worm, the common vinegar eel, Anguillula,[74] affords an example. This is found in polluted water, bog-moss, and moist earth, as well as in vinegar; also in the alimentary canal of the pond-snail, the frog, fish, &c. Another species is met with in the ears of wheat affected with a blight termed the “cockle”; another, the A. glutinis, in sour paste. If grains of the affected wheat are soaked in water for an hour or two before they are cut open, the so called “eels” will be found. The paste-eel makes its appearance spontaneously just as the pasty mass is turning sour; the means of securing a supply for microscopical examination consists in allowing a portion of the paste in which they show themselves to dry up, and laying it by for stock; if at any time a portion of this is introduced into a little fresh-made paste, and the whole kept warm and moist for a few hours, it will be found to swarm with these wriggling little worms. A small portion of paste spread over the face of a Coddington lens is a ready way of viewing them.
Trichina spiralis.--One of the smallest and most dangerous of all human internal parasites is T. spiralis, since it finds its way into the muscles throughout the human body. The young animal presents the form of a spirally-coiled worm in the interior of a minute oval-shaped cyst (Plate IV., No. 104), a mere speck scarcely visible to the naked eye. In the muscular structure it resembles a small millet seed, somewhat calcareous in composition. The history of the development of Trichina in the human muscle is briefly that in a few hours after the ingestion of infected pork, Trichina, disengaged from the muscle, will be found in the stomach: hence they pass into the small intestine, where they are further developed. Continuing their migrations, they penetrate far into the interior of the primitive muscular fasciculi, where they will be found, in about three days after ingestion, in considerable numbers, and so far developed that the young entozoa have almost attained a size equal to that of the full-grown Trichina (Plate IV., No. 105). They quickly advance into the interior of the muscular fasciculi, where they live and multiply in continuous series, while the surrounding structures as well as the muscular tissue undergo a process of histolysis. The destructive nature of the parasite is very great.
The number of progeny produced by one female may amount to several thousands, and as soon as they leave the egg they either penetrate through the blood-vessels, or are carried on by the circulation, and ultimately become lodged in the muscles situated in the most distant parts of the body. Here, as already explained, they become encysted.
Professor Virchow draws the following conclusions:--“1. The ingestion of pig’s flesh, fresh or badly dressed, containing Trichinæ, is attended with the greatest danger, and may prove the proximate cause of death. 2. The Trichinæ maintain their living properties in decomposed flesh; they resist immersion in water for weeks together, and when encysted may, without injury to their vitality, be plunged in a sufficiently dilute solution of chromic acid for at least ten days. 3. On the contrary, they perish and are deprived of all noxious influence in ham which has been well smoked, kept a sufficient length of time, and then well boiled before it is consumed.”
A more minute Filarian worm has been detected in the human blood-vessels, known as Filaria sanguinis hominis. This worm carries on its work of destruction throughout the night; during the day it remains perfectly passive. It increases rapidly, and produces swellings of the glandular structures of the body, somewhat after the nature of those characteristic of the Bombay plague, with a slight difference, that after death the swellings are seen to be due to the vast accumulations of the Filaria sanguinis blocking the blood-vessels. The accompanying Fig. 377 shows a similar infiltration of monads in the blood of rats dying of plague in Bombay.
Trematode Worms.--In the order Trematoda, to which the fluke belongs, the body is unsegmented, and to the naked eye smooth throughout, with a blood circulatory system, and two suctorial discs at the hinder end. There is a distinct digestive canal, usually forked, furnished with only one aperture, the mouth. The excretory organs open out as in tape worms, and the male and female organs co-exist in the same individual.
The Fluke (shown in Plate IV., No. 103) is cone-shaped, and is the Amphistome conicum of Rudolphi. This parasite is common in oxen, sheep, and deer, and it has also been found in the Dorcas antelope. It invariably takes up its abode in the first stomach, or rumen, attaching itself to the papillated folds of the mucous membrane. In the full-grown, adult stage, it rarely exceeds half an inch in length. It is certainly one of the most remarkable in form and organisation of any of the internal parasites.
The larger fluke (Fasciola hepatica) often attains to an inch or more in size. It is not only of frequent occurrence in all varieties of grazing cattle, but has likewise been found in the horse, the ass, and also in the hare and rabbit and other animals. Its occurrence in man has been recorded by more than one observer. The oral sucker forming the mouth leads to a short œsophagus, which very soon divides into two primary stomachal or intestinal trunks, the latter in their turn sending off branches; the whole together forming that attractive dendritic system of vessels so often compared to plant-venation. This remarkably-formed digestive apparatus is represented in Plate IV., Nos. 106 and 107, Fasciola gigantea of Cobbold, and should be contrasted with the somewhat similarly racemose character of the water-vascular system. Let it be expressly noted, however, that in the digestive system the majority of the tubes branch out in a direction obliquely downwards, whereas those of the vascular system slope obliquely upwards. A further comparison of the disposition of these two systems of structure, with the same systems figured and described as characteristic of the Amphistoma, will at once serve to demonstrate the important differences which subsist between the several members of the two genera, if we turn to the consideration of the habits of Fasciola hepatica, which, in so far as they relate to excitation of the liver disease in sheep, acquire the highest practical importance. Intelligent cattle-breeders, agriculturists, and veterinarians have all along observed that the rot, as this disease is commonly called, is particularly prevalent after long-continued wet weather, and more especially so if there have been a succession of wet seasons; and from this circumstance they have very naturally inferred that the humidity of the atmosphere, coupled with a moist condition of the soil, forms the sole cause of the malady. Co-ordinating with these facts, it has likewise been noticed that the flocks grazing in low pastures and marshy districts are much more liable to the invasion of this endemic disease than are those pasturing on higher and drier grounds; a noteworthy exception occurring in the case of those flocks feeding in the salt-water marshes on our eastern shores. Plate IV., No. 106, Fasciola gigantea: the anterior surface is exposed to display oral and ventral suckers, and the dendriform digestive apparatus injected with ultra-marine; No. 107 shows the dorsal aspect of the specimen and the multiramose character of the water-vascular system, the vessels being injected with vermilion.
In their larval condition the Amphistoma live in or upon the body of the pond-snail. This we infer from the circumstance that the larvæ, or cercariæ, of a closely-allied species, the Amphistoma subclavatum, are known to infest the alimentary canal of frogs and newts, and have also been found on the body of the Planorbis by myself. The cercariæ larvæ are taken, it is believed, by the sheep and the cattle while drinking. The earliest embryotic stage in which I have found the embryo fluke is represented at Fig. 378, No. 1. In the year 1854, whilst observing the habits of Limnœa and other water-snails, I brought home specimens from the ornamental water in the Botanic Gardens; upon these were discovered thousands of minute thread-like worms, subsequently met with on other embryos, and at first taken to be simple infusorial animals, but upon placing them in a glass vessel these minute bodies were observed to detach themselves and commence a free-swimming existence. A fringe of cilia was seen to surround the flask-shaped body (No. 1).
The study of these embryos throws a flood of light upon the obscure history of Cercariæ. After a short period of wandering, their embryos fasten upon the water-snail, and compel it to act as a wet-nurse, and prepare it for a further and higher stage of life. The earliest condition in which I have discovered them concealed about the body of the water-snail is shown at No. 2; in appearance, a simple elongated sac filled with ova or germs, and which in a short time develop into the caudate worms already spoken of; their tails gradually attaining to the length of the mature embryos, Nos. 3 and 4, the latter being a full-grown Cercaria ephemera.
Diesing described no less than twelve species of Cercariæ, some of the most curious of which live on the puddle-snail, in colonies of thousands. All throw off their tails at the moment of changing into a fluke. On placing some Cercaria furcata (Nos. 6 and 7) under the microscope, they were seen to plunge about in frantic attempts to escape from confinement. Suddenly I saw them shed their tails and their bodies divide into two parts, each half swimming about as vigorously as before, quite indifferent as to the severance, and apparently dying from exhaustion. Those represented in Nos. 6 and 7 have a highly-organised nervous system, forming a continuous circuit throughout the body and tail. The mouth is furnished with a sucker and hooklets, which can be projected out some distance, while a digestive apparatus and ventral opening or sucker can be differentiated. The tail is bifurcated and articulated with the body by a sort of ball-and-socket joint, and when broken off, the convexity of one part is seen to accurately fit into the concavity of the other; it lashes about this appendage with considerable dexterity, rarely attaching itself to any of the small aquatic plants.[75]
There is yet another Filarian worm, a pest to the poultry-yard, the Gape-worm, Sclerostoma syngamus. This parasite is widely distributed, and is invested with special interest, since it produces disease, and kills annually thousands of young chickens, pheasants, partridges, and many of the larger kinds of wild birds. The worms find their way into the windpipe or tracheæ, through the drinking water, while in the embryotic or cercarian stage of existence, and their increase is so rapid, the birds quickly die of suffocation. The female gape-worm often attains to a considerable size, and when full grown resembles the well-known mud-worm of the Thames (Gordius aquaticus). She measures full six-eighths of an inch in length, while the male only measures one-eighth. So insignificantly small is he that the female carries him about tucked into a side pocket. The ova sac occupies a considerable portion of the internal body space, and is always found loaded with eggs in all stages of development, numbering some five hundred or a thousand. In shape these are ovoid. On cutting open the windpipe of chicken and partridges, I have found their tracheæ literally swarming with the gape-worm.[76]
A remarkable form of the Trematode worm is Bilharzia hæmatobra of Cobbold, Distomia hæmatobium of other authors (Plate IV., No. 102). This genus of fluke, discovered by Dr. Bilharz in the human portal system of blood vessels, gives rise to a very serious state of disease among the Egyptians. So common is the occurrence of this worm, that this physician expressed his belief that half the grown-up population of Egypt suffer from it. Griesinger conjectures that the young of the parasite exist in the waters of the Nile, and in the fish which abound. Dr. Cobbold thinks “it more probable that the larvæ, in the form of cercariæ, rediæ, and sporocysts, will be found in certain gasteropod mollusca proper to the locality.” The anatomy of this fluke is fully described by Küchenmeister in his book on parasites, by Leuckart,[77] and by Cobbold. The eggs and embryos of Bilharzia are peculiar in possessing the power of altering their forms in both stages of life; and it is more than probable that the embryo form has been mistaken for some extraordinary form of ciliated infusorial animal, its movements being quick and lively. We cannot fail to notice the curious form of the male animal, and, unlike the Filarian previously described, it is he who carries the female about and feeds her. The whip-like appendage seen in the figure is a portion of the body of the female. The disease produced by this parasite is said to be more virulent in the summer months, probably owing to the greater abundance of cercarian larvæ at this period of the year.
There are also double parasitic worms, which may be described as a sub-order of Trematoda, differing very much from those previously described. These live on the gills of several species of fresh-water fish, the gudgeon and minnow, for instance. Among them is a most remarkable creature well deserving the name of Diplozoum paradoxum which has been bestowed upon it. It consists of two complete mature similar halves, each possessing every attribute of a perfect animal (a). Each of the pointed front ends has a mouth aperture, and close to it two small sucking discs; while each individual has a separate intestine, consisting of a medium tube and innumerable side-branches. At the hinder end of the body are two suckers sunk in a depression, and protected by four hard buckle-shaped organs. The eggs are elongated, and provided at one end with a fine thread-like appendage (b). In this egg the young (c)--which at the time of hatching is only about one-hundredth of an inch--takes about a fortnight to develop. It is covered with cilia, has two eyes and two suckers; after quitting the egg, the larvæ are very lively and restless in their movements, gliding about and then swimming off with rapidity. If unable to find the fish into which they are destined to live, they grow feeble and perish, but if successful they grow into the Diporpa (d), which is flattened and lancet-shaped, and bears a small sucking disc on the under surface and a conical excrescence on the back. After living in this state for some weeks, and gaining nourishment by sucking the blood from the fish’s gills, the worms begin to join together in pairs, one specimen seizing the conical excrescence of another by its ventrical sucker; then, by a truly acrobatic feat, the second twists itself to the dorsal excrescence of the first, and in this state an inseparable fusion takes place between the suckers and the excrescences involved in the adhesion.[78]
In the group Vermes, the more highly-organised Annelida must be included. These, for the most part, live either in fresh or salt water. The Annelids are various, while the Planaria, a genus of Turbellaria, are very common in pools, and resemble minute leeches; their motion is continuous and gliding, and they are always found crawling over the surfaces of aquatic plants and animals, both in fresh and salt water. The body has the flattened sole-like shape of the Trematode entozoa (Fig. 378, No. 9), the mouth is surrounded by a circular sucker; this is applied to the surface of the plant from which the animal draws its nourishment; it is also furnished with a rather long proboscis, which is probably employed for a similar purpose.
Planariæ multiply by eggs, and by spontaneous fissuration in a transverse direction, each segment becoming a perfect animal. Professor Agassiz believes that the infusorial animals, Paramæcium and Kolpoda, are simply planarian larvæ.
Hirudinidæ, the leech tribe, are usually believed to form a link between the Annelida on the one hand, and the Trematoda on the other; their affinities place them closer with the latter than the former. Although deprived of the characteristic setæ of the Annelida, and exhibiting no sectional divisions, they are provided with a sucker-like mouth possessed by Trematoda, but they present no resemblance to them in their reproductive organs. On the other hand, in the arrangement of the nervous system and in their vascular system, the Hirudinidæ resemble Annelida. The head in most of the Annelida is distinctly marked, and furnished with eyes, tentacles, mouth, and teeth, and in some instances with auditory vesicles, containing otolithes. The nervous system consists of a series of ganglia running along the ventral portion of the animal, and communicating with a central mass of brain.
Hirudina medicinalis puts forth a claim for special attention on the ground of services rendered to mankind. The whole of the family live by sucking the blood of other animals; and for this purpose the mouth of the leech is furnished with a number of strong horny teeth, by which they cut through the skin. In the common leech three rows of teeth exist, arranged in a triangular, or rather triradiate form, a structure that accounts for the peculiar appearance of leech bites. The most interesting part of the anatomy of the leech to microscopists is certainly the structure of the mouth (Fig. 380). This is a muscular dilatable orifice, within which three beautiful little semi-circular saws are situated, arranged so that their edges meet in the centre. It is by means of these saws that the leech makes the incisions whence blood is to be procured, an operation which is performed in the following manner. No sooner is the sucker firmly fixed to the skin, than the mouth becomes slightly everted, and the edges of the saws are thus made to press upon the tense skin, a sawing movement being at the same time given to each, whereby it is made gradually to pierce the surface, and cut its way to the capillary blood-vessels beneath.
In Clepsinidæ the body is of a leech-like form, but very much narrowed in front, and the mouth is furnished with a prehensile proboscis. These animals live in fresh water, where they may often be seen creeping over aquatic plants. Their prey is the pond-snail.
Tubicola.--The worms belonging to this series of branchiferous Annelida are all marine, and distinguished by their invariable habit of forming a tube or case, within which the soft parts of the animal can be entirely retracted. This tube is usually attached to stones or other submarine bodies. Externally it is composed of various foreign materials, sand, crystalline bodies, and the débris of shells; internally it is lined with a smooth coating of sarcode, sometimes of a harder consistency. The Tubicola generally live in societies, winding their tubes into a mass which often attains a considerable size; only a few are solitary in their habits. They retain their position in their cases by means of tufts of bristles and spines; the latter, in the tubicular Annelids, are usually hooked, so that by applying them to the walls of the case, the animal is enabled to oppose a considerable resistance to any effort made to withdraw it. In the best known family of the order (Sabellia), the branchiæ are placed in the head, and form a circle of plumes, or a tuft of branched organs. The Serpulidæ form irregularly twisted calcareous tubes, and often grow together in large masses, when they secure themselves to shells and similar objects; other species, Terebellidæ, which build their cases of sand and stones, appear to prefer a life of solitude. The best known form is Terebella littoralis.[79] The curious little spiral shells seen upon the fronds of seaweeds are formed by an animal belonging to the Spirorbis.
If the animals be placed in a vessel of sea-water a very pleasing spectacle will soon be witnessed. The top part of the tube is seen to open, and the creature cautiously protrudes a fringe of tentacles; these gradually spread out two beautiful fan-like rows of tentacles, surrounded by cilia of a rich purple or red colour. These serve the double purpose of breathing and feeding organs. When withdrawn from its calcareous case, the soft body is seen to be constructed of a series of rings, with a terminal prehensile foot by which it attaches itself.
Many Annelids are without tubes or cells of any kind, simply burying their bodies in the sand near tidal mark. The Arenicola, lob-worm, is a well-known specimen of the class; its body is so transparent that the circulating fluids can be distinctly seen under a moderate magnifying power. Two kinds of fluids flow through the vessels, one nearly colourless, the other red; the vessels through which the latter circulate are described as blood-vessels.
Not very much interest attaches to the developmental stage of the Annelida. They issue forth from ova, and the embryo so closely resemble ciliated polypes, that competent observers have mistaken them for animals belonging to a lower class; a few hours’ careful watching is sufficient to dispel a belief of the kind, when the embryonic, globular, or shapeless mass is seen to assume a form of segmentation, and soon the various internal organs become more and more developed, eye spots appear, and the young animal arrives at the adult stage of its existence.
Crustacea.
The crustaceans comprise a large assemblage of Arthropods, presenting great diversity of structure. Some of the parasitic species have become so simplified in organisation that they appear to present no relationship with the higher members of the class, yet it is certain that all the species, whether terrestrial or aquatic, belong to the same stock, and may have had origin in the same fundamental plan of structure. Essentially, the body consists of a large number of segments, to each of which is attached a pair of two-branched appendages; the external branch is termed the exopodite and the internal the endopodite. Five segments at the front end of the body unite to form a head, the appendages of the first two being situated in front of the mouth, and performing the office of feelers or antennæ, while those of the remaining three segments are transformed into jaws, the first pair of jaws being the mandibles and the following two pairs the maxillæ. The rest of the appendages are variously modified and to some are attached respiratory organs in the form of gills. Crustaceans are broadly divided from Centipedes, Millipedes, Insects, &c., by the presence of two pairs instead of one pair of antennæ, and by the possession of branchial and not tubular (tracheal) respiratory organs. Arachnida and some other species are again widely separated. The majority of the young on leaving the egg are quite unlike the parent, and only acquires their definite form after undergoing a series of changes. The earliest stage, which has been called the Nauplius, already referred to in connection with the barnacle, is a minute body showing no trace of segmentation, and provided with a single eye, and three pairs of swimming appendages, which become the two pairs of antennæ and the mandibles of the adult. This stage is by no means of invariable occurrence, but is chiefly characteristic of the lowest members, the Entomostraca, and is rare in the higher, Malacostraca. The typical crustaceans are shrimps, crayfish, &c., so familiarly described by Huxley. The zoæa stage of the crab, a minute transparent creature, which undergoes several changes, swims about flapping its long jointed abdomen, like some of the Entomostraca, and the shrimp in particular. The larva of crayfish, the so-called glass-crab, is very peculiar and interesting. The sessile-eyed series, in which the compound eyes are never mounted on a movable stalk, and to which the Isopoda belong, exhibits great diversity of structure as well as of habits and habitat. Some live in fresh water, most are marine, while others live on land and take to a parasitic life.
This genus contains Gnathia, in which the male and female are so dissimilar, that they are frequently referred to as members of two families. In the adult male the mandibles are powerful and prominent, and the head is large, squared, and as wide as the thorax. In the female, on the contrary, the head is curiously small and triangular, without visible mandibles, and the thorax is much dilated. The creatures are about one-sixth of an inch long, and of a greyish colour, and the destruction they bring about is due to their habit of boring into timber below water mark. Fig. 382 represents an enlarged view of the male Gnathia. These crustaceans are vegetarians, and feed on wood. Other members of the group, known as fish lice, are much larger in size, and chiefly infest the cetacea, and bear in addition two large eyes. By means of their powerful fore feet the Cymothordæ attach themselves to both marine and fresh-water fish, showing a preference for the inside of the mouth of their host.
The bar-footed group Copepoda are free living, and the thorax bears four or five swimming feet; the abdomen is without appendages. The best known fresh-water form is Cyclops, the structure of which serves as a type of the order. The body is, as is well known to microscopists, broad in front and tapering behind, being thus, when viewed swimming, pear-shaped in outline. The dorsal elements of the head are fused to form a carapace, which bears a single eye, from which circumstance it derives its name. The eggs are carried by the female in a couple of ova-sacs attached to the last segment of the thorax, and so prolific are these creatures that a female will produce over four thousand million young. The young when hatched is an oval Nauplius, which after two or three moults acquires the adult state. In the family of the Apodidæ we have an equally well-known crustacean, the Branchipus. In the Branchipodidæ the body is also elongated, but there are no appendages to the abdomen, which consists of nine segments, while there are eleven pairs of thoracic appendages. The head shield is not developed backwards, and the large separated eyes are supported on distinct stalks. In the male the second antennæ are converted into claspers. These crustaceans swim upside down (Fig. 383).
=Cladocera= (Daphniadæ of Dr. Baird).--The water-flea (Daphnia pulex) may be taken as the best known example of the order. The body of this little active animal is narrowed in front, and at the posterior end, where the carapace is deeply notched, is the tip of the abdomen bearing the pair of rigid barbed setæ from which the genus takes its name. At the front of the head is a large compound eye and two pairs of branched plumed appendages, antennæ. The first pair of these are small and simple. The jaws consist of the mandibles and the first pair of maxillæ, the second pair of maxillæ being obsolete in the adult. The thorax comprises five segments, each bearing a pair of leaf-like swimming limbs. The abdomen consists of three segments, and is destitute of limbs. The males are usually smaller than the females, and much rarer, being rarely met with before the end of summer.
Eggs are laid both in summer and winter, and are passed into a brood-pouch, separating the upper surface of the thorax from the backward extension of the carapace. Here the summer eggs hatch, but the winter set are enclosed in a kind of capsule developed from the carapace. This capsule, termed the ephippium, is cast off with the next moult of the mother’s integument (a process necessary for the gradual growth of the crustacean), and falling to the bottom of the water, gives exit to the embryos, which hatch in its interior, and the young born from these “ephippial” eggs produce young, which in their turn become mothers. It appears, then, the winter eggs are enclosed in capsules of more than usual hardness to enable them to withstand any degree of cold that might otherwise prove fatal to the parent. Dr. Baird found, on examining ponds that had been again filled up by rain after remaining two months dry, numerous specimens of Daphnia and Cyclops quadricornis in all stages of growth.[80]
We learn also from his investigations that the Daphnia have many enemies. “The larva of the Corethra plumicornis, known to microscopical observers as the skeleton larva, is exceedingly rapacious of Daphnia. Pritchard says they are the choice food of a species of Nais; and Dr. Parnell states that the Lochleven trout owes its superior sweetness and richness of flavour to its food, which consists of small shell-fish and Entomostraca.” These crustaceans abound in fresh and salt water. Artemiæ are formed exclusively in salt water, in salt marshes, and in water highly charged with salt. Myriads of these Entomostraca are found in the salterns at Lymington, in the open tanks or reservoirs where the brine is deposited previous to boiling. A pint of the fluid contains about a quarter of a pound of salt, and this concentrated solution destroys most other marine animals. During the fine days in summer Artemiæ may be observed in immense numbers near the surface of the water, and, as they are frequently of a lively red colour, the water appears tinged with the same hue. The movements of this little animal are peculiar. It swims about on its back, and by means of its tail, its feet being at the same time in constant motion. They are both oviparous and ovoviviparous, according to the season of the year. At certain periods they only lay eggs, while during the hot summer months they produce their young alive. In about fifteen days the eggs are expelled in numbers varying from 50 to 150. As is the case with many of the Entomostraca, the young present a very different appearance from the adult animals; and they are so exactly like the young of Chirocephalus, that with difficulty are they distinguishable one from the other. The ova of other species are furnished with thick capsules, and imbedded in a dark opaque substance, presenting a minutely cellular appearance, and occupying the interspace between the body of the animal and the back of the shell; this is called the ephippium. The shell is often beautifully transparent, sometimes spotted with pigment; it consists of a substance known as chitine, impregnated with a variable amount of calcium carbonate, which produces a copious effervescence on the addition of a small quantity of a strong acid to the water in which the shell is immersed. When boiled, Artemiæ turn red as their congeners, lobsters. Their shells may be said to consist of two valves united at the back, resembling the bivalve shell of a mussel, or simply folded at the back to appear like a bivalve, but are really not so; or they may consist of a number of rings or segments. The body of Cypris presents a reticulated appearance, somewhat resembling cell structure. Entomostraca should be narcotised and prepared for examination under the microscope as directed by Mr. Rousselet at pages 345, 346.
INSECTS’ EGGS, ETC.