IV. Arthropoda--Insecta…

IV. Arthropoda--Insecta

Distinctive Characters of Insects.--The term Insect, although originally and according to the meaning of the word correctly employed in a wide sense to embrace all those articulate creatures in which the body is externally divided into a number of segments, including, of course, flies, butterflies, beetles, bugs, spiders, scorpions, crabs, shrimps, &c., is now by common consent used in a much more restricted sense to apply only to such of these animals as have six walking legs. Insects belong to a class of Arthropoda, and are distinguished by having the head, chest, and abdomen distinctly marked out and separable; by having not more than three pairs of legs in the adult state; by having the legs borne by the thoracic segments only; by having usually two pairs of wings; by the possession of tracheæ, or air-tubes, as respiratory organs; and by being provided with a single pair of antennæ, or feelers. The insect class is one exhibiting uniformity of type and structure. Extreme variations are no doubt seen within certain limits, but these variations are sharply marked off from the groups we have been previously considering. The examination of insects may be pursued according to a defined order, and it will be found that no class of animals will afford the microscopist a more wonderful field of observation and a greater variety of interesting objects than that of the insect tribes.

In the insect, as in the crustacean, the hard parts of the body form an outer and protecting covering, and also serve for the attachment of muscles. The casing, however, in insects is purely of a chitinous, or horny nature, and has in its composition only a trace of calcium carbonate. Each somite, or joint of the body, is usually composed of six pieces; the upper, or dorsal half of each segment is named the tergum, the lower half the sternum, the side pieces pleura, the sternum being further sub-divided into epimeral and espisternal pieces. The body as a whole consists of some twenty segments, of which five or six form the head, the thorax of three joints, while the abdomen may number from nine to eleven. The head segments are united to form apparently a single mass, and the appendages of this region are modified for sensory purposes, and also serve as cutting and masticatory organs. The appendages of the head, examined in order, will be found to consist of eyes, antennæ, or feelers, and organs of the mouth. The antennæ of insects rarely exceed two in number, but these present great variations in form and size. In their simplest form they exist as straight jointed filaments, but in many insects they are forked, in others club-shaped, while in others they mimic forms of vegetation, and for the most part are extremely interesting objects for the microscope.

The principal use of these antennæ is that of organs of touch, but it is quite probable that they may subserve other functions, as of taste or even hearing. The eyes of insects consist of either a pair of ocelli, or of a great number, when they are termed compound eyes, formed of an aggregation of external hexagonal facets and lenses, and nerve filaments, all of which have a distinct connection with the mass of ganglia recognised as the brain, as will be seen in Fig. 384, a section of the eye of a fly. The number of facets varies very greatly in these compound eyes; ants, for example, have fifty facets, flies two thousand or more, and butterflies as many. Dr. Hooke counted seven thousand, and Leuwenhoeck as many as twelve thousand in the eye of a dragon fly. The eyes of some insects are supported on short stalks or pedicles, but these are never movable, as, for example, in Stalk-eyed crustaceans.

The organs of the mouth in insects present a striking homology or similarity in their fundamental structure. Two chief types of mouth are found. The biting or masticatory, as in beetles, includes a labium or upper lip, a pair of mandibles or lower jaws, a pair of lesser jaws or maxillæ, which bear one or two pairs of palpi, and a lower lip or labium, also with palpi. This latter and primitive condition of the labium is seen in Orthropterous insects and some Neuroptera. Other structures occurring in those of the mouth are the ligula, this being sometimes divided, as in bees, into three lobes, of which the two outer are the paraglossæ and the middle process the lingua or tongue. There is a second form of mouth, termed the suctorial. This is seen in Lepidoptera (butterflies), and is adapted for extracting the pollen and juices of flowers, and in which the palpi are greatly developed, and form two hairy pads or cushions, between which the proboscis is coiled up when at rest. Thus we find in the Lepidoptera the same fundamental condition of mouth as in some Coleoptera. In Hymenoptera (wasps and bees), a variety of mouth is found which presents a combination of the masticatory with the suctorial types. The labium and mandibles exist as in the beetle, the maxillæ being developed to form long sheaths protecting the labium, which now takes the form of a tongue. In Hemiptera (bugs and their allies), the mandibles and maxillæ exist as sharp lancets, while the labium forms a protective sheath. In the Diptera (flies, gnats, &c.), the labium undergoes a great development, and forms a very prominent tongue, the other parts of the mouth being developed simply as sheaths to the labium. See Figs. 389 and 390.

The thorax or chest of insects consists of three segments, named from before backwards: the prothorax, mesothorax, and metathorax. The first bears the anterior pair of legs; the mesothorax, the second pair of legs and the first pair of wings; and the metathorax, the third pair of legs and second pair of wings. The last joints of the leg constitute the tarsus or foot-claws. The nervures of the wings are in reality hollow tubes, and are extensions of the spiracles, or respiratory apertures.

The muscles of insects lie concealed beneath the integument; they are not gathered into distinct bundles as in the higher animals, although they exhibit in many cases a striated or striped structure. This is well seen in some of the beetle tribe, the water-beetle in particular. In certain larvæ the muscles are exceedingly complicated. Lyonnet found in the larva of the goat-moth, two hundred and twenty-eight muscles in the head alone, and in the whole body no less than three thousand nine hundred and ninety-three. The muscular power of insects is, relatively to the size of the body, very great. The flea, for instance, leaps two hundred times its own height. There are beetles weighing a few grammes that will escape from a pressure of from twenty to thirty ounces.

Professor Schäfer infers that the structure of the wing-muscles of insects furnishes the key to the comprehension of the more intricate muscular structure of vertebrates. The sarcode element, however, is not made up of a bundle of rods, but of a continuous sarcous element, readily made out by staining with hæmatoxylin. This substance is then seen to be pierced by minute tubular canals, and the longitudinal striation of muscle is due to this canalisation. The whole is connected and enclosed by a membrane of extreme delicacy.

The digestive system of insects varies with their habits and food. In Stylops, bee-parasites, and in young bees living on fluids, the intestine ends in a blind sac. There are three coats of structure throughout the digestive system. The œsophagus or gullet is provided with a crop in flies, bees, and butterflies; a true analogue of the gizzard in birds. There is in some respects a curious likeness between the conformation of the digestive organs of birds and that of insects. No true liver, but salivary glands in the mouth have been made out; the heart lies dorsally, and consists of a pulsating sac divided into compartments, and the fluid flows through it towards the head, whence it circulates freely to other parts of the body. Each trachea is an elastic tube formed of two delicate membranes, between which the spiral filament is coiled up, and is of sufficient density to prevent the collapse of the tube by the movements of the body. These tracheæ are distributed throughout the muscular tissue and the whole of the body. Thus the insect, like the bird, may be said to breathe in every part of the body, and is in this way rendered light and buoyant for flight. The air is admitted to the tracheæ by apertures termed spiracles, which the insect can close at will, and these are distributed to the number of eleven on each side of the body. The nervous system consists of a chain of ganglia or nerve-knots, which unite towards the head to form a single cord, as seen in the section made through the spider (Fig. 409).

The reproduction of all insects takes place by ova, and they are diæceous--that is, have two distinct sexes. In some few instances, as that of Aphides, or plant-lice, we have the peculiar phenomenon of parthenogenesis, the process of reproduction being performed by imperfect wingless females. These bring forth living young ones, which begin to feed the moment they are born, and constitute a viviparous brood; in other cases females lay eggs, and the process proceeds in the ordinary way, and nearly all the year round. The former is provided with a lancet-like beak for piercing and sucking the juices of the leaf, and a pair of curious honey-tubes. Insects generally undergo a transformation or metamorphosis in passing from the egg to the adult stage. While within the egg the body may be seen to become segmented, and in the course of time--in such insects as flies, bees, beetles, and butterflies--issue forth from the egg as larvæ, or caterpillars. This worm-like creature makes for itself an investing case or cocoon, in which it passes into the pupa stage of its existence. Within the pupa case a wonderful transformation takes place; the larval body being literally broken down by the process of histolysis, while its elements are rebuilt and transformed into that of the imago, or perfect insect. In grasshoppers, crickets, dragon-flies, bugs, &c., the metamorphosis is incomplete (hemimetabolic). Some few lower insect forms (lice, spring-tails, &c.) undergo no change of the kind, and in no way differ from the adult except in size. These are termed ametabolic insects. Others again, as the cockchafer and gold beetle, pass three years in the larval stage. Development in all cases is arrested or retarded by cold. Reaumur kept a butterfly pupa for two years in an ice-house, and it exhibited no tendency towards a change until removed to a warm temperature.

From the short natural history of insect life I have endeavoured to sketch out, it will have been surmised that insects offer a wide field of research, and an almost endless number of objects of interest for the microscope. The variety of material is great, and the structure and adaptation of means to an end is of the most fascinating kind. Most cabinets abound in preparations gathered together with some care and mounted with all the skill at the command of the collector, affording, as a rule, as endless an amount of pleasure to the tyro as to the more practised entomologist. It may be surmised, then, that to enter fully into a description of the several parts of insect structure would require a volume[81] of very large bulk, and occupy months and years. I will, therefore, take some points of interest in the structural characteristics of insects, and take them in the order in which they have already been brought to notice. The head, eyes, and other appendages of these insects we are more or less acquainted with.

We will take for examination a typical member of Muscidæ, a family embracing a large and varied assortment of species, among which the house-fly and the blow-fly are the best known forms. Musca domestica needs no description. An interesting part of the house-fly to the microscopist is the wonderful component parts of the head. On examination we find a couple of protuberances, more or less prominent, and situated symmetrically one on each side. Their outline at the base is for the most part oval, elliptical, circular, or truncated; while their curved surfaces are spherical, spheroidal, or pyriform. These horny, round, and naked parts are the corneæ of the compound eye of the fly, and they are appropriately so termed, from the analogy they bear to the larger transparent tunics in the higher classes of animals. They differ, however, from the latter, as when viewed by the microscope they display a large number of hexagonal facets, which constitute the medium for the admission of light to several hundred simple eyes. Under an ordinary lens, and by reflected light, the entire surface of one cornea presents a beautiful reticulation, like very fine wire gauze, with minute papilla, or at least a slight elevation, in the centre of each mesh. These are resolved, however, by the aid of a compound microscope, and with a power of from 80 to 100 diameters, into an almost incredible number (when compared with the space they occupy) of minute, regular, geometrical hexagons, well defined, and capable of being computed with tolerable ease, their exceeding minuteness being taken into consideration.

Fig. 386 represents a vertical section of the eye, showing the hexagonal faceted arrangement of cylindrical tubes.

In this section it appears to be questionable whether the normal shape of the lenses is not round, assuming the hexagonal shape during the process of growth in consequence of their agglomeration. The corneal surface can be peeled off, and if carefully flattened out and mounted it will be seen that each lens is not a simple lens, but a double-convex compound one, composed of two plano-convex lenses of different densities or refracting power joined together.

Experiments made on the eyes of insects, and also of crustaceæ, show that in the insect a real and reversed image of external bodies is formed in each ommatidium; it coincides with the internal face of the crystalline cone in immediate contact with the retina. Although small, the retinal image is distinct and subtends an angle of nearly forty-five. In the same way in the crustacean, the crystalline lens forms on the retinula a reversed image, but the refractive media have a longer focus, and the retinal membrane is not connected with the lens, the interval being filled up by a substance analogous to the vitreous of vertebrates. In both cases it would appear that light does not act directly on the rods; these latter can only receive impressions through the intermediary retinal cells. The retinal images of arthropods, as might have been surmised, are much less perfect than those of the higher orders; on the other hand, their eyes seem to be better adapted for seeing objects in relief and the movements of bodies. The shyness of butterflies and moths is certainly an inherited instinct as a protection against danger from their many enemies.

In the accompanying Fig. 387, A is a vertical section of the eye of Melolontha vulgans, the fan-like arrangement of the facets, together with the transparent pyramidal gathering of the retinal rods proceeding towards the brain; B is a few of the corneal tubes more highly magnified, the darker portion representing the pigment layer of the corneal tubes. In Plate VI., No. 133, the under surface of the head and mouth of the “Tsetse” fly, Glossina morsitans, is shown. The proboscis of this fly is long and prominent, and the antennæ are peculiar, inasmuch as the third segment is long, and produced almost as far as the flagellum, which is furnished with barbed hairs along its outer surface only. Although this fly barely equals the blow-fly in size, it is one of the greatest pests to the domestic cattle of Equatorial Africa. The palpi, although arising from two roots, are seen joined together when the fly is at rest, but when in the act of piercing or sucking they divide and the sheath is thrown directly upwards. The palpi are furnished on their convex sides with long and sharply-pointed dark-brown setæ or hairs, while the inner concave sides, which are brought into contact with the proboscis, are perfectly smooth and fleshy. Three circular openings seem to indicate the tubular nature of what in the house-fly is a fleshy, expanded, and highly-developed muscular proboscis (seen in Fig. 388, Musca domestica). The proboscis (labium) forms the chief part of the organ, dilates into wonderful muscular lips, and enables the insect to employ the tongue as a prehensile organ. The lips are covered with rows of minute setæ, directed a little backwards and arranged rather closely together.

There are very many rows of these minute hairs on each of the lips, and from being arranged in a similar direction are employed by the insect in scraping or tearing delicate surfaces. These hairs are tests for the best of high powers. It is by means of these that it teases human beings in the heat of summer, when it alights on the hand or face, to sip the perspiration as it exudes from the skin. The fluid ascends the proboscis, partly by a sucking action, assisted by the muscles of the lips themselves, which are of a spiral form, arranged around a highly elastic, tendinous, and ligamentous structure, with other retractile additions for rapidity and facility of motion.

The beautiful form of the spiral structure of the tongue should be viewed under a high magnifying power, when it will be seen that no continuing spiral structure really exists; each ring, apparently detached, does not extend quite round; their action is that of sucking tubes. Fluids are evidently drawn up through the entire fissure caused by the opening between the ends of the whole series of rings. It may well be pronounced a marvellous structure. The mounting of the tongue must be done with a considerable amount of care to show this structure, imperfectly represented in my woodcut.

These insects are of some service in the economy of nature, by their consumption of decaying animal matter, found about in quantities ordinarily imperceptible to most people, and that would not be removed by ordinary means during hot weather. It was asserted by Linnæus that three flies would consume a dead horse as quickly as a lion. This was, of course, said with reference to the offspring of such three flies; and it is quite possible the assertion may be correct, since the young begin to eat as soon as hatched, and a female blow-fly will produce twenty thousand living larvæ (one of which is represented in Plate VI., No. 141). In twenty-four hours, each will have increased in weight two hundred times, in five days it attains to its full size, and changes into the pupa, and then to the perfect insect.

In the drone-fly (Eristalis tenax), the mouth organs are larger than in the house-fly, and differ in many respects. The tongue is split up for a certain distance, and then again united, as represented in Fig. 390. The labium, mandibles, and maxillæ are converted into well-developed lancet-shaped organs; these both pierce the skin of animals, and form tubes by which their blood may be sucked up. Next to the maxillary palpi a couple of lancets are seen to project out; these again are associated with two other instruments, one resembling in appearance a two-edged sword, and a peculiar one with pincers or cutting teeth at the extremity. It is very peculiar, and resembles an instrument used in surgery for enlarging the wound, and in this case to increase the flow of blood. This remarkable compound piercing apparatus of the drone-fly is of exquisite finish, and must strike the observer with amazement, while it greatly transcends the work of human mechanism. The fleshy tongue itself projects some distance from the apparatus described, and is furnished with setæ or hairs, shorter and fewer in number than those of the house-fly, and while its spiral structure is not so fully developed, its retractor, muscles, and ligaments are even more so.

The further development of the mouth organs must be looked for in other members of the insect tribe, when it will be seen many assume a more or less modified form of structure, that, for example, in Hymenoptera (the bee and wasp), in which insects the mouth and tongue are divided into lobes which are used to extract the nectary (as Linnæus termed it) from the plants on which they feed. The tongue in most species is capable of extension and contraction.

In Fig. 391 the under-surface of the wasp’s tongue is shown, together with its two pairs of antennæ, and pair of brushes on either side, for brushing off the gathered pollen and honey from the broad tongue. It is amply provided with muscular structure. The antennæ, or feelers, are as curious in form as they are delicate in structure. Those of the male differ from those of the female.

Both the bee and the wasp are armed with an exceedingly venomous sting, as is well known. This structure takes the form of a well-adapted mechanical contrivance, and is a weapon of offence as well as of defence. The sting consists of two barbed needle-points, of a sufficient length to pierce the flesh to some depth. From the peculiar arrangement of their serrated edges their immediate withdrawal cannot take place, and it is this circumstance, with the drop of poison injected into the open wound, that renders their sting of the most painful and irritating kind. The gland containing the poison is contained in a minute sac situated at the root of the piercing apparatus. In Fig. 392 is shown the sting of the wasp and the bee.

Very many insects are provided with instruments for boring into the bark or solid wood itself. The female Cynip bores into the oak-apple for the purpose of depositing her egg. The larva, when full grown, eats its way out of the nut, and drops to the ground, where it attains the form of the perfect fly (Fig. 393).

There are numbers of species living exclusively upon the leaves of plants, to which they do much damage by the excrescences or galls they form. Each tree seems to be infested by its own species of gall-mite, the so-called nail-gall of the lime being caused by a species named Phytoptus tibiæ. These galls take the form of a pointed column, standing erect on the upper side of the leaf. Galls of much the same structure occur in the sycamore, maple, elm, and various fruit trees.

The gnat (Culex pipiens) is furnished with a sting curiously constructed (Fig. 394), and enclosed in a perfectly clothed sheath covered throughout by scales or feathers. This is folded up when not in use. The mouth is provided with a complete set of lancets for piercing the flesh; after having inflicted a severe wound, it injects an acid poison through the proboscis. The scales of the gnat vary in structure accordingly as these are found on the wing, the body, or the proboscis. A magnified wing is shown at No. 2, Fig. 394, and a magnified scale from the proboscis at No. 3. In Fig. 405, Nos. 3 and 5, more highly magnified wing and body scales are given. The proboscis is protected on either side by antennæ and feelers.

The giant-tailed wasp, Sirax gigas, is furnished with an even more curious mechanical boring apparatus (Fig. 395) than its congeners. This is a boring ovipositor, skilfully contrived for piercing the bark of trees, in which the insect deposits her eggs, and where the larva, when hatched, will find an ample supply of food to carry it through this stage of existence. The boring tube, it will be seen, is a perfect muscular structure (c, c, a, and x); in short, it is an endless form of drill, well known to the mechanic, such as is employed in fine work for drilling holes. The females are of some size, and may be surprised and taken in the act of boring through the bark of the pine tree, for which they have a preference.

There is also a species of the broad-bodied saw-fly, Lyda campestris. These bore the Scotch fir, and deposit their eggs. The larvæ from these eggs, when hatched out, feed upon the pine-needles, first spinning a fine web to conceal their work of depredation. A better known saw-fly, Abraxas grossulariata, plays havoc among our gooseberry trees. The female is provided with a curious mechanical apparatus as an ovipositor, with which she cuts into the thicker under-leaf of the plant. This penetrating and cutting tool consists of a double-saw (Fig. 396) of elaborate construction, which when not in use is kept concealed in a long narrow case situated beneath the abdomen. It is further protected by two horny plates. The saws pass out through a deep groove so arranged that the saws work side by side backwards and forwards, without a possibility of running out of the groove. When the cut is made, the four are drawn together and form a central canal, through which an egg is forced into the leaf. The cutting edges of the saws are provided with about eighteen or twenty teeth; these have sharp points of extreme delicacy, and together make a serrated edge of the exact form given to the finest and best-made surgical saws of the present day. In the summer-time the proceedings of the female insect may be witnessed, and the method of using this curious instrument seen, by the aid of a hand magnifier. These insects are not easily alarmed when busy at work.

Before bringing my remarks on proboscides of insects to a conclusion, attention must be given to that of the honey bee (Apis mellifica), and its curious accessories. The mouth of bees exhibits a combination of the suctorial and the masticatory form of oral apparatus. Thus the labial, or upper lip, and the mandibles, or large pair of jaws, are well developed, while the maxillæ, or lesser pair, are elongated to form a tubular organ, through which, together with the tongue, the flower juices, “honey-dew,” may be sucked up. The labium, lower lip, is also rather prolonged, and the palpi, or organs of touch, with which it is endowed form a useful protective apparatus. The mandibles are employed by bees in the construction of their abodes, while the suctorial portion of the mouth is devoted to the reception of nourishment and to prehension. The sting of the bee, already noticed, is in fact an ovipositor, the female alone being provided with this weapon as an egg-depositing organ, although better known as an aculeus or sting; but it forms no part of the oral apparatus (as shown in Fig. 397). The proboscis itself will be seen to be curiously divided; the divisions are elegant and regular, beset with numerous setæ or hairs. The two horny outside lancets are spear-shaped and partially set with short hairs; at the base of each is a hinge articulation; this permits of considerable motion in several directions, and is much used by the busy insect for forcing open the more internal parts of flowers, thus facilitating the introduction of the proboscis. The two shorter feelers are closely connected with the proboscis, and terminate in three-jointed articulations. The structure of the proboscis is so arranged that it can be enlarged at the base, and thus made to contain a greater quantity of the collected honey-dew; at the same time it is in this cavity the nectar appears to be converted into pure honey. The proboscis tapers off to a little nipple-like extremity, and at its base is seen two shorter and stronger mandibles, from between which is protruded a long and narrow lance-like tongue, the whole being most curiously connected by a series of strong muscles and ligaments. The basal or first joint of the hind leg in the neuter or working bee is developed into an enlarged form of pocket, used by the insect for conveying the pollen of flowers and the propolis to the hive. Indeed, both the tibia and the first joint of the tarsus are broadened out into plates, but the two sides of the plates are differently furnished. On one side is a thick coating of hairs, those on the tarsus taking the form of a brush, evidently used for brushing out the pollen, as these special developments are not found on the hind legs of the drones or of the queen.

The wax used in the formation of cells is a secretion that exudes through certain portions of the body of the bee, since it is found in little pouches situated on the under part of the body, but it is not brought home ready for use. The walls of the cells are strengthened when completed by a kind of varnish, already referred to as the propolis, collected from the buds of poplar and lime trees, and this is spread over the walls of the cell by that wonderful pair of broad spatulæ, represented in the drawing.

Many interesting variations will be found in the legs and feet of flies, as well as in those of other orders of insects (Lepidoptera). One or two typical forms are represented in Plate VI., and in Fig. 398.

The tarsus, or foot of the fly (Fig. 398), consists of a deeply bifid, membranous structure, pulvillus; anterior to its attachment to the fifth tarsal joint, or the upper surface, are seated two claws, or “tarsal ungues”; these are freely movable in every direction. These ungues differ greatly in their outline, size, and relative development to the tarsi, and to the bodies of the insects possessing them, and in their covering; most are naked over their entire surface, having however a hexagonal network at their bases, which indicates a rudimentary condition of minute scale-like hairs, such as are common on some part of the integument of all insects. Flexor and extensor muscles are attached to both ungues and flaps; the flaps are either corrugated or arranged on the ridge and furrow plan, in other cases they are perfectly smooth on their free surface, while others are covered with minute scale-like hairs. The thickness of the divided membrane on the blow-fly does not exceed the 1/2000th of an inch at the margin; they somewhat increase in thickness towards the point of attachment. Projecting from the flap are organs which have been termed “hairs,” “hair-like appendages,” “trumpet-shaped hairs.” These are doubtless the immediate agents in holding on to a smooth surface, as that of glass, and are termed “tenent-hairs,” in allusion to their office. The under surface of left forefoot of Musca vomitoria is shown with tenent-hairs (Plate VI., No. 140); a and b are more magnified hairs, a from below, b from the side. No. 142 is the left forefoot of Amara communis, showing the under surface and form of tenent appendages, one of which is seen more magnified at a; No. 143, under surface of left forefoot, Ephydra riparia. This fly is met with in immense numbers on the surface water in salt marshes. It does not possess the power of climbing glass; this is explained by the structure of the tenent-hairs; the central tactile organ is also very peculiar, the whole acting as a float, one to each foot, to enable the fly to rest on the surface of the water; a is one of the external hairs, No. 135, under surface of left forefoot of Cassida viridis (tortoise-beetle), showing the bifurcate tenent appendages, one of which is given at a more magnified. These, in ground beetles, are met with only in males, and are used for sexual purposes. The delicacy of the structure of these hairs in the fly and the elastic membranous expansion of the foot are marvellous. When the fly is climbing, a minute quantity of some glutinous fluid is exuded, so that the tubular nature of the tenent-hairs hardly admits of a doubt.

“At the root of the pulvillus, or its under surface, is a process, which in some instances is short and thick, in others long and curved, and tapering to its extremity (Scatophaga), setose (Empis), plumose (Hippoboscidæ), or, in one remarkable example (Ephydra), closely resembling in its appearance the very rudimentary pulvillus with which it is associated. Just at the base of the fifth tarsal joint, on its under surface, there is present, in Eristalis, a pair of short, very slightly curved hairs, which point almost directly downwards.”[82]

Tenent-hairs are usually present in some modification or other. It is really difficult to name a beetle which has not some form of them; the only one I yet know that seems to me really to possess nothing of the kind is a species of Helops, living on sandy heaths. I suppose the dense cushion of hairs on the tarsi to be for the protection, simply, of the joints to which they are attached. I have detected them on the tarsal joints of species of Ephydra, and on the first basal tarsal joint of the drone of the hive-bee. A very rudimentary form of tenent-hairs is present on the under surface of some of the tree-bugs (Pentatomidæ), which have in addition a large, deeply-cleft organ at the extremity of the tarsus; this appears to be a true sucker.

When walking on a rough surface, the foot represents that of a Coleopterous insect without any tenent appendages. The ungues are always attached to the last joint of an insect’s tarsus. They are not attached to the fifth tarsal joint of a Dipterous insect, neither are they attached to the fifth tarsal joint of a Hymenopterous insect, but to the terminal sucker, which again, in this great order, is a sixth tarsal joint, membranous, flexible, elastic in the highest degree, retractile to almost its fullest extent within the fifth tarsal joint--a joint modified to an extraordinary degree for special purposes.

In plantula of Lucanus, with its pair of minute claws, the ungues are hairs modified for special purposes; and they have the structure of true hairs. The sustentacula of Epeira, the analogous structures on the entire under surface of the last tarsal joints in Pholcus, the condition of the parts in the hind limbs of Notonecta, in both its mature and earlier conditions, as well as in Sarcoptes, Psoroptes, and some other Acari, all may be cited in proof of this fact. The various orders of insects have, for the most part, each their own type of foot. Thus there is the Coleopterous type, the Hymenopterous type, the Dipterous type, the Homopterous type, &c.; each so very distinctive, that in critical instances they will sometimes serve at once to show to which order an insect should be referred. Thus, amongst all the Diptera, I have as yet met with but one subdivision which presents an exception to the structure described. This exception is furnished by the Tipulidæ, which have the Hymenopterous foot. With hardly an exception, then, I believe the form of foot described will be found universal among the Diptera.

It may be desirable to add a few words on the best plan of conducting observations on the feet of insects. Their action should be studied by placing the insect under the influence of chloroform. It is of advantage to carefully preserve the parts examined, and for this purpose Deane’s medium or glycerine jelly suits very well; some of the more delicate preparations, however, can only be kept unchanged in a solution of chloride of zinc. The plan of soaking in caustic potash, crushing, washing, putting into spirits of wine and then into turpentine, and lastly into Canada balsam, is perfectly useless, excepting in rare instances where points connected with the structure of the integument have to be made out. Of course, the parts should be viewed from above, from below, and in profile, in order to gain exact ideas of their relations. The binocular microscope diminishes the difficulties which formerly had to be encountered, as by its aid many parts may be clearly viewed without preparation of any kind.

Moths and butterflies supply the microscopist with some of the most beautiful objects for examination. What can be more wonderful in its adaptation than the antenna of the moth (represented in Fig. 400, No. 1), with a thin, finger-like extremity almost supplying the insect with a perfect and useful hand, moved throughout its extent by a muscular apparatus of the most exquisite construction. The tongue of butterfly (No. 2) is evidently made for the purpose of dipping into the interior of flowers and extracting the juices; this act is assisted by a series of fine muscles. An enlarged view of a portion is given at No. 3; see Plate VI., Nos. 132 and 133, antennæ of Vapour Moth.

The inconceivably delicate structure of the maxillæ or tongues (for there are two) of the butterfly, rolled up like the trunk of an elephant, and capable, like it, of every variety of movement, has been carefully examined and described by Mr. Newport. “Each maxilla is convex on its outer surface, but concave on its inner; so that when the two are united they form a tube, haustellium, by their union, through which fluids may be drawn into the mouth. The inner or concave surface, which forms the tube, is lined with a very smooth membrane, and extends throughout the whole length of the organ; while that of each maxilla is hollow in its interior, apparently forming a tube ‘in itself,’ but this is not so; the mistake has arisen from the existence of large tracheæ, or breathing tubes, in the interior of the proboscis. In some species the extremity of the haustellium is studded externally with a number of minute papillæ, or fringes--as in Vanessa atalanta--in which they become small elongated barrel-shaped bodies, terminated by smaller papillæ at their extremities. On alighting on a flower, the insect makes a powerful expiratory effort, by which the air is expelled from the interior air-tubes, and from those with which they are connected in the head and body; and at the moment of applying its proboscis to the food, it makes an inspiratory effort, by which the central canal in the proboscis is dilated, and the food ascends it at the same instant to supply the vacuum produced; and thus it passes into the mouth and stomach, the constant ascent of the fluid being assisted by the action of the muscles of the proboscis, which continues during the whole time that the insect is feeding. By this combined agency of the acts of respiration and the muscles of the proboscis we are also enabled to understand the manner in which the humming-bird sphynx extracts in an instant the honey from a flower while hovering over it, without alighting; and which it certainly would be unable to do were the ascent of the fluid entirely dependent upon the action of the muscles of the organ.”

The trachæal or respiratory system of insects varies, or rather is found to exist in modified forms to suit their varied conditions of life. While in the larval stage the breathing apertures are seen to recur at intervals on each side of the abdomen (as that of the silkworm, Fig. 401), thus ensuring a continuous supply of air to the circulating fluids throughout the whole body. These spiracles are usually nine or ten in number, and consist of a membranous ring of an oval form. The air-tubes are exquisitely composed of two thin membranes, between which a delicate elastic thread or spiral fibre is interposed, forming a cylindrical opening and keeping the tube in a distended condition, thus mechanically preserving the sides from collapse or pressure in their passage through the air, which otherwise might occasion suffocation. Fig. 402 represents the double spiral arrangement of a portion of a trachea of Hydrophilus, which ensures both elasticity and strength.

There are other points of interest confined to the water-beetle tribe, among the more striking of which is the foreleg of the Dytiscus marginalis. Here the first three joints of the tarsus are expanded into a broad surface, and fringed throughout with curved hairs. From the surface of these spring a number of short hairs, with cup-like discs at their extremities, one of which is seen highly magnified in Plate VI., No. 142. These are so cup-like in form that they have been hitherto described as “suckers,” but it is believed they are simply a special apparatus for the development of the hairs seen on the leg and foot of the beetle. Another curious example occurs in the Gyrinus, or whirligig-beetle. The front pair of legs are of the ordinary kind, but the under pair are furnished with expanding paddles. The trochanter, femur, and tibia, are flat plates of a triangular shape, pointed at their outer angles, from which the apex springs. But the tarsus is jointed on the inner angle of the furthermost end of the tibia, and each of its four joints expands into a flat paddle blade. In the accompanying Fig. 403 one paddle is seen expanded, the other closed.

These paddles are adapted with much precision to ensure the most effectual application of the propelling power; as the beetle strikes out in the act of swimming, the membranous expansion described enables it to move about with great rapidity; upon the legs being drawn back towards the body, the membrane closes up, and thus offers no resistance to the water. The eyes are not the least curious part of the merry little beetle: the upper section is fitted for seeing in the air, and is adapted to the upper or superior part of the head; the lower portion, for seeing under the water, being placed at a lower angle, a thin division only separating the two.

Wings of Insects.--These exhibit variety of form and structure, as well as of beauty of colouring. At an early period the orders of insects were mainly founded upon these interesting appendages. The Orthoptera were the straight wings; the Neuroptera the nerved; the Trichoptera the hairy wings; the Coleoptera the cased or sheathed wings; the Diptera the two wings; the Hymenoptera the married wings; and the Lepidoptera the scaled wings. A number of wings are small and membranous, and may be mounted dry for examination under the microscope. Others are better seen mounted in benzol-balsam. The elytra, iridescent wing cases of the diamond, and other beetles, as well as the wings of the more highly coloured butterflies, make pretty objects mounted dry for opaque illumination by the Lieberkühn or reflector. The thicker horny cases of other members of the beetle tribe require long soaking, as described in a former chapter.

The wings of moths and butterflies are covered with scales or feathers, carefully overlapping each other, as tiles are made to cover the tops of houses. The iridescent variety of colouring on insects’ wings arises from the peculiar wavy arrangement of the scales. Figs. 404 and 405 are magnified representations of a few of them. No. 1, a scale of the Morpho menelaus, taken from the side of the wing, is of a pale-blue colour; it measures about 1/120th of an inch in length, and exhibits a series of longitudinal striæ or lines, between which are disposed cross-lines or other striæ, giving it very much an appearance of brick-work (better seen in Fig. 405, No. 1).

Polyommatus argiolus, azure-blue (Fig. 404, Nos. 2 and 6), are large and small scales taken from the under-side of the wing of this beautiful blue butterfly; the small scale is covered with a series of spots, and exhibits both longitudinal and transverse striæ, these should be clearly defined, and the spots separated by a quarter-inch object-glass. No. 3, Hipparchia janira, is a scale from the meadow-brown butterfly: on this brown spots, having an irregular shape with longitudinal striæ, are seen. No. 4, Pontia brassica, cabbage butterfly, was at one time taken to be an excellent criterion of the penetration and definition of an object glass. It is seen to have a free extremity or brush-like appendage. With a fairly good power, the longitudinal markings appear like rows of small beads. Chevalier selected for his test object the scale of the Pontia brassica. Mohl and Schacht extolled Hipparchia janira as a good test of penetration in an objective of moderate angular aperture. Amici’s test object is Navicula rhomboides, the display of the lines forming the test.

The Tinea vestianella, clothes-moth, is furnished with unique scales. Small and destructive as this moth is, it suffers much from a parasitic mite, and from which it is unable to free itself.

The Podura scale (Fig. 405), with its delicate transparent membrane and curiously inserted “notes of admiration,” as they were called, was long believed to be an excellent test object for the highest powers of the microscope, but I believe it is no longer regarded in that light: indeed, most insect scales have declined in the value and estimation of the skilled microscopist. This is in part due to the improvements made in the objective. The high-angled glasses have cleared up obscure points in the structural characters of the minuter forms of life, and the scales of insects are no longer found to be difficult test objects for the modern objective of a Zeiss or a Powell to resolve. Nevertheless, the scale of the Podura belonging to the order Thysanura, a curious little insect commonly known by the name of springtail, usually found living in most obscure places, and too small to attract attention, is not likely to be entirely thrust aside. The springtails (Collembola) are furnished on the under-side of the first abdominal segment with a curious tube or sucker, from the orifice of which glandular process a secreted viscid matter is protruded; they are remarkable also from the fact that in most of them no trace of a tracheal system has yet been discovered. The eyes when present are in the form of simple or grouped ocelli, the antennæ number six joints, and the abdomen has but six segments, often only three. The forked tail is a curious process turned forward and attached to one of the tender segments and held in position under the body; when released it springs back and bounds up to a very considerable height. Fig. 406 represents Podura villosa. There are several species, one of which (P. aquatica) is found floating in patches on pools of water on bright summer days.

Lepisma saccharina belongs to the same genus as Podura. This minute springtail derives its name from having been discovered in old sugar-casks. It has a spindle-shaped body covered with silvery scales, long used as test objects. The sides of the abdomen are furnished with a series of appendages with long bristle-like setæ, or hairs, at their extremities. The head is concealed under a prothorax, the antennæ are long, and the maxillary palpi are either five or seven-jointed, and very conspicuous, to enable them to cut the dry wood on which they principally feed. The scales must be mounted under thin cover-glasses; oblique illumination shows up some portions to advantage, while central light from an achromatic condenser and a wide-angled objective renders their markings more distinct. Portion of a scale more highly magnified is shown in Fig. 405.

Eggs of Insects (Plate VI., Nos. 124-139).--In form, colour, and variety of design, the eggs of insects are more surprisingly varied than those of the feathered tribes; but as from extreme smallness they escape observation, an acquaintance with their structure is not so familiar as it might be. Although the eggs of the bird tribe differ much in their external characteristics, they closely resemble each other while yet a part of the ovarian ova, and prior to their detachment from the ovary. At one period of their formation all eggs consist of three similar parts:--1st. The internal nucleated cell, or germinal vesicle, with its macula; 2nd. The vitellus, or yolk-substance; and 3rd. The vesicular envelope, or vitelline membrane. The germinal vesicle is the first produced, then the yolk substance, which gradually envelops it, and the vitelline membrane, the latest formed, incloses the whole. The chemical constituents of the egg are the same in all cases, albumen, fatty matters, and a proportion of a substance precipitable by water. The production of the chorion, or shell membrane, does not take place till the ovum has attained nearly its full size, and it then appears to proceed, in part at least, from the consolidation over the whole surface of one or more layers of an albuminous fluid secreted from the wall of the oviduct.

The embryo cell is so directly connected with the germinal vesicle that at a certain period it disappears altogether, and is absorbed into the germinal yolk, or rather becomes the nucleus of the embryo, when a greater degree of compactness is observed in the yolk, and all that remains of the germinal vesicle is one or more highly refracting fat globules and albuminoid bodies. Towards the end of the period of incubation, the head of the young caterpillar is said to lie towards the dot or opening in the lid, termed the micropyle,[83] from its resemblance to a small gate, or opening through which the larva emerges forth as a butterfly.

The germinal vesicle is comparatively large and well-marked while the egg is yet in the ova-sac. By preparing sections after Dr. Halifax’s method,[84] we find that the germinal vesicle in the bee’s egg is not situated immediately near or even below the so-called micropyle, but rather more to the side of the egg; just in the position which the head of the embryo is subsequently found to occupy at maturity.

The egg membrane, or envelope, of all the Lepidoptera is composed of three separate and distinct layers: an external slightly raised coat, tough and hard in its character, a middle one of united cells, and a fine transparent vitelline lining membrane, perfectly smooth and homogeneous in structure, imparting solidity, and giving a fine iridescent hue to the surface. The germinal vesicle is of a proportionately large size for the egg, and its macula is at first single, then multiple. In the egg of the silkworm the outer membrane is comprised of an inner reticulated membrane of non-nucleated cells, in the outer layer the cells are arranged in an irregular circular form, also non-nucleated, with minute interstitial setæ or hairs projecting outward.

The outer surface of the egg-shell of Coccus Persicæ is covered by minute rings, of which the ends somewhat overlap. These rings are thought to be identical in their character with the whitish substance which exudes through pores on the under-side of the body; it is more than probable that a succession of layers of rings fully accounts for the beautiful prismatic hues they present viewed as opaque objects under the microscope, and illuminated by Lieberkühn or side-condenser. This white substance, it should be observed, forms a part of the intimate structure of the egg-shell, and is in nowise affected by methylated spirit or dilute acids. Sir John Lubbock[85] states that in the greenish eggs of Phryganea, “the colour is due to the yolk-globules themselves. In Coccus, however, this is not so; the yolk-globules are slightly yellow, and the green hue of the egg is owing to the green granules, which are minute oil globules. When, however, the egg arrives at maturity, and the upper chamber has been removed by absorption, these green granules will be found to be replaced by dark-green globules, regular in size, and about 1/8000th of an inch in diameter, and which appear to be in no way the same in the yolk of Phryganea eggs.” Another curious fact has been noticed, which partially bears on the question of colour: the production of parasite bodies within the eggs of some insects. In the Coccus, for instance, parasitic cells of a green colour occur, “shaped like a string of sausages, in length about the 1/2000th of an inch by about the 1/7000th in breadth.”

The eggs of moths and butterflies present many varying tints of colour; in speaking of this quality I do not restrict the term solely to those prismatic changes to which allusion has been made, and which are liable to constant mutations according to the accident of the rays of light thrown upon them; but I more particularly refer to the several natural transitions of colour, the prevailing tints of which are yellow, white, grey, and a light-brown. In some eggs the yellow, white, and grey are delicately blended, and, when viewed with a magnifying power of about fifty diameters, and by the aid of the side-reflector (parabolic-reflector), exhibit many beautiful combinations. The more delicate opalescent, or rather iridescent, tints appear on the eggs of insects, while those of the feathered tribes furnish no like example. The egg of the mottled umber moth, Erannis defoliaria (Plate VI., No. 137), is in every way very beautiful. It is in shape ovoid, with regular hexagonal reticulations, each corner being studded with a knob or button; the space within the hexagon is finely punctated, and the play of colours is exquisitely delicate. In this egg no micropyle can be seen. The egg of the thorn moth, Ennomos erosaria (Plate VI., No. 138), is of an elongated brick-looking form, one end of which is slightly tapered off, while the other, in which the lid is placed, is flattened and surrounded by a beautifully white-beaded border, having for its centre a slightly raised reticulated micropyle. The empty egg-shell gives a fine opalescent play of colours, while that containing the young worm is of a brownish-yellow.

The egg of the straw-belle moth, Aspillates gilvaria (Plate VI., No. 139), is delicately tinted, somewhat long and narrow, with sides slightly flattened or rounded off, and irregularly serrated. The top is convex, and the base a little indented, in which are seen the lid and micropyle. The young worm, however, usually makes its way through the upper convex side: the indentation represented in the drawing shows the place of exit.

An example of those eggs possessing a good deal of natural colour is presented in that of the common puss-moth, Cerura vinula, a large spheroidal-shaped egg, having, under the microscope, the appearance of a fine ripe orange; the micropyle exactly corresponds to the depression left in this fruit on the removal of the stalk. The surface is finely reticulated, and the natural colour a deep orange.

The egg of the mottled rustic moth, Caradina morpheus (No. 124), is subconical, and equally divided throughout by a series of ribs, which terminate in a well-marked geometrically-formed lid. The egg of the tortoise-shell butterfly, Vanessa urticæ (No. 125), is ovoid and divided into segments, the ribs turning in towards the micropyle. The common footman, Lithosia campanula (No. 126), produces a perfectly globular egg covered with fine reticulations of a delicate buff colour. The egg of the shark moth, Cucullia umbratica (No. 127), is subconical in form, with ribs and cross-bars passing up from a flattened base to the summit, and turning over to form the lid. No. 136 is the egg of blue argus butterfly, Polyommatus argus. That of the small emerald moth, Jodis Vernaria (No. 134), is an egg of singular form and beauty--an oval, flattened on both sides, of silvery iridescence, and covered throughout with minute reticulations and dots. It is particularly translucent, so much so that the yellow-brown worm is readily seen curled up within. The lid or micropyle is not detected until the larva eats its way out of the shell. It should be noted that the series of eggs in Plate VII. are somewhat over-coloured, and consequently lose much of their natural transparency. The eggs of flies and parasites also present much variety in form, colour, and construction. Many of their eggs are provided with a veritable lid, which opens up with a hinge-like articulation. This lid is seen in the egg of bot-fly, Plate VI., No. 144, from which the larva is just escaping; No. 146, egg of Scatophaga; No. 147, egg of parasite of magpie.[86] Still more remarkable in the delicate and beautiful forms are some of the parasities which infest birds in particular: Plate VI., No. 145, the egg of parasite of pheasant; No. 147, that of the magpie, while that of the peacock is curiously interesting. In Fig. 407 the larvæ of the horn-bill are seen just about to emerge from their eggs.

The larvæ of most Hymenoptera are footless grubs, furnished with a soft head, and exhibiting but little, if any, advance upon those of Diptera (Plate VI., No. 141). In the saw-fly, however, the larva, instead of being as above described, a mere footless maggot, presents the closest resemblance to the caterpillar of the Lepidoptera; it is provided with a distinct head, with six thoracic legs, and in most cases from twelve to sixteen pro-legs are appended to the abdominal segments.

One other conspicuous object represented in Plate VI., No. 128, is the maple Aphis, also known as the leaf-insect, averaging in size about the one-fiftieth of an inch in length. Although recognised and described under the name of the leaf-insect, nothing was known of its origin and history, with the exception of what the Rev. J. Thornton published in 1852, and to whom we owe its re-discovery on the leaves of the maple. Subsequently it attracted the attention of the Dutch naturalist, Van der Hoeven, who regarded it as the larval form of a species of Aphis, and named it Periphyllus. It has more recently engaged the attention of Dr. Balbiani and M. Siguoret, whose united investigations will be found in “Comptes Rendus,” 1867. These observers assigned it definitely to Aphis. A brown species is also met with during a great part of the year feeding upon the young shoots of the maple. The female produces two kinds of young, as do all the genus Aphis, one normal the other abnormal; the first are alone capable of reproducing their species, while the latter retain their original form, which is not changed throughout their existence. They increase so slowly in size that it may appear doubtful whether they eat, the mouth being rudimentary; they undergo no change; do not acquire wings, and their antennæ always retain the five joints peculiar to all young Aphides before the first moult. Neither are they all of the same colour, some being of a bright green, as represented in Plate VI., while others are of a darker, or brownish-green colour. The brown-green embryos differ from the adult female only in those characters analogous to all other species, and this chiefly with regard to the minute hairs, which are long and simple. In the green embryos, in the place of setæ, the body is surrounded by transparent lamellæ, oblong in shape. These scales not only cover the body, but also the anterior portion of the head, the first joint of the antennæ, and the outer edge of the tibiæ of the first pair of legs. The dorsal surface in these insects is covered with a mosaic of hexagonal plates, very closely resembling the plates of the carapace of the tortoise. In this particular my artist has fallen into a slight error. Another peculiarity is that the body is much flattened out, and looks so much like a scale on the surface of the leaf that it requires considerable practice, as well as quickness of sight, to detect the young maple Aphis. One of the lamellæ is seen highly magnified at c, and a tenent-hair at b. The antennæ, tapering off towards the apex, are serrate on both edges, and terminate in a fine lancet (shown at a), with which it penetrates the leaf of the plant. Beneath the insertions of the antennæ is a complex form of sucking mouth, and on either side of the head are two brilliant scarlet-coloured eyes.

Aphides, as is well known, live upon the juices of plants, which they suck, and when they occur in great numbers cause considerable damage to the gardener and farmer. Many plants are liable to be attacked by swarms of these insects, when their leaves curl up, they grow sickly, and their produce is either greatly reduced or utterly ruined. One striking instance is presented in the devastation caused by the hop-fly (Aphis humuli).

The Aphrophora bifasciata, common frog-hopper, is a well-known garden pest. The antennæ of this insect are placed between the eyes, and the scutellum is not covered; the eyes, never more than two in number, are occasionally wanting. These pests are furnished with long hind legs, that enable them to perform most extraordinary leaping feats. The best-known British species is the cuckoo-spit, froth-fly (Fig. 408). The names cuckoo-spit and froth-fly both allude to the peculiar habit of the insect, while in the larva state, of enveloping itself in a kind of frothy secretion, somewhat resembling saliva.

Arachnidæ.--In this class of insects, spiders, scorpions, and mites are included, all of which belong to a sub-class of Arthropoda, and are appropriately placed between the Crustacea on the one hand and the Insecta on the other. The highest Crustaceans have ten feet, the Arachnidæ eight, and insects six. The Arachnidæ are wingless, have no antennæ, and breathe by means of tracheal tubes, or pulmonary sacs, these performing the function of lungs. As a rule they have several simple eyes, have no proper metamorphosis, and they are essentially predaceous, the females being larger than the males. Most of the Arachnidæ live on insects, and may therefore be regarded in the light of a friend to the florist and gardener.

The Epeira diadema is the best known member of the species; in summer spiders abound on every shrub, and spin out their wonderful webs from branch to branch.

The body, seen in my illustration, Fig. 409, in section, consists of two parts; the foremost is the cephalothorax, or head, upon which is mounted four pairs of eyes (two of which are seen in section), while to the thorax is attached eight jointed well-developed legs terminating in feet, with claws adapted for climbing and holding on. The other half consists, of the abdomen, together with spinnerets and glands, which secrete the fluid out of which the web is spun, and this, although it hardens to some extent on exposure to the air, retains its viscid nature for the purpose of entangling its prey. The spinnerets are the most interesting feature in the anatomy of the Epeira (Figs. 410 and 411).

Five kinds of spinning glands are found in spiders. The glandulæ aciniformes are those which consist of a proper tunica and an epithelium; these exhibit in all parts the same reaction to staining agents. The glandulæ pyriformes consist of a tunica proper and an epithelium, which in their lower parts (or those near the efferent ducts) stain more deeply than the upper. The glandulæ ampullaceæ and glandulæ tubuliformes have similar coverings, the latter terminating in a large spool. The glandulæ aggregatæ have a wide and branched lumen, the efferent duct of which is provided with cells and an accessory piece, which draws out to a tip. All the glands have secreting portions, which serve as collecting cavities for the spinning material. The spools are two-jointed basal and one-jointed accessory pieces. In addition to the five glands enumerated, there are also lobate and cribelleum glands; these are variously distributed, and exercise different functions, one set preparing the so-called moist filaments from the moist droplets, another spins the egg-cocoon, as nearly all spiders envelop their eggs in a covering of silken threads and store them up in some sheltered place awaiting the warm weather of spring to hatch them out. The bag that holds the eggs is not one of the least curious efforts of skill and care. The mother uses her body as a gauge to measure her work, precisely as a bird uses her body to gauge the size and form of its nest. The spider first spreads a thin coating of silk as a foundation, taking care to have this circular by turning its body round during the process. In the same manner it spins a raised border round this till it takes the form of a cup; it is at this stage of the work the female begins to lay her eggs in the cup, and not content to fill it up to the brim, she also piles up a heap as high as the cup is deep. Here, then, is a cup full of eggs, the under half covered and protected by the silken sides of the cup, but the upper still exposed to the air and the cold. She now sets to work to cover this; the process is similar to the preceding--that is, she weaves a thick web of silk all round the top, and instead of a cup-shaped nest, like those of the bird tribe, the whole partakes of the form of a ball much larger than the body of the spider.

The eight legs and feet of the spider (one only is represented Fig. 410, No. 4) are curiously constructed. Each foot, when magnified, is seen to be armed with strong horny claws, with serrations on their under-surface. By this arrangement the spider is enabled to regulate the issue of its web from the spinnerets. In addition, a remarkable comb-like claw is provided for the purpose of separating certain threads which enter into the composition of the delicate web, so that everything is arranged and planned in the most geometrical order, while the mouth or jaws with their two movable poison-fangs convert the Arachnidæ into formidable and dangerous foes. The maternal industry and instincts of spiders, the ballooning habits of others, the cave dwellers, with their limited vision, combined with an increased delicacy of touch and hearing, their disguise of feigned death when a strong enemy approaches, are all of the most interesting character.

One of the more remarkable, the Argyroneta aquatica (diving spider), weaves itself a curious little bell-shaped globule, which it takes with it to the bottom of the water, whither it retires to devour its prey. Notwithstanding its aquatic habits, this, like the rest of its species, is fitted only for aerial respiration; it therefore carries down, entangled amongst the hairs of its body, a small bubble of air. This contrivance presents us with the earliest form of diving-bell.

Mites and Ticks constitute a group which for diversity of structure, number of species and individuals, and minuteness of size, has no equal. The typical genus of the family--Ixodidæ--being wholly parasitic in their habits, are so modified in organisation, so marked by degeneration, that some authors have proposed to remove them into a class by themselves. One leading character distinguishes the whole: the abdomen rarely presents a trace of segmentation, but is confluent with the cephalothorax, the fusion between the two being so complete that, as in the harvest spiders belonging to Palpatores, the anterior sternal plates of the abdomen are thrust far forward between the coxæ of the cephalothoracic limbs. As in Arachnidæ, however, the mouth is adapted for sucking, but the jaws are often partially united, and form, with a plate termed the epistome and the labium, a beak. The mandibles are either pincer-like, or simply pointed at the tip, forming piercing organs; the palpi have their basal segments, or maxillæ, united, which form a conspicuous plate, or hypostomes, constituting the floor of the mouth. These organs are often seen to be separated from the rest of the cephalothorax by a membranous joint, and constitute a kind of head, the capitulum. In most cases no trace of special respiratory organs can be found. Another characteristic of value in separating ticks from harvest-spiders is that in the former the young undergo a metamorphosis in the course of growth, being hatched as six-footed larvæ, and acquiring later in life a fourth pair of legs.

The Acariæ include a number of families, all distinguished by the position of the respiratory stigmata and the form of the mandibles and palpi. In the velvety mites (Trombidiidæ), the integument is soft and covered with variously-coloured fine hairs, and the legs are adapted for walking, running or swimming. The latter live in fresh-water ponds, creeping over the leaves of aquatic plants. The fresh-water mites (Atax spinipes, Fig. 412) swim about freely by means of vigorous strokes of their legs, which act as oars. In the adult the body is more or less spherical, and usually of a bright red or greenish colour. The males of one species have a curious blunt tail-like prolongation from the hinder end of the abdomen. The eggs are laid in the spring on the stems of water plants, and the six-footed larvæ when hatched attach themselves to water-bugs (Nepa) or water-beetles (Dytiscus) by means of a large sucker developed on the front of the head.

Of all the Acari, the best known and most troublesome are those belonging to the family Ixodidæ; these infest the whole animal creation. They are furnished with a long cylindrical beak, armed with recurved hooks, formed of the two mandibles above and the long slender labium below. They have no eyes, nor apparently any dermaploptic sense, but there are various seemingly sensitive setæ distributed over the body and on the appendages. The whole of the mites will be found suitable objects for the study of development, as the process is slow and their eggs do not require much care. The segmentation of the eggs differs; some of the cells are distinguished by their large nuclei, which stain feebly by carmine. During the cleavage of the egg no division of the so-called yolk has been observed, but later on this breaks up into several minute pieces.

The accompanying Fig. 413 shows the under surface of the body and the mouth parts of the common English dog and sheep tick, Ixodes ricinus, with its six formidable legs. The upper surface is shown in Fig. 415; the head (capitulum) and mouth organs in Fig. 414, c, d, e, f, g, together with the four segments of the palpi; h the labial process armed with hooks forming the lower side of the beak, and i indicating the tips of the two mandibles forming the upper side, and projecting beyond the apex of the labium. By means of this beak, which is thrust to its base into the integument, the tick adheres firmly to its host, and in detaching them care must be taken that the head is not left behind buried in the skin. This tick is found in all stages of growth; the females, gorging themselves with blood, swell up to the size of a pea, as seen in Fig. 413, but the male, formerly regarded as a distinct species, is of a much smaller size. In distribution these pests are almost cosmopolitan, and in tropical countries they grow to much greater dimensions, the females sometimes attaining the size of a large gooseberry.

The family of true mites is that of the Sarcoptidæ; these are either free or parasitic. They have no breathing organs; the palpi are basally fused to the rostrum, the mandibles are pincer-like, and the tarsi are often furnished at their tips with a sucker. The most familiar is the cheese mite, Tyroglyphus, which feeds upon decaying matter.

The well-known cheese mite attains to a size plainly visible to the naked eye, but when first hatched out from the egg (shown in its several stages of development in Fig. 417), requires a moderate amount of magnification. Its growth, however, is rapid and the young begin to feed as soon as they leave the egg. The body is partially covered over by setæ, or hairs, and the feet terminate in hooklets, as seen in the full-grown acarus. The mandibles are cutting, but as a rule they prefer soft and partially-decayed kinds of food. It also feeds upon damaged flour, sugar, and other domestic articles. The Dermestes lardarius, one of the minute beetle tribe (Fig. 418), commits even greater depredations among insect and other collections during the larval stage of its existence.

Birds suffer much from mites living parasitically upon them belonging to Sarcoptidæ; these likewise infest mankind, and give rise to a disease known as the itch (Fig. 416, No. 3). This malady and the irritation accompanying it are caused by the mite excavating tunnels under the skin. In these the eggs are laid and hatched, and the young then start burrowing on their own account; their burrows are traced as whitish lines on the surface of the skin.

Fig. 416, No. 4, Demodex folliculorum, is another remarkable parasite found beneath the skin; this is usually obtained from a spot where the sebaceous follicles or fat glands are abundant, such as the forehead, the side of the nose, and the angles between the nose and lip. If the part where a little black spot or a pustule is seen be squeezed rather hard, the oily matter there accumulated will be forced out in a globular form. This minute mite is less than one-fiftieth of an inch in length; if it be laid on a glass slide, and a small quantity of glycerine added to cause the separation of the harder portions, the parasite in all probability will float out, and, by means of a fine-pointed pencil or brush, can be transferred to a clean slide and mounted in Canada balsam. An allied species is found in the skin of dogs suffering from mange.

The Stylopidæ are remarkable parasites, living upon the bodies of wasps, bees, and bugs, and present a type of structure quite distinct from beetles or the ticks described. The male (Xenos peckii, Fig. 420) is a winged insect with coarsely faceted eyes, large fan-shaped wings, extremely small inconspicuous elytra, the two first thoracic rings short, while the metathorax is elongated and covers the base of the abdomen, and the hind legs are placed a long way behind the middle pair. The female, on the other hand, is a grub-like creature, without legs, wings, or eyes; she never leaves the body of her host, and from her eggs active little larvæ develop and get carried into the nests of bees and wasps.

Mites are very numerous, differ in form, and are interesting objects under the microscope. The body of the common flea (Fig. 421) is divided into distinct segments, those about the thorax being separated. Although apterous, the flea has the rudiments of four wings in the form of horny plates on both sides of the thoracic segments. Its mouth consists of a pair of sword-shaped mandibles, finely serrated; these, with a sharp, penetrating, needle-like organ, constitute the formidable weapons with which it pierces through the skin.

The neck is distinctly separated, and the body covered with scales, the edges of which are beset with short setæ; from the head project a short pair of antennæ, below which are a proboscis and a lance-shaped cutting apparatus. On each side of the head a large compound eye is placed; it has six many-jointed powerful legs, terminating in two-hooked claws; a pair of long hind legs are kept folded up when the insect is at rest, which, in the act of jumping, it suddenly straightens out with great muscular force. The female flea (Fig. 421) lays a great number of eggs, sticking them together with a glutinous secretion; the flea infesting the dog or cat glues its eggs to the roots of the hairs. In about four days the eggs are hatched out, and a small white larva or grub is seen crawling about, and feeding most actively. Plate VI., No. 141, is a magnified view of one covered with short hairs. After nine or ten days the larva assumes the pupa form; this it retains four days, and in nine days more it becomes a perfect flea. The head of the flea found in the cat (Fig. 415, No. 3) somewhat differs in form from that of the species infesting the human being; its jaws are furnished with more formidable-looking mandibles, and from between the first and second joints behind the head short strong spines project.

Two small and obscure groups of the mites and ticks have been associated with the latter, but for no better reason than that their affinities are unknown. The first of these are the Tardigrada, or bear animalcules, which comprise microscopical animals living in damp, sandy, and mossy places; the body is long and oval in shape, and possesses four pairs of bud-like unjointed appendages, each tipped with claws: the last pair of legs project from the hinder part of the body. The mouth is much subdued, and only a trace of jaws is found as a pair of stylets; there appear to be no organs of respiration or circulation, and, unlike what obtains in all true Arachnida, the sexes are united in each individual. These curious infusorial creatures have been found by myself in an infusion of cow manure.

Injurious Insects.--In describing some of the more interesting points in connection with insect life, I have only quite incidentally referred to the destructive habits of the larger number of insects and the ravages annually inflicted, chiefly by the smaller parasitical tribes, upon our cultivated crops of all kinds.

Here we have a wide field of research open to the microscopist, whose investigations must be carried out systematically, day by day, and for which a moderate power will effectually serve his purpose.

There are some ten or twelve species of injurious insects that attack the hop plant. By way of example, I will select one of the least known among them, the hop-flea, or beetle (Haltica concinna). This is sufficiently minute to require the aid of the microscope, and very closely resembles the turnip-flea proper, H. nemorum. Under the microscope the former will be seen to differ considerably. Its colour is brassy, whereas the colour of its congener is dusky or black, and its wing-cases are striped. They both have wonderful powers of jumping. H. concinna has a curious toothed formation of the tibia, with a set of spines, while the tibia of the turnip-flea is without any curve. It presents other points of difference. The hop-flea is, in fact, a winged beetle, and passes the winter in the perfect state under clods, tufts of grass, or weeds outside the hop-plantation, and here it lays its eggs. In the early spring the larvæ are hatched out as a little white maggot, which immediately makes its way to the hop-plant and burrows into the young leaves and feeds upon its tissues. Here we have an insect taken at random from among thousands of others of the most destructive kinds which annually destroy crops of enormous value to the nation.

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V. Vertebrata

The most complicated condition in which matter exists is where, under the influence of life, it forms bodies with a structure of tubes and cavities in which fluids are incessantly in motion, and producing continuous changes. These have been rightly designated “organised bodies,” because of the various organs they contain. The two principal classes into which organised bodies have been divided are recognised as vegetable and animal. It was Bichat who taught that our animal life is double, while our organic life is single. In organic life, to stop is to die; and the life we have in common with vegetables never sleeps, and if the circulation of the fluids within the animal body ceases for a few seconds, it ceases for ever. In the vertebrate body, however, the combination of organs attains to the highest development, in striking contrast with that of the class we have previously considered, the Invertebrata, the animal kingdom being divided into Vertebrates and Invertebrates.

The Vertebrata are distinguished from all other animals by the circumstance that a transverse and a vertical section of the body exhibits two cavities completely separated from one another by a partition. A still more characteristic feature separates the one from the other; it is the specialisation of the chief nervous centres, and their peculiar relation to the other systems of the body.

The dorsal cavity of the body contains the cerebro-spinal nervous system, the ventral, the alimentary canal, the heart, and usually a double chain of ganglia; these pass under the name of the sympathetic system. It is very probable that this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. In any case, the central parts of the cerebro-spinal nervous system--i.e., the brain and the spinal cord--would appear to be unrepresented among invertebrate animals. Likewise, in the partition between the cerebro-spinal and visceral tubes, certain structures which are not represented in Invertebrates are contained. During the embryonic condition of all Vertebrates, the centre of the partition is occupied by an elongated cellular cylindrical mass, the notochord, or chorda dorsalis. This structure persists throughout the life in some Vertebrata, but in most it is more or less completely replaced by a jointed, partly fibrous, cartilaginous, and bony vertical column. All vertebrate animals have a complete vascular system. In the thorax and abdomen, in place of a single perivisceral cavity, in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior, recalling in the latter case the atrial cavities of the Mollusca. In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess a hepatic portal system, the blood of the alimentary canal never being wholly returned directly to the heart by the ordinary veins, but being more or less completely collected into a trunk (the portal vein), which ramifies through and supplies the liver.

With reference to one other point of importance, the development of the ova of Vertebrates, these have the same primary composition as those of other animals, consisting of a germinal vesicle containing one or more germinal nuclei, and included within a vitellus. But as this forms a part of general anatomy, and as my object is simply the investigation of the fundamental and microscopical structure of animal organisms, I shall not further pursue the morphological part of the subject, especially as so many excellent text-books are within reach of the student who desires to fully acquaint himself with precise information.

Notwithstanding, then, the apparent diversity in the structure of the vertebrate and the invertebrate and the various tissues of which animals and vegetables are constituted, microscopical research has satisfactorily demonstrated that all textures have their origin in cells; in fact, when the formative process is complete, the animal cell is seen to consist of the same parts and almost the same chemical constituents as the typical cell of the plant--namely, a definite cell-wall enclosing cell contents, of which the nature may be diverse, but the cell nucleus is precisely the same and is the actual seat and origin of all formative activity. The cell and nucleus grow by assimilation or intersusception, that is, by inflowing of nutrition among all parts, the new replacing the old, yet maintaining its original structure and composition. That which was once thought special to animals is now found to be common to both plants and animals: they are found to be alike fundamentally in internal structure, and in the discharge of the mysterious processes of reproduction and of nutrition, although the latter forms a convenient line of separation. Life in plants goes on indefinitely; cuttings may be taken without injury to their vigour and duration of life. The same may be said of some of the lower forms of invertebrate life; for example, the hydra, the anemone, and some other well-known animals, may be cut up, divided into several parts, each one of which will form a new animal, provided a nucleus be included in the section. Nevertheless, the organisation of the amœba and the hydra is as complete for its purpose as that of man for his, and the evidence of continuity forbids the drawing of hard and fast lines, as was formerly done between the two kingdoms, the animal and vegetable. The amount of similarity or agreement in the organisation of animals is various. Animals indeed differ from each other in slight points only, for the discovery of which the microscope must be brought into requisition. Living matter in its earliest stage and simplest form appears to the naked eye as a homogeneous structure, but when placed under the highest powers of the microscope, it is seen not to be so.

But perhaps the most marked feature of the age has been the increasing attention given to the study of the lower forms of life, using their simpler structures and more diffuse phenomena to elucidate the more general properties of living matter. To understand life we must understand protoplasm. Of this there can be no doubt, as we have seen in a previous chapter that a whole family, the Monera, consists of this simple living, microscopic, jelly-like substance, which has not even begun to be differentiated, as in the amœba, which has as yet no special organs, and every speck becomes a mouth or a stomach, and which can be turned inside out and shoot out tongues of jelly to move and feel with. “Reproduction is the faculty most characteristic of life, and sharply distinguishes the organic from the inorganic.” It is, then, the corpuscles of protoplasm, called cells (cellulæ), which have so much interest for the physiologist, and these, like the cytods, may form independent organisms, which are then termed unicellular. Again, cells form other cells, and a multicellular organism results, and goes on increasing in geometrical progression. In the Vertebrata the cell retains its characteristic spheroidal shape, as seen in Fig. 423, and undergoes division by virtue of its living protoplasmic mass.

Epithelial Cells.--All free surfaces of the human body, both internal and external, are to a very considerable extent covered by epithelium cells. These cells are everywhere the same, but with modifications in shape and arrangement. Epithelial cells are nucleated and always joined by their surfaces or edges, without, on the external surfaces, the intervention of connective tissue.

There are four essential varieties:--1. Tesselated; 2. Columnar; 3. Spheroidal; 4. Ciliated; in all of which the nucleus remains remarkably uniform in its characters, is either round or oval, and flattened out, measuring 1/6000th to 1/4000th of an inch in diameter. They are insoluble in acetic acid, colourless, or slightly tinted by the structure with which they are in contact, and usually contain one or more nucleoli with a few minute irregular granules, as represented in Fig. 424.

The simplest and most commonly distributed variety is the tesselated, known also as the scaly, squamous, pavement, and flattened epithelium, always arranged in single layers, lining serous cavities, many parts of the mucous membrane, and the interior of ducts and blood vessels. Upon the external surface of the body it occurs in superimposed layers, forming the “stratified epidermis.” To obtain specimens of lamellar epithelium it is only necessary to collect a little saliva, or pass a glass slide over the lining membrane of the cheek, cover it with a thin cover glass, and examine it with a 1/4-inch objective. Pavement epithelium is the elementary structure of hair, nails, and horn.

Columnar epithelium exists upon the mucous membrane of the stomach, on the villi of the intestines, and in the several canals. It occupies either a vertical or horizontal position, and may be detached in rows, as shown in Plate XIX., No. 2, a section taken from the intestine of a rabbit. This variety, when more highly magnified, as in Fig. 424, is seen to consist of club-shaped nucleated cells, the thicker end being turned towards the surface. The protoplasm of the cell is granular, and the presence of minute vacuoles and fatty globules occupy a great part of the space. The nucleus is now seen to contain a fine network. At times the outer end of the cell is distended, as in Fig. 3. This form of columnar epithelium (known as the “goblet” cell) presents a close and remarkable resemblance to the cilio-flagellate “collared” infusorial monad in its extended “wine-glass” form.

Spheroidal epithelium is confined to the closed cavities of the body, and in the internal structure of the ducts of secreting glands. The cells are, for the most part, circular, although some are flattened out at the sides in which they are in contact with each other (Plate XIX., No. 1_a_). Specimens of this form may be taken from the internal surface of one of the lower animals with a scalpel. The collected matter must be placed in a drop of distilled water and examined with a high power.

Ciliated epithelium is characterised by the presence of those fine hair-like filaments (cilia) attached to the free surface of the cell. During life, and for some time after death, the cilia are seen to retain their constant waving motion. The cilia all move in one direction and rhythmically, thus giving rise to the appearance of a succession of undulations. Ciliated epithelium is found lining the mucous membrane of the air passages and nasal ducts, and wherever it is necessary to urge on a secretion by mechanical means, ciliated epithelium exists. Specimens for examination are easily obtained from the oyster, and with care will show the characteristic motion. A portion of a gill separated from the mollusc will live on for a considerable time if kept in a little of its natural secretion. The parameciæ, rotifera, and all the ciliata, are furnished with cilia as a means of locomotion and obtaining sustenance. By snipping off a small piece from the gills of the mussel, always accessible to the microscopist, and covering it over with thin glass to prevent evaporation of the animal juices, its cilia will continue to work for hours.

Lymph and Blood, Fig. 425 B, a a.--There are other cells in the animal body which possess a certain amount of resemblance to those confined to the more superficial structures--i.e., the lymph, chyle, and blood. These fluids present in one respect a physical uniformity of composition, and a resemblance in the size of their characteristic corpuscles. Chyle contains besides the corpuscles of lymph, a quantity of minute granules which imparts a white colour to the fluid. Intermixed are oil globules, free nuclei, and sometimes a few red blood discs. Chyle may be had for microscopic examination by squeezing a little juice from the lymphatic gland of a sheep just slaughtered.

Blood Corpuscles or cells vary considerably in mammals, birds, reptiles, and fishes. Fig. 102 (page 143) is a microphotograph of a drop of blood magnified 3,500 times; and Fig. 425, A, shows both red and white discs drawn to scale, magnified 1,200 diameters. The red corpuscles of human blood are distinguished by their clearly defined outlines and dark centres. Each disc is biconcave in form, and hence the whole surface cannot be focussed at the same time. When the circumference is well illuminated the centre is dark, but by bringing the objective nearer to the object, the concavity of the disc is brought into focus. It generally happens that blood corpuscles, on being first drawn, run together, and present the appearance of rolls of coins; or they may be scattered about over the field. There is a considerable difference in the form of the discs; they are circular in all mammals, except the camel, dromedary, and llama, these being oval. In profile blood corpuscles are biconcave, their investing membrane is homogeneous and elastic, and will readily move along the smallest capillary vessels. There is no trace of a nucleus in the blood-discs of the adult Mammalia, while in size they bear no proportion to the bulk of the animal in whose blood-vessels they circulate. The corpuscles of Mammalia in general are like those of man in form and size, being either a little larger or smaller. The most marked exception is the blood of the musk-deer, in which the corpuscles are of extreme smallness, about the 1/12000th of an inch in diameter. In the elephant they are large, about 1/2700th of an inch in diameter. The goat, among common animals, has very small corpuscles, but they are, withal, twice as large as those of the musk-deer. In the Menobranchus lateralis they are of a much larger size than in any animal, being the 1/350th of an inch; in the proteus, the 1/400th of an inch in the longest diameter; in the salamander, or water-newt, 1/600th; in the frog, 1/900th; lizards, 1/1400th; in birds, 1/1700th; and in man, 1/3200th of an inch. Of fishes, the cartilaginous have the largest corpuscles; in gold-fish, they are about the 1/1700th of an inch in their longest diameter.

The large size of the blood discs in reptiles, especially in the Batrachia, has been of great service to physiologists by enabling them to ascertain many particulars regarding structure which could not have been otherwise determined with certainty. The value of the spectroscope in the chemical examination of the blood has been already referred to. See page 252.

White corpuscles or leucocytes (Fig. 425, I) differ materially from the red. They are large, spheroidal, finely granular masses of about 1/2800th of an inch in diameter. In a cubic millimètre of human blood there are about 10,000 white corpuscles. They have a lower specific gravity than the red, have no cell-wall, and their substance mainly consists of protoplasm. The internal granular appearance is now believed to be due to a fine intercellular network having small dots at the intersections of the web. In the meshes of the net a hyaline substance is interspersed. They possess one or more nuclei; these are seen on the application of a few drops of acetic acid. When examined in a perfectly fresh state, especially if the glass slide be placed on the warm stage of the microscope, they exhibit a spontaneous change of shape, amœba-like, such movements being accordingly termed amœboid. The movements referred to consist in the protrusion of processes of protoplasm which are retracted and other processes protruded as represented (Fig. 425, I). Both in human blood and in newts there are colourless corpuscles which contain coarser granules than others; these are called granular corpuscles. Some are shown near the amœboid bodies. The white corpuscles are readily found in various tissues of the body, as in the lymphatic glands. In inflammatory diseases these leucocytes pass through the walls of the capillaries into the tissues, and form morbid products, pus-cells.

Sections of blood discs are made by dipping a fine needle in a drop of blood as it exudes from a prick of the finger and drawing thin lines across the glass slip, allowing time to dry, and then cutting the lines across in all directions with a razor. The loosened portions should be removed with a camel’s-hair brush.

In birds, the blood discs are oval in shape and possess a nucleus, shown in Fig. 425 B, in the blood of the fowl; this is rendered more apparent on adding a drop of acetic acid. The blood of fishes is also oval and nucleated, rather more pointed than that of birds. In reptiles generally the red blood discs are large, oval, nucleated bodies, the white corpuscles still preserving their invariable circular form and granular appearance. In the salamander and proteus the discs attain to their greatest size. In the former they measure 1/700th of an inch, and in the latter 1/400th.

Blood Crystals.--In addition to the elements described, the blood contains various crystalline forms, represented in Fig. 425, C to H. In connection with the micro-spectroscope (p. 253), the spectra of certain blood crystals are given; although varying in different animals, sufficient uniformity prevails as to render them characteristic. The crystals are formed when a little blood is mixed with water on the slide, allowing a short time for crystallisation. Near the edge of the cover-glass, where crystals begin to form, they are more distinct, but a high power is required for their examination. In human blood the crystals are prismatic; in that of the guinea-pig, tetrahedral; in the blood of the mouse, octahedral. Other forms may be obtained by the aid of chemical reagents.

In human blood there are at least three distinct forms of crystals: Hæmatin is formed in normal blood, is made visible on the addition of a little water to blood, or by agitation with ether, so as to dissolve the cell-wall of the blood corpuscles, and allow the contents to escape. A drop of blood will furnish crystals large enough to be seen with a moderate power. Hæmatoidin crystals are abnormal products, found in connection with certain diseased conditions. These crystals are seen as represented at D. Hæmin crystals must be regarded as artificial chemical products, the result of treating blood with glacial acetic acid; the acicular crystals at E, reddish-brown in colour, are artificially produced.

Basement Membrane--Connective Tissue System.--Connective or areolar tissue is present almost throughout the whole of the human body, and serves to connect the various organs with one another, as well as to bind together the several parts. The muscles are surrounded by a connective tissue sheath; this penetrates into their substance, and binds together fasciculi and fibres. The same tissue is present in the skin and the mucous membranes; it also forms a sheath for the arteries, veins, and nerves. It is plentifully supplied by blood-vessels, and nerves pass through its substance. Microscopically, four different elements can be clearly made out:--1. Connective tissue cells or corpuscles; 2. White fibrous tissue; 3. Yellow fibrous tissue; 4. Ground substance.

On examining the connective tissue cells of young animals, various cells will be seen with fine granular contents, together with nuclei, lying in spaces in the ground substance, some branched, others flattened or rounded. Even tissues supposed to be homogeneous in structure, are on staining seen to have connective tissue cells, such as those represented in a section of the cornea of the eye (see p. 31). In this case the connective tissue cells are termed corneal corpuscles; the branched cells, it will be noticed, are united by branches.

The cells in the fibrous tissue of tendons are square or oblong, and form continuous rows. White fibrous tissue is distributed throughout the animal body, but in a variety of forms; it is found in the skin and other membranes, and in all parts where strength and flexibility are necessary. The structure of white and yellow fibrous tissues is shown in Figs. 426 and 427.

White fibrous tissue presents silver-lustre bundles, running for the most part in parallel directions through and over the muscles and tendons. For examination under the microscope, obtain a fragment of fresh meat cut in the longitudinal direction; place it in water, and tease it out with needles as directed in a former chapter. The smallest fragment will suffice for examination under a quarter or one-sixth inch objective. These filaments are exceedingly minute, measuring 1/3000th to 1/2500th of an inch in diameter, and do not interlace through the bundles, although they intersect each other occasionally. Transverse sections may be made by drying a piece of tendon until it becomes sufficiently firm to cut with a razor or microtome, and mounted as a permanent specimen. From the cut ends of the fibres small dark points will be seen, especially in the denser structure of the tendons; these are termed “connective tissue corpuscles.”

Yellow elastic fibrous tissue is remarkable in contradistinction to the white for its elasticity and capability of extension. It is found on the coats of blood-vessels, between the vertebral arches, and in quadrupeds it forms a strong elastic band, extending from the occiput, throughout the spines of the vertebra, and enabling the animal to support the head in the pendent position, without muscular exertion. These fibres can only be separated from each other with difficulty, and their elasticity is shown by a tendency to curl up. These yellow fibres are somewhat coarser than the white, and they remain unaffected by acetic acid of the ordinary strength. Elastic tissue is a constituent of the skin, mucous, and serous membranes, and of the areolar or cellular tissue.

In order to microscopically examine this structure, take a small portion of the strong ligament of the neck of the ox, place it as before in water, and tease it out with needles; place a fragment on a glass slip, cover with a thin cover-glass, and submit it to a high magnifying power. Transverse sections made as directed in the case of white tissue will be seen to be hexagonal in form.

Adipose Tissue.--Fat is found in many situations in the animal body, and on examination is seen to consist entirely of vesicles, distributed through a delicate membrane of connective tissue, shown in Plate XIX., Nos. 4 and 5. On pressure, the circular or oval form of the cells becomes polyhedral; occasionally the fatty acids in the interior of the vesicles crystallise, and give rise to a star-like appearance. For the examination of adipose tissue, take a portion of the mesentery of any small animal--a mouse, or rat.

Retiform Tissue.--Adenoid, or retiform tissue, consists of a delicate network of connective tissue corpuscles, joining their branches together. This forms the stroma or framework of lymphoid tissue. It is found in connection with all the lymphatic glands, spleen, &c. Plate XIX., No. 3, a b, shows small sections of a lymphatic, together with capillary vessels.

Muscular Fibre.--There are two varieties of muscular fibre in the body--i.e., striated, and non-striated. The striated is formed in muscles attached to bony structures, as those of the arm and leg, and in some of the soft structures, as the tongue, palate, œsophagus, in short, all muscles under the control of the will. Striped muscle is of a dull red colour and marked with peculiar longitudinal furrows on its surface. Voluntary muscle consists of:--1, a connective tissue sheath; 2, fasciculi; 3, fibres and sarcolemma; 4, discs, fibrilla and sarcous elements. These are shown in connection with other tissues in Plate XIX., Nos. 11 and 12, and also in Fig. 428 (1, 2, 3).

In Plate XIX., Fig. 11, the muscular fibre taken from the tongue of a lamb shows the continuity of the upper portion with the connective tissue membrane. In Fig. 12, a branching-out bundle of muscular fibre, taken from the upper lip of the rat, is seen to end in stellate connective cells. The delicate homogeneous sheath that binds the fibres together is termed sarcolemma. This is readily seen in prepared muscle of the frog and water-beetle, less plainly in man. Each muscle is provided with a sheath of connective tissue; this surrounds it, binds the fasciculi together, and supports the blood-vessels; it is called the perimysium, and sends fine prolongations in between the fibres, termed endomysium. The intervals seen on high amplification between the dark striæ are called Kruse’s membrane. On breaking up the striated structure it is resolvable into fibrillæ and furthermore into discs.

Among mammalia the pig furnishes the best examples of muscle fibrillæ; among insects the water-beetle and the thorax of the housefly. A power of 600 or 800 diameters is required to separate the fibrillæ. Blood-vessels are well supplied with striated muscle, but none of their minuter branches penetrate the sarcolemma. The involuntary or non-striated variety of muscular fibre exists in all parts of the body where movements occur independently of the will, also in the ciliary muscle and the iris of the eye, as well as in the middle coats of the arteries. Non-striated fibres are pale in colour, prismatic in shape, and easily flattened by pressure. In size, they vary from 1/7000th to 1/3500th of an inch in diameter, and are marked at short intervals by oblong corpuscles.

The Integument or Skin consists of epidermis or cuticle, dermis, corium or cutis vera, sweat-glands, nails, hairs, sebaceous glands, and numerous nerves and vessels. The epidermis forms a protective covering over the whole surface of the body, and is moulded on to the surface of the corium beneath, covering the ridges, depressions and papillæ. It is made up of three principal layers: the horny layer or stratium corneum, the most superficial, this consists of layers of flattened cells, which are without a nucleus; the stratum lucidum, composed of layers of nucleated cells, more or less indistinct in section; the rete mucosum or malpighian layer; is composed in its upper part of layers of “prickle cells” and its inferior of a single stratum of columnar cells. Pigment is principally found in the lowest layer, Fig. 429.

The gradations of colour in the skin are due to the granular contents of the pigment cells. This is seen on steeping sections cut from the skin of a negro in chlorine; the colour is discharged. In Plate XIX., No. 13, the pigment cells of the choroid coat of eye are shown. Here the pigment is darker in colour, and its function is the absorption of light and the prevention of disturbing effects occasioned by circles of dispersion.

The Dermis, or true skin, consists of an interlacing network of connective tissue, yellow elastic tissue corpuscles, vessels, and nerves. There are also small muscular fibres in connection with the hair follicles, and beneath the subcutaneous tissues contain an abundant supply of fat adipose tissue. Numerous ridges are seen on the surface, especially on the palm of the hand and sole of the foot, caused by rows of little elevations of the cutis vera, termed papillæ. These are more or less conical, and contain a capillary loop, nerve, and touch corpuscle, which serve to increase the sensitiveness of the part, lodging a touch corpuscle in a favourable position for receiving sensations of touch, Fig. 430.

Sweat glands are situated in the subcutaneous tissue, and consist of fine tubes, which form the duct (seen in the section, Fig. 430); these are continuous with a blind extremity, coiled up into a ball one-sixtieth of an inch in diameter, and surrounded by a plexus of capillaries to form the gland (Fig. 431, No. 2). Between the layer of columnar cells and the limiting membrane is a layer of non-striated muscle, and beneath the rite mucosum there are several layers of polyhedral cells, and an external and internal limiting membrane; the epithelium of the duct is at its mouth continuous with the epithelium of the epidermis.

Nails consist of a root and body, the lunular of which is the whitish portion of the body near the root, where the skin beneath is less vascular than any other portion of the finger. The nail closely resembles the epidermis, and consists of hard and thin layers of cells on the surface, and round, moist cells beneath. Posteriorly the nail fits into a groove which lodges its root. The part to which the nail is attached is known as the nail-bed. The stratified appearance produced by the coalescence of the cells, and their lying over each other, is shown in Plate VII., No. 149, the toe of the mouse; while the special arrangement of tissue is better seen under polarised light (Plate VIII., No. 174).

Hairs consist of a shaft and root. The shaft is cylindrical, and covered with a layer of imbricated scales, arranged with their edges upwards. The substance of the hair consists of fibres, or elongated fusiform cells, in which nuclei are seen. There are present in some hairs (Fig. 432) small air spaces or lacunæ. In the coarser hair of the body there is a pith (medulla), occupied by small angular cells and fat granules.

The root of the hair is seen to dilate that it may fit more firmly into the skin hair-follicle. The latter consists of two coats, an outer and an inner, continuous with the epidermis, and this is called the root sheath. The outer portion consists of three layers, formed of connective tissue, blood-vessels, and nerves. The inner, or epidermic, coat comes away when the hair is pulled out, and hence is called the root sheath. This again is made up of two layers, the outer of which corresponds with the horny layer, and is composed of flattened cells. The bulbous root of the hair is connected with the papilla. In the cat the tactile nasal hairs are very large. Small bundles of involuntary muscular fibres connect the corium with the root, so that in contracting they elevate or expand the hair.

The hair of the lower animals presents a diversity of structure, especially on the outer surface, and with reference to the arrangement of the scales. The hair of the Indian bat, for instance, consists of a shaft invested with erectile scales, placed at regular intervals; these stand out from the shaft, as in Fig. 433, No. 1. This form of scale varies considerably in the different species of these animals, and a portion of hair near the root is nearly divested of scales. Many of the scales are not unlike those of certain of the insect tribe, seen in that of Dermestes, No. 3, while the hair of the mouse has a series of transverse imbricated scales arranged as tiles on a house, due to accumulated pigment. Hairs taken from various animals form interesting objects of study for the microscope, as already noticed. Other hairs are shown in Fig. 434. No. 1 is a transverse section of a hair from the ant-eater; the central part consists of air-cells, the outer of a granular pith. No. 2 is a transverse section of hair of peccary, with a diversified arrangement of the cortical envelope, sending outward a set of radial prolongations and air-cells; this kind of structure is also found in the quills of the porcupine. No. 3 is a transverse section of a hair of the elephant, which shows a combination of a number of tubes united together, somewhat resembling the arrangement of the hoof-horn of some of the ruminants, and the denser horny growth on the snout of the rhinoceros, No. 4. The curious modification of these horny structures is seen in the horns of other animals, and which may be likened to a bundle of hairs. On making a transverse section, as in Fig. 434, and submitting it to polarised light, on rotating the analyser, the dark central spot shown is replaced by a bright one with a play of colours due to the interference of light (Plate VIII., No. 178). The scales of fish are also of interest (Fig. 435). These have been shown to afford an unerring guide in the classification of fishes and in the examination of their fossil remains. As a class of objects for the microscope, they are found to be both curious and beautiful. Plate VIII., No. 176, is a scale of the grayling, seen under polarised light.

Of the harder outgrowths of the dermal structures, the teeth afford the chief example among animals. The rough anatomy of the tooth in mankind consists of a crown, that projects from the gum; a root, or fangs, fixed in a socket of the jawbone, and a short intermediary neck. Each tooth is supplied with an artery and nerve, and has a central cavity filled with a soft, vascular, sensitive substance, the pulp. On making a vertical section of a tooth, we recognise the several structures in the order of, pulp, crusta, petrosa, dentine, and enamel. A section through a human molar tooth (shown in Fig. 436) will convey some idea of the arrangement of the denser structures referred to above.

Blandin was the first to demonstrate that teeth are developed in the mucous membrane, similar to that of hair and nails. Teeth are formed in grooves of the mucous membrane, and subsequently converted into closed sacs by a process of involution, and their final adhesion to the jaw is a later process. It is very generally conceded that teeth belong to the muco-dermoid, and not to the periosteal, series of tissues; that, instead of standing in close relation to the endo-skeleton, they are part of the dermal or exo-skeleton; their true analogues being the hair, and some other epidermic appendages. Huxley proved that, although teeth are developed in two ways, they are mere varieties of the usual mode in the animal kingdom. In the first, which is typified by the mackerel and the frog, the pulp is never free, but from the first is inclosed within the capsule, seeming to sink down as fast as it grows. In the other, the pulp projects freely at one period above the surface of the mucous membrane, becoming subsequently included within a capsule formed by the involution of the latter; this occurs in the human subject. The skate offers a sort of intermediate structure.

The enamel forms a continuous layer, and invests the crown of the tooth; it is thickest upon the masticating surface, and decreases towards the neck, where it usually terminates. The external surface of the enamel appears smooth, but is always marked by delicate elevations and transverse ridges, and covered by a fine membrane (Nasmyth’s membrane), containing calcareous matter. This membrane is separable after being subjected to hydrochloric acid; it then appears like a network of areolar tissue, shown in Fig. 438, No. 6; Huxley’s “calcified membrana,” which commence at the pulp cavity, and pass up to the enamel.

Czermak discovered that the curious appearances of globular conglomerate formations in the substance of dentine depend on its mode of calcification and the presence of earthy material; and he attributed the contour lines to the same cause. Contour markings vary in intensity and number; they are most abundant in the root, and most marked in the crown. Vertical sections exhibit them the best; as Fig. 440, No. 1. In preparing a specimen, first make the section accurately, then decalcify it by submersion in dilute hydrochloric acid; dry it and mount in Canada balsam; place the specimen in the hot chamber for some time to soak in the fluid resin before it cools. The white opacity at the extremity of the contour markings gives the appearance of rings to the tooth-fang.

“The tooth-substance appears,” says Czermak, “on its inner surface, not as a symmetrical whole, but consisting of balls of various diameter, which are fused together into a mass with one another in different degrees, and in which the dentine tubes in contact with the germ cavity terminate. By reflected light, dark-ground illumination, one perceives this stalactite-like condition of the inner surface of the tooth-substance very distinctly, by means of the varied illumination of the globular elevations, and by the shadows which they cast.” To see this structure to advantage the preparation should be made from a tooth root, the growth of which is not complete. With such preparations, the ground-substance of the last formed layer of the tooth-substance is seen to be, at least partly, in the form of globular masses, fused together with those of the penultimate layers.

The cementum is the cortical layer of osseous tissue, forming an outer coating to the fangs, which it sometimes cements together. Its internal surface is intimately united with the dentine, and in many teeth it would appear as if the earliest determined arrangement of the fibres of the dentine started from the canaliculi, as they radiate from the lacunæ in the cement. The inter-lacunar layer is often striated, and exhibits a laminated structure: sometimes it appears as if Haversian canals were running in a perpendicular direction to the pulp cavity. The canaliculi frequently run out into numerous branches, connecting one with another, and anastomising with the ends of the dentine fibres. The thick layers of cement which occur in old teeth show immense quantities of aggregated lacunæ of an irregular and elongated form.

Compact Tissues, Cartilage and Bone.--Cartilage is a bluish or yellowish-white, semi-transparent, elastic substance, without vessels or nerves, and surrounded by a membrane, termed pericondrium, of a dense fibrous nature. That kind, however, known as articular cartilage, receives a layer of epithelium from the synovial membrane, but this is confined to marginal portions, in consequence of the central wear which occurs as soon as the parts are subjected to friction, during the movement of the limbs. Cartilage covers the ends of all bones in apposition to form joints, and thus lessens the effects of concussion. Besides the ordinary kind of cartilage, temporary and permanent, there are two modifications of the tissue, confined to certain portions of the body: cellular cartilage, composed of cells lying close together, in a mesh formed of fine fibres; and fibro-cartilage, cells distributed in a matrix of fibrous tissue.

Examined with a low power, cartilage appears to be homogeneous in structure, studded over with numerous round, oval, oblong, semilunar, and irregular-shaped corpuscles, as seen in Plate XIX., No. 8, a vertical section of animal cartilage, arranged in columns, and condensed at the lower surface previous to its conversion into bone. The greater opacity of this portion is owing to the increase of osseous fibres, and the multiplication of oil globules, and the intercellular spaces becoming filled with vessels. No. 9 shows a small transverse section of the same, with a further change of the cartilage cells at a into bone cells, and at b with the characteristic canaliculi and lacunæ. No. 7 further shows a section of the large tendon fixed to the back of the heel of the foot, near the juncture of the tendo-Archillis with the cartilage. For the examination of these several changes a high power is necessary, and for the purpose pieces taken from the ox may be easily obtained from the butcher, and fine sections cut with a razor parallel to the surface.

The better specimens for microscopical examination are those taken from very young animals, in whom the ossific process is still incomplete. In order to examine cellular cartilage, the ear of the mouse should be taken and just dried sufficiently to enable fine sections to be cut by the microtome transversely (Fig. 440).

Cartilage forms the entire skeleton of a certain number of fishes, as the skate, lamprey, ray, shark, &c., the cells of which are embedded in a matrix of granular matter, which has been properly termed intercellular. The nearest approach to ossification of cartilage in fishes is that of the cuttle-fish; in this stellate cells are freely distributed, as shown in Fig. 441, No. 3.

White fibro-cartilage occurs between the bodies of the vertebræ as a connecting medium. In this kind the cells are more widely distributed, specimens of which may be taken from the central portion of an interarticular disc of any animal. The oval or circular corpuscles will be seen surrounded by an abundance of fibrous tissue.

An acquaintance with the degeneration of the textures with which we have been dealing may be of service to the student, as he may, in the course of his examination, meet with an abnormal condition altogether different to those described. The process of degeneration is usually a slow one, except in the case of fatty infiltration, an example of which is furnished by the fatty degeneration of the liver in Strasburg geese. Muscular tissue is very prone to fatty degeneration, and fatty heart is often met with. Calcareous degeneration of the muscles, ligaments, and cartilages, as well as morbid deposits, are not at all uncommon in these structures. In Plate XIX., No. 9, a small section is given of an enchondroma, and in which the round or ovoid cells of the cartilage are seen degenerated and converted into granular masses of a calcareous nature. Fig. 442 is a somewhat more highly magnified section of a calcareous or morbid growth, taken from a human subject in which a morbid growth was seen to be gradually destroying the bone and cartilage cells.

Bone.--Bone is a hard unyielding structure, and which in the vertebrata forms the skeleton of the adult. It is the framework for the support of the soft tissues of the body, and forms various cavities for the reception of important organs, as the brain, spinal cord, eyes, heart and lungs, and acts as levers for the action of the muscles and joints. The partial elasticity of bone is seen in the ribs, and the rebound when the skull is dropped on the ground. Bone consists of earthy and animal matters intimately combined; the removal of either, however, does not destroy the form of the bone, if the process of separation be carefully conducted. The earthy constituents may all be dissolved out by hydrochloric acid, but the form of the bone is preserved in its minute particular, and in this state sections may be cut for microscopical examination. If allowed to become dry it shrivels, and assumes the density of horn. The interior of a bone is of a spongy or cancellated structure, particularly at the ends. The outer portion of the bone is more dense than the internal part. The study of bone should commence with sections of the softened structure. Directions for making sections of bone are given in the chapter on Practical Microscopy.

The intimate structure of bone will be studied in connection with Plate XX. Two series of lamellæ may be demonstrated in bone after maceration in acid, a larger system surrounding the medullary canal, and a smaller surrounding the Haversian canals, both of which are seen in Nos. 1 and 2. In macerating bones, the lamellæ of the layer concentric system may be peeled off in layers; these are seen to be pierced with fine apertures, caused by the canaliculi. In some parts larger apertures are seen through which bundles of fibres pass, pinning, as it were, the several layers together; these are the perforating fibres. The outermost of the layers, being near the periosteum, the membrane covering the bone, are termed periosteal layers; the innermost, being close to the canal, are called medullary layers. No. 1 is a transverse section of a flat bone, the clavicle, and it shows the Haversian canals, varying in size from 1/2000th to 1/200th of an inch in diameter, the largest being near the medullary canal. In shape they are round, oval, or oblong, according to the line of section. Each canal is surrounded by rings, none of which are complete, and running one into the other at various parts. Under a higher power, those irregular shaped bodies termed lacunæ, with fine radiating fibres, are seen to be smaller canals, canaliculi.

By means of this complete and intricate distribution of the canals of the Haversian system, the nutritive fluids pass into the most compact parts of the osseous tissue. Longitudinal sections of the long bones show these canals as continuous branching-out cells.

In many of the lower animals the bony structure differs from those of man, as will be seen in Plate XX. No. 3 shows a transverse section of the femur, or leg-bone of an ostrich, magnified ninety-five times, in which the Haversian canals are much smaller and more numerous, and many of them run in the transverse direction. No. 4, again, is a transverse section of the humerus, or fore-arm bone of a turtle (Chelonia mydas). This exhibits traces of Haversian canals, with a slight tendency to a concentric arrangement of bone-cells around them, the bone-cells being large and numerous, and occur, for the most part, in parallel rows. In No. 5, a horizontal section of the lower jaw-bone of a conger-eel exhibits a single plane of bone-cells arranged in parallel lines. There are no Haversian canals present, and when this specimen is contrasted with that of No. 4, it will be noticed that the canaliculi given off from each of the bone-cells of this fish are very few in number in comparison with that of the reptile. No. 6 is a section of a portion of the cranium of a siren (Siren lacertina), remarkable for the large size of the bone-cells, and of the canaliculi, which are larger in this animal than in any other yet examined; and as in the preceding specimen, no Haversian canals are present. No. 7 is a section of bone taken from the exterior of the shaft of the humerus of a Pterodactyle; this exhibits the elongated bone-cells characteristic of the order Reptilia. No. 8 is a horizontal section of a scale, or flattened spine, from the skin of a Trygon, or sting ray; this exhibits large Haversian canals, with numerous wavy parallel tubes, like those of dentine, communicating with them. This specimen shows, besides wavy tubes, numerous bone-cells, whose canaliculi communicate with the tubes, as in dentine.

The following points may be noted with regard to the several sections of bone described. That of the bird, for instance, contrasted with that of the mammal, exhibits the following peculiarities: the Haversian canals are more abundant, much smaller, and often run in a direction at right angles to that of the shaft, by which means the concentric laminated arrangement is in some cases lost; the direction of the canals follows the curve of the bone; the bone-cells are much smaller and more numerous; while the number of canaliculi sent off from the cells is less than in those of mammals. No. 3 is the average length of a bone-cell of the ostrich, 1/2000th of an inch, in breadth 1/6000th.

In the Reptilia, the bones may be either hollow, cancellated, or solid; and their specific gravity is less than that of birds or mammals. The short bones of most of the chelonian reptiles are solid, and the long bones are either hollow or cancellated; the ribs of the serpent-tribe are hollow, the medullary cavity performing the office of a Haversian canal; the bone-cells are accordingly arranged in concentric circles around their canals. The vertebræ of these animals are solid; and the bone, like that of certain birds, is remarkable for density and whiteness. When a transverse section is taken from one of the long bones, and contrasted with that of a mammal or bird, the difference will be noticed; there are very few, if any, Haversian canals, and these are large; and at one view, in the section, No. 7, the canals and bone-cells are arranged both vertically and longitudinally. The bone-cells are remarkable for the great size to which they attain; in the turtle they are 1/375th of an inch in length, the canaliculi are extremely numerous, and are of a size proportionate to that of the bone-cell.

In fishes a greater variation occurs in the minute structure of the skeleton than in either of the three preceding classes. A rare structure is that of the sword of the sword-fish (Istiophorus). In this, Haversian canals and a concentric laminated arrangement of the bone are found, but no bone-cells. The Haversian canals, when they are present, are of large size, and very numerous, and then the bone-cells are, generally speaking, either absent or but few in number, their place being occupied by tubes or canaliculi, which are often of a very large size. The bone-cells are remarkable for their graduate figure, and the canaliculi derived from them are comparatively few in number. In a thin section of the scale of an osseous fish, the cells lie nearly all in one plane, and the anastomoses of the canaliculi are more distinctly seen; in the hard scales of many, as the Lepidosteus and Calicthys, and in spines of the Siluridæ, the bone-cells are well differentiated. In the true bony scales comprising the exo-skeleton of cartilaginous fishes the bone-cells are seen in great numbers.

Now, if we proceed at once to the application of the facts which have been laid down, and make a fragment of bone of an extinct animal the subject of investigation, it will be found that the bone-cells in Mammalia are tolerably uniform in size; and if we take 1/2000th of an inch as a standard, the bone-cells of birds fall below that standard; but the bone-cells of reptiles are much above either of the two preceding, while those of fishes are essentially different, both in size and shape, and are not likely to be mistaken for one or the other; so that the determination of a minute yet characteristic fragment of fishes’ bone is a task easily performed. If the portion of bone does not exhibit bone-cells, but presents either one or other of the characters indicated, the task of discrimination is equally easy. We have now the mammal, the bird, and the reptile to deal with. In consequence of the very great size of the cells and their canaliculi in the reptile, a portion of bone of one of these animals can readily be distinguished from that of a bird, or a mammal. The only difficulty lies between these two last; but, notwithstanding that on a cursory glance the bone of a bird appears very like that of a mammal, there are certain points in their minute structure in which they differ; and one is the difference in size of the bone-cells. To determine accurately, therefore, between the two, we must, if the section be a transverse one, also note the comparative sizes of the Haversian canals, and the tortuosity of their course; for the diameter of the canal bears a certain proportion to the size of the bone-cells, and after close examination the eye will readily detect differences.

Arteries and Veins.--The circulation of the animal frame is maintained by arteries, veins, and capillaries. The arteries are elastic and contractile tubes; these convey the blood from the heart to the capillaries. The larger arteries are exceedingly elastic, but feebly contractile on account of the muscular tissue in their walls. The veins ramify throughout the body, are more numerous than the arteries, and of greater capacity. They usually accompany the arteries and correspond to them in structure, the larger veins possessing semi-lunar valves; these project into their interiors, and thus prevent the regurgitation of the blood. They have four coats, consisting of areolar tissue, yellow fibres combined with muscular fibres, and white fibrous tissue, two layers of yellow fibres arranged longitudinally, and a single layer of epithelial cells. Intermediate between the arteries and veins there are exceedingly fine tubes, termed capillaries, in which the arteries terminate, and from which the veins arise. These are composed of a fine homogeneous membrane, with here and there a nucleus. The capillary circulation of the blood is readily seen in the tail of the newt and the foot of the frog, Fig. 443.

A network of capillaries conveying blood to the lungs, and ramifying throughout the structure, is shown in Fig. 444, and in Plate XIX., No. 6, the termination of a capillary of a blood-vessel in the fat-cells of the human body. Plate VII. illustrates the distribution of the arteries and veins to various parts of the animal body. This coloured plate, however, is designed to show the value of injected preparations in the delineation of animal structures. By thus artificially restoring the blood and distending the tissues, a much better idea is obtained of the relative condition of parts, the appearance presented by the erectile papillæ, &c. In the section of foot of mouse (No. 149), the bone is seen surrounded by its vascular supply, arterial and venous; in No. 150, the papillæ of the tongue are distended and seen erect; in No. 152, a vertical section of the fungi-form papillæ on the tongue of cat, with capillary loops passing into them, is demonstrated; in No. 151, the vertical section of brain of a rat, the vascular supply is shown; No. 153, the malpighian tufts (circular bodies) and arteries ramifying about the structure; in No. 154, the vertical section through the intestine of the rat, shows villi (arteries and veins) surmounted by epithelium, and supported on a layer of the mucous membrane; in No. 155, the vascular supply sent to the roots of the whisker of the nose of the mouse; in No. 157, a tangential section cut through the several textures, the sclerotic coat and retina of the eye of a cat is clearly made out although not highly magnified; again, in No. 156, the beautiful vascular arrangement of the internal gill of the tadpole could scarcely be so strikingly illustrated in any other way; while in the central, No. 158, the vascular system throughout the whole of the body of a fully developed tadpole, with the way in which the blood is carried from the remotest part of the tail to the heart, and sent to the gills, the brain, &c., it is quite unnecessary to enlarge upon. These are seen under a low power, but for the purpose of studying the basement membrane, together with the intimate association and termination of the nerves accompanying the arteries and veins, it is absolutely necessary to resort to a staining process, and cutting fine sections with the microtome. Small portions of a nerve may be cut off with fine scissors, teased out with needles, and a drop of acetic acid added to render the sheath more transparent; in a few seconds the connective tissue corpuscles will be brought into view. For the microscopical examination of nerve-fibrillæ take a small section from the leg of a frog, and tease it out in blood serum or white of egg. In size the fibrillæ vary, even in the same nerve, from the 1/12000th to the 1/1500th of an inch in diameter.

To show the circulation of the blood in the frog’s foot, and without causing the animal pain or much inconvenience, it is better to enclose it in a black silk bag, and draw out the foot as shown at a a a, Fig. 445. The bag provided should be from three to four inches in length, and two and a half inches broad, shown at b b, having a piece of tape, c c, sewn to each side, about midway between the mouth and the bottom, and the mouth itself capable of being closed by a drawing-in string, d d. Into this bag the frog is placed, and only the leg which is about to be examined kept outside; the string d d must then be drawn sufficiently tight around the small part of the leg to prevent the foot from being pulled into the bag, but not to stop the circulation; three short pieces of thread, f f f, are now passed around the three principal toes; and the bag with the frog must be fastened to the plate a a by means of the tapes c c. When this is accomplished, the threads f f f are passed either through some of the holes in the edge of the plate, three of which are shown at g g g, in order to keep the web open; or, what answers better, in a series of pegs of the shape represented by h, each having a slit, i, extending more than halfway down it; the threads are wound round these two or three times, and then the end is secured by putting it into the slit i. The plate is now ready to be adapted to the stage of the microscope: the square opening over which the foot is secured must be brought over the aperture in the stage through which the light passes from the mirror.

The tadpole circulation is readily seen by placing the creature on its back, when we immediately observe the beating heart, a bulbous-looking cavity, formed of delicate, transparent tissue, through which the blood alternately enters by one orifice and leaves by a more distant exit. The heart, it will be noticed, is enclosed within its pericardium, this being the more delicate part of the creature’s organisation. The binocular microscope should be used for viewing the circulation. Passing along the course of the great blood-vessels to the right and left of the heart, the eye is arrested by a large oval body, of a more complicated structure. This is the inner gill, formed of delicate, transparent tissue, traversed by arteries, and a network of blood-vessels. It is almost unnecessary to say the tadpole has a respiratory and circulatory system resembling those of fishes.

In nearly all fish the heart has but two cavities, an auricle and ventricle; the blood is returned by the veins to the auricle, passes into the ventricle, and is then transmitted to the gills, where, being exposed to the air contained in the water, it becomes deprived of carbonic acid, aerated, and rendered fit to breathe. In the reptile we find a modification of plan. The heart has three cavities, two auricles and one ventricle; by this contrivance there is a perpetual mixture in the heart of the impure carbonized blood which has already circulated through the body, and flows into the ventricle from the right auricle, with the purer aerated blood returned from the lungs, which flows at the same instant into the ventricle from the left auricle.

For the purpose of subsequent observations the tadpole should be selected at a period in which the skin is perfectly transparent, otherwise the appearances already described of the form and situation of the heart, and the three great arterial trunks (proceeding right and left), will not be clearly made out. The anatomical arrangement of the vessels will be seen to be closely connected with the corresponding gill, the upper one (the cephalic) running along the upper edge of the gill, giving off, in its course, a branch which ascends to the mouth, with its accompanying vein; this is termed the labial artery and vein. The cephalic artery continues its course around the gill, until it suddenly curves upwards and backwards, and reaches the upper surface of the head, when it dips down between the eye and the brain.

It must not be supposed that this can be made out in the average tadpole, the obstacle to which is the large coil of intestines, usually distended with dark-coloured food. This must first be reduced by making your tadpole live on plain water for some days. Plate VII., No. 158, affords a view of the vessels obtained under the influence of low diet, and whereby we are enabled to trace the course of the three large arteries. The third trunk, traversing the lung, is seen to emerge from the lower edge and descend into the abdomen to form the great abdominal aorta. A small half-starved tadpole shows the heart beating and the blood circulating, but the latter is quite colourless, not a single red globule visible anywhere. The heart is a colourless globe, the gills two transparent ovals, and the intestines a colourless, transparent coil. Through the empty coil the artery is seen on either side leaving the gills, and converging towards the spine, and uniting to form the abdominal aorta, the large central vessel coloured red in the figure. After the aorta has supplied the abdominal viscera, a prolongation, or caudal artery is seen descending to the tail, the all-important organ of locomotion in the tadpole. This artery, entering the root of the tail, is imbedded deeply in the flesh, whence it emerges, and then continues its course, closely accompanied by the vein, to within a short distance of the extremity, where, being reduced to a state of extreme fineness, it terminates in a capillary loop, composed of the end of the artery and the beginning of the vein. The artery, in its course, gives off branches continually to supply the neighbouring tissue. The blood-current in the tail is often seen, even in the main artery or vein, to be sluggish. This occurs independently of the heart, which will continue to beat as usual; it happens, because the circulation in the tail depends very much on the motion of the organ. When this is suspended (as in the confined tadpole under the microscope), the blood moves sluggishly, or stops, till the tail regains its freedom and motion, when the activity of the current is restored.

Having traced the arterial system which conveys the blood from the heart to the extremities, we will now note its return by the veins back again to the heart.

The caudal vein runs near the artery during the greater part of its course, with its stream of blood towards the heart. This stream is swollen by perpetual tributaries from numerous vessels. As the vein approaches the root of the tail it is inclined towards the artery, and diverges from it at the point of entering the abdomen. Here it approaches the kidneys and sends off branches, while the main trunk continues its course onward; and, passing upwards behind a coil of intestine, it approaches the liver, and runs in a curved course along the margin of that organ. The blood is now seen to enter the vena cava by several channels, that converge towards the great vein as it passes in close proximity to the organ. Beyond the liver the vena cava continues its course upwards and inwards to its termination in the sinus venosus or rudimentary auricle of the heart. This termination is the junction of not less than six distinct venous trunks, incessantly pouring their blood into the heart. The circulation in the fringed lips forms a most complicated network of vessels, out of which proceeds a vein corresponding to the artery already traced. This descends in a direct course till it joins the principal vein of the head, which corresponds to the jugular in the mammalia.

Thus it will be seen the blood is driven by the heart into each inner gill through three large blood-vessels, which arise directly from the truncus arteriosus, and may be called the afferent vessels of the gill. In Plate VII., No. 156, an enlarged view of a gill is shown.

On closer examination “each internal gill or entire branchial organ is seen to consist of cartilaginous arches, with a piece of additional framework of a triangular form, stretching beyond the arches, composed of semi-transparent, gelatinous-looking material. These form the framework of the organ and support upon their upper surface the three rows of crests with their vascular network, and the main arterial and venous trunks lying parallel to and between them. The three systemic arteries arising, right and left, from the truncus arteriosus, enter each gill on its cardiac side, and then follow the course of the crests, lying in close proximity to them. The upper of these branchial arteries runs alone on the outside of the upper crest, and another branch leaving the trunk and passing into the network of the crest, whence a returning vessel may be traced carrying back the blood across the branchial artery, and to a vessel lying close to and taking the same course as the artery itself. Carrying the eye along the latter vessel we find, at a short distance from the first of these crest branches, a second, leaving the main trunk and entering the crest, when a corresponding returning vessel conveys the blood across the arterial trunk into the vessel lying beside it, as in the former instance. A number of these branches may be traced from one crest to the other. But it is now seen that the trunk from which these arterial branches spring diminishes in size as it proceeds in its course (like the gill artery in fishes), while the vessel running parallel to it and receiving the stream as it returns from the crest enlarges to some extent. Thus, the artery or afferent vessel which brings the blood to the gill is large at its entrance, but gradually diminishes and dwindles to a point at the opposite end of the crest; while the venous or efferent vessel, beginning as a mere radical, gradually enlarges, and thus becomes the trunk that conveys the blood out of the gill to its ultimate destination. This vessel is the upper branchial vein so long as it remains in contact with the gill; subsequently it changes its name on leaving the gill and as it passes upwards for distribution to the head, when it is designated the cephalic artery. The middle branchial artery and vein proceed in like manner in connection with the middle crest, and the lower artery and vein in connection with the lower crest. The middle and lower venous trunks, having reached the extremity of the crests, curve downwards and inwards, and leave the gill. The former trunk, converging towards the spine, meets its fellow, and with it forms the ventral aorta. The latter gives origin to the pulmonary artery, and supplies also the integuments of the neck. Curious and interesting is the final stage of the metamorphosis, when the waning tadpole and incipient frog coexist, and are actually seen together in the same subject. The dwindling gills and the shrinking tail--the last remnants of the tadpole form--are yet seen, in company with the coloured, spotted skin, the newly formed and slender legs, the flat head, the wide and toothless mouth, and the crouching attitude of the all but perfect reptile.”[87]

To observe the circulation and how it is carried on during life in the gills, the outer covering must be carefully raised, or even stripped off. This will be better accomplished by putting the tadpole under the influence of cocaine or chloroform--a drop of the fluid is sufficient for the purpose.

The metamorphosis in the embryo of the frog is by no means exceptional. The ascidian begins life in the form of a tadpole, with a muscular tail; subsequently it fixes itself by its head to a rock, and its tail disappears. The changes the tadpole of the frog passes through are in every respect, except in one or two minor details, similar to those of adult amphibia which pass their whole lives in water. The newly-hatched flat-fish is symmetrical, an eye being placed on each side of its head, with the adults of other fishes. The fœtal whale has well-developed hind limbs, and which, after passing into a condition almost perfect in proportion to the rest of the body, gradually dwindle away again to the merest rudimentary structures. In all these, and a number of similar cases, it is seen that the earlier condition of existing animals represents, and is in agreement with that of its adult ancestor of a remote period in the past. Collected facts bearing upon this question have been made the groundwork of a theory of hereditary properties in the germ, and a disposition to go through the same phases of life as the parent.

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