THE tissues of which animals consist, like those of plants, are primarily derived from cells; in fact the essential part of the egg or Óvum, from which all perfect animals originate consists at first only of a simple cell, with its nucleus and nucleolus.

The animal cell-wall differs from that of the vegetable cell in its softness and delicacy—also in its chemical composition,—the former consisting of albÚminous (albÚmen, white of egg) matter, while the latter is composed of cellular or vegetable-cell substance.

There is also a striking difference between vegetable and animal tissues, in the circumstance that, while the former retain their cellular condition to a very great extent, the cells of the latter are frequently so altered by compression and fusion together, or are obscured by the great development of the cell-contents, that the cell-form is obliterated, or can only be discovered by the application of chemical reagents; and in many instances, the relation of the tissues to the cell can only be discovered by tracing the growth or development of the latter from its earliest stages. Hence the examination of the elements and tissues of animals is not well adapted for those who are unpractised in the use of the microscope; and in treating of them, we shall simply notice a few which are most easily examined, beginning with those found in animals belonging to the subkingdom VertebrÁta (ver´tebra, a spine-bone).

MammÁlia.—The animals belonging to this class suckle their young; and their blood-vessels contain red blood.

Blood.—This blood consists of a yellowish liquid, in which very numerous red blood-corpuscles or globules (Pl. IX. fig. 1) are suspended, and to which the red colour is owing. The blood-corpuscles are not globular, but discoidal, i. e. they are circular and flattened, the sides being slightly sunk in. Their form is best seen as they roll over on a slide, after the application of a glass cover. The coloured corpuscles are cells; they appear yellowish red under the microscope, the deep red colour of the blood depending upon the large number of them seen at once and crowded together. It need scarcely be stated that a drop of blood may easily be obtained, by puncturing the wrist with a clean needle. The blood is contained in the blood-vessels. These consist of the ar´teries, which convey the blood from the heart; the veins, which return it to the heart; and a very fine set of intermediate vessels, called the cap´illaries. If a little water be added to a drop of blood on a slide, colourless corpuscles, rather larger than the coloured disks, will be seen scattered among the latter. These are the colourless or lymph-corpuscles of the blood. They are truly spherical, and granular on the surface.

Bone.—In examining a transverse section of a bone, one or several very large cavities will be seen with the naked eye in the centre of the section; these contain the marrow, or medulla. In the long bones, the medullary cavity is single, and runs longitudinally down the bone; whilst in the flat bones the cavities are numerous, forming cancelli. Under the microscope, thin transverse sections of bone exhibit oval or rounded holes, or foramina (Pl. IX. fig. 15), which are sections of canals conveying blood-vessels through the bone; these are the Haver´sian canals. Around the sections of these canals are seen numerous concentric rings, indicating layers or lamellÆ of bony matter. The substance of bone presents numerous black, somewhat elongated bodies (fig. 15 a), called the lacÚnÆ (lacÚna, a little hollow) or bone-corpuscles, which are however hollow, therefore not truly corpuscles, as they were formerly considered. Between the adjacent lacunÆ run numerous fine, dark, branched lines, consisting of very minute canals, or canalic´uli. If the section of bone be viewed by reflected light, the lacunÆ and canaliculi will appear white. In the dried bone they contain air.

The structure of bone is best seen when viewed as a transparent object in the dry state; for when the section is immersed in liquid, the lacunÆ and canaliculi become filled up.

The size and form of the bone-corpuscles and canaliculi vary in different animals, so much so that the Class or Order to which an animal belongs may be determined by reference to these particulars.

Bone consists of earthy salts deposited in a finely granular form throughout the substance of cartilage. By soaking a piece of bone in vinegar or other dilute acid, the earthy salts will be dissolved, the soft cartilage being left. But the structure of cartilage may be best observed by making thin sections of the gristle covering the ends of bones. It exhibits a bluish-white basis (Pl. IX. fig. 16), in which are imbedded numerous cells or cartilage-corpuscles, often undergoing cell-division. In some kinds of cartilage, the basis is composed of fibres.

Muscle.—On examining a piece of the red flesh of an animal under a low power, the mass will exhibit a number of coarse, parallel, longitudinal, dark lines (Pl. IX. fig. 20), the substance between these lines being marked with cross or transverse striÆ, or lines, and with fine longitudinal lines. The coarse longitudinal lines indicate the intervals between bundles of slender fibres, or fibril´lÆ, of which muscle consists. The fibrillÆ (fig. 20 b, c) are very difficult to separate; but when perfectly separated, they are seen to be exceedingly slender, and to consist of alternately light and dark portions in regular series. When the fibrillÆ of the bundles are in close apposition, as in the natural muscle, the dark portions, being in the same lines, by their coincidence form the transverse striÆ. The bundles into which they are combined are surrounded by a delicate skin or membrane, with a little cellular tissue.

The structure of muscle may be observed in a piece of ham which has been soaked for a day or two in spirit of wine, the mounted needles being used to pick it to pieces.

The above-mentioned transversely striated muscular fibre is that found in the voluntary muscles, or those under the influence of the will. But there are other muscles in animals which are involuntary, or not subject to the will; in these the fibrillar structure is absent, the muscular tissue consisting of simple elongated and nucleated cells.

Cel´lular tissue.—This fills the interstices between the other tissues and organs of animals, in the same manner that the vegetable parenchyma does those of plants. It is not, however, composed of cells, but of very fine, soft, colourless, and wavy fibres (Pl. VIII. fig. 20 a), aggregated into bundles, which interlace so as to leave spaces or arÉolÆ between them.

The cellular or areolar tissue may be found in a piece of beef or mutton, in the intervals of the muscular fibres.

Skin.—The skin is composed of cellular tissue, its outer surface presenting a number of projecting blunt points, called papil´lÆ. It contains a large number of blood-vessels; and when the capillaries are well filled by injection with a coloured composition, it forms a beautiful microscopic object.

The skin is covered by the epider´mis or cuticle, which consists of several layers of cells. It is the epidermis which is raised and covers the bladders formed by the action of a blister applied to the skin.

Hair.—The hair consists of long solid filaments (Pl. IX. fig. 9), and not of hollow tubes, as was formerly supposed. It presents varieties of structure in different animals, which agree generally in animals belonging to the same Orders.

Hairs are implanted in pits in the skin; each is swollen at the base to form the bulb, which is seated upon a papilla of the skin, by which it is formed or secreted. The hair is an epidermic formation, consisting of epidermic cells more or less flattened and altered in shape by mutual pressure.

The colour of the hair is usually seated in the outer or cortical portion of the stem or shaft, and arises from the presence of aggregations of minute granules of colouring-matter or pigment, as the colouring-matter of animals is called: in the human hair it forms short longitudinal stripes (fig. 9). In the central pith or medullary portion of the hair the cellular structure is more open and distinct than in the cortical portion, in which the cells are so compressed and consolidated as only to exhibit the cell-structure after treatment with reagents; and the medullary cells often contain air.

In grey or white hairs, the whiteness depends mainly upon the presence of air in the cells of the pith. In the gnawing or rodent animals, as the mouse or the rabbit, the pigment is partially at least situated in the cells of the medulla.

In the hairs of many animals, the cuticular or surface-cells of the shaft are distinctly imbricated (fig. 5), and form beautiful microscopic objects.

The principal interest in the structure of the hair relates to the three points above mentioned, viz. the position of the pigment, the arrangement of the cuticular cells or scales, and that of the cells of the pith.

The pigment is best examined in hairs moistened with a little spirit of wine, which displaces the air from the cells of the pith, and renders the hair transparent; a little water should be subsequently added. The cuticular scales are also well shown by this proceeding. Towards the root of the hairs in the mouse, they project beyond the margin, giving it a toothed or dentate appearance; in the hair of the mole, the bat (fig. 5), or the wolf, this dentation may also be seen. In the hairs of some of the foreign bats, the scales are whorled, forming very beautiful objects.

The cells of the pith (Pl. IX. fig. 7) also present interesting varieties, being sometimes arranged in a single row, at others in two or more rows (fig. 6). These are best seen in hairs recently immersed in spirit or in oil of turpentine; for if the hair be too long soaked in these liquids, the air will be entirely displaced by them. The cells of the pith appear black by transmitted and white by reflected light, in the dry hairs, from the presence of air. They may be well examined in the hair of the mouse (figs. 6 & 7), or in that of the mole. Wool, which is the hair of the sheep, consists of curled fibres (Pl. IX. fig. 10), in which the imbricated arrangement of the surface-scales is very distinctly seen.

In Pl. IX. figs. 10-13 the fibres of wool, flax or woody fibre, cotton, and silk are represented together, to allow of comparison; for the microscope is of great assistance in discriminating these substances when existing in textile fabrics. The fibres of wool (fig. 10) are distinguished by their solidity, wavyness, and the imbricated scales; those of flax (fig. 11) by their thick walls, great length, acute ends, and their knotty appearance at intervals. The fibres of cotton (fig. 12) are soft, flaccid, flattened, and often twisted; and those of silk (fig. 13) are solid and very slender. By a little chemical testing, the discrimination is made still more easy; but for an account of this I must refer to the Dictionary.

Birds.—In the Class of Birds, the structure of the feathers deserves special notice. Feathers are epidermic formations, or consist of aggregations of epidermic cells, yet so altered by compression and fusion together that the cell-structure is in most parts difficult to detect. In a feather three parts are distinguishable,—the transparent cylindrical quill; its opake continuation, which is more or less flattened at the sides, forming the shaft; and the vanes or beards, which arise from the sides of the shaft, consisting of numerous closely set, parallel, flattened fibres, called the barbs. The structure of the barbs forms the interesting object to the microscopist. On examining a piece of the coloured vane of a somewhat large feather (Pl. IX. fig. 14), a row of fine parallel colourless filaments (pinnÆ) will be observed, arising from the opposite sides, the filaments of one side lying obliquely across those arising from the other; and while the filaments or pinnÆ of one side present a row of little teeth (fig. 14 c) near their base, those of the opposite side (fig. 14 b) are provided with as many hooks near their apex, which curve over the teeth to connect the barbs together. This curious arrangement is adapted to keep the parts of the feather firmly united, and yet to allow of their play and flexibility. To observe this structure, a portion of a vane should be soaked in oil of turpentine, and mounted in balsam.

In the downy feathers (Pl. IX. fig. 17) the barbs are not furnished with the pinnÆ, but present simply whorls of minute spines (fig. 18).

The bones of birds present the same general structure as that of mammals, the lacunÆ being, however, more numerous and smaller.

The blood of birds (Pl. IX. fig. 2) differs entirely from that of mammals, in the red corpuscles being oval instead of circular, and convex instead of concave; and each contains a distinct oval and granular nucleus.

Reptiles.—In reptiles, as the frog, toad, or water-lizard (TrÍton), the bone-corpuscles or lacunÆ are larger and more numerous than in either of the former classes; and the blood-corpuscles (Pl. IX. fig. 3) are comparatively very large, oval, rather concave, and contain a large granular nucleus.

The smooth water-newt or triton, properly called LissotrÍton punctÁtus, is a very interesting animal in a microscopic point of view. It may be found in most ponds; and if several are removed in a net, and kept in a large glass jar, with water-plants, they will live for a long period. In the spring or early summer they will deposit their eggs upon the aquatic plants, generally on the under surface of a leaf, which they bend downwards, so as to protect them. The eggs or ova, are about half the size of a pea, and consist of a sac containing a transparent liquid, with a yellowish globule within. After a time these eggs will hatch, and the larvÆ or young newts must be removed from the water, otherwise the parents will devour them.

If one of these larvÆ, which resemble little fish in appearance, be placed with a little water in the “live-box,” and the cap be very gently pressed down, so as to fix the body of the animal, the circulation of the blood may be very beautifully seen in either the fringe-like gills, which are placed on each side of the neck, or in the tail, a low power being used; at the same time the beautiful stellate pigment-cells of the skin will be observed. The structure of the rudimentary spinal column, which runs down the middle of the back, and consists of simple large cartilage-cells, may also be made out, when the animal is dead, by a little dissection with the aid of needles.

Fishes.—In the fourth class of vertebrate animals, which consists of the fishes, we find interesting structures in the blood, the scales, and the roe. The corpuscles of the blood (Pl. IX. fig. 4) differ from those of the Mammalia, but agree with those of birds and reptiles, in being oval instead of round. The scales of fishes (Pl. IX. figs. 22, 23) are usually rounded or oval, as in most of our freshwater fishes, when they are called cyc´loid (??????, circle); but sometimes they are toothed at one end (fig. 23 a), forming ctÉnoid (?te??, a comb) scales, as in the perch. Most scales exhibit a number of concentric rings, which are the indications of laminÆ; and many of them are lobed at the margin, sometimes also having radiate furrows. In the centre are often seen little rounded solid bodies, having somewhat the appearance of cells, which are very well seen in the scales of the perch; and in some scales these bodies are arranged in concentric rows throughout the substance, as in those of the eel or the cod (fig. 24). The substance of which scales consist is generally cartilaginous; in some of them, however, true bony matter is present. Fish-scales are contained within the substance of the skin, and not merely attached to it by one end, as appears to be the case in many fishes. In most of our common fishes, as the roach or perch, the scales project beyond the level of the skin; but the projecting portion is covered by a thin layer of the skin; and when the scales are scraped off, this layer, with its elegant stellate pigment-cells, is usually found adherent to it. In some other fishes, as the cod and eel, the scales are entirely sunk below the surface; and these are commonly supposed to have no scales. They may, however, be easily found by dissection, or by drying a piece of the skin under pressure between two plates of glass, and mounting a portion in balsam.

The beautiful silvery lustre of the skin of fishes depends upon the presence of innumerable very minute and thin crystals; these may be well examined in the skin of a sprat.

The roe of fishes consists of the ova or eggs, and the spermatozoa,—the ova being contained in the hard, the spermatozoa in the soft roe. The eggs consist of a cell surrounded by one or two membranes; and the latter are often traversed by numerous fine radial canals, or present a funnel-shaped tube leading to the ovum. The spermatozoa of the soft roe consist of exceedingly slender filaments (fig. 25), terminated at one end by a kind of head. The reader will not fail to detect the analogy between the ovum of the animal and that of the ovule of the plant; and it need scarcely be stated that the spermatozoa of the animal fertilize the ova, in the same manner that the pollen-tubes and spermatozoa of plants fertilize the ovules existing in them. In the case of fishes, the spermatozoa of the soft roe escaping into the water, and moved by the ciliary action of the filament, enter the micropyle-like canals of the ova, which are deposited by the fish upon the bottom of rivers.

The scales of fishes may be prepared for examination by scraping them off and macerating them in water until the adherent portion of the skin is softened and decomposed, so that it may be washed away. They should be dried between glass plates, and viewed under a low power, as dry transparent objects.

The structure of muscle can be more easily made out in fishes than in other animals. A portion of the flesh should be macerated in spirit as directed above.

Mollus´ca.—We shall now leave the vertebrate animals, and pass to the subkingdom Mollusca, the marine kinds of which are popularly called shellfish: three of their structures form interesting objects for examination—the shell, the tongue, and the gills.

Shell.—The general structure of the shell of the Mollusca may be illustrated by reference to that of the oyster. Two kinds of shell-substance are at once distinguishable in an oyster-shell, an outer brown, and an inner pearly or nacreous. The brown portion exhibits under the microscope the appearance of a cell-structure (Pl. IX. fig. 28), the angular forms from mutual pressure being very distinct. The component bodies of this portion are seen to be more or less elongated and flattened in the side view, forming prisms (fig. 29). The structure of the pearly part of the shell is more difficult of examination, and can only be seen distinctly in ground and polished sections. In these, under a high power, it exhibits numerous fine, somewhat parallel wavy lines (fig. 19), which are the indications of thin layers, or laminÆ, of which it is composed.

Shell consists of a basis of animal matter in which carbonate of lime (chalk) is deposited, the whole being poured out or secreted by the skin or mantle of the mollusk.

Pearls, which possess the same structure as the nacreous part of shell, consist of the nacre formed around some foreign body, as a grain of sand, &c., by which the mantle has been wounded.

Tongue.—The structure of the tongue of the Mollusca is very interesting, on account of the curious teeth which are found upon it. It may be illustrated by the common Whelk (Buc´cinum unda´tum), which is sold at the street-stalls. As, to one unacquainted with the anatomy of the Mollusca, there is some difficulty in finding the tongue, it may be well to point out how it is to be found. If the shell containing the animal be placed so that its orifice is directed upwards, the point or apex of the spire being towards the reader, the lid (oper´culum) which closes the shell will be at once evident. On drawing the animal from the shell by means of the lid, the foot or portion which is applied by the animal to the surface upon which it creeps will be seen. At the upper part of this is the head, with its two horns (ten´tacles). Below the roots or bases of the tentacles, and between them and the upper part of the foot, is the little round mouth. On slitting this up with scissors, a cavity will be opened, and in it will be seen a reddish tube (the proboscis), about as large as a goose-quill, with an aperture at the end. This must be carefully slit up, when the tongue, which is of about the size of a crow-quill, will come into view. The tongue is moveable in the proboscis, and can be protruded or withdrawn by the animal at will. If the surface of the tongue be viewed under a handlens, the rows of teeth will be seen at once. It is better not to pull the tongue out with forceps, as the teeth are easily displaced and injured. The best plan is to dissect away the muscular structures with forceps and a pair of fine-pointed scissors, then to cut off the tongue at its root, and to soak it in water for some hours, when the skin or epidermis containing the teeth can be separated with the mounted needles under a simple lens or microscope. After any loose particles have been washed away with a hair pencil, the object may be spread flat on a slide, and dried between two slides. The upper slide should then be removed, the tongue soaked in oil of turpentine, and mounted in balsam with the least possible heat.

As thus prepared, the horny teeth (Pl. IX. fig. 21) are seen to be arranged in rows, united by a colourless membrane, so as to form a long ribbon. The teeth form three longitudinal parallel rows, a central and two lateral. Each tooth, considering the separate pieces as constituting distinct teeth, has little teeth or denticles at its lower edge. These are curved inwards, four in number, and connected by a basal plate in the side teeth; while the middle teeth have six or seven straight denticles. These teeth serve to enable the animal to scrape or rasp the algÆ, and other matters forming their food, from the surfaces upon which they grow. And if some water-snails are placed in a glass jar the inside of which is covered with confervoid growths, the curious patterns left after the action of the snails’ tongues will be found to present a very curious appearance.

Gills.—The gills or “beards” of the oyster or mussel exhibit very strikingly the phenomenon of ciliary motion. The gills (branchiÆ) are respiratory organs, consisting of folds of the skin, covered with cilia, by means of which the water in which the animal lives is set in motion, and constantly changed to aËrate the blood within them. The currents thus induced serve also to bring the food which floats in the water towards the mouth of the animal. By snipping off a thin portion of one of the brown beards of a fresh oyster, laying it upon a slide, adding a drop of the “liquor” contained within the shell, and lightly pressing a cover upon the whole, the remarkable phenomenon to one who has not before viewed it will be seen under a somewhat high power—about ¼-inch. The whole field will appear in motion, and the lashing or whip-like action of the cilia will be seen, especially towards the edges of the bars (Pl. IX. fig. 36) of the gills. The rapid motion of any floating particles present will also be noticed, showing the direction of the currents of liquid, which, as the liquid is transparent, would not otherwise be recognizable.

BryozÓa (????, moss, ????, animal).—The animals included in this Class, which belongs to the Mollusca, are mostly marine. They are microscopic, and contained in horny or calcareous sacs or cells, aggregated together to form polyp´idoms (pol´ype, and d?a, a house). They are sometimes plant-like or leafy (Pl. IX. fig. 27), at others filamentous and branched, or they form a layer or crust upon the objects to which they are attached. The polypidoms, which are often some inches in length, are frequently met with on the seashore, the cells (fig. 26 a) having slit-like valvular orifices. The bodies of the animals are soft and polype-like, and are furnished at one end with a circle of tentacles, covered with rows of cilia, by which the water is changed for respiration, and particles of food are brought to the mouth. The tentacles can be protruded or withdrawn at the will of the animal. The Bryozoa are what are called compound animals, each individual body having its own set of organs; yet the whole are connected together.

The two species figured are very common. Flus´tra foliÁcea (Pl. IX. fig. 27) is found everywhere upon the sea-shore. The polypidom has cells upon both sides; and they are narrowed at one end, and rounded at the other. Membranip´ora pilÓsa (Pl. IX. fig. 39) occurs upon sea-weeds and other marine bodies, forming a closely adherent layer. The orifices of the cells are surrounded with teeth, and are usually furnished below with a very long bristle—the polypidom appearing to the naked eye as a white hairy crust. In the variety figured, the long bristles are replaced by a spine; and this is not uncommon.

The polypidoms of the Bryozoa form interesting microscopic objects, the cells being furnished with variously arranged spines and punctures or dots. In some the cells are erect and arranged in rows upon the branches of a plant-like stem, while in others they are scattered irregularly over a creeping filament.

For examination they should be prepared by maceration in fresh water, and drying between glass plates or sheets of paper, and either viewed as opake objects or, after soaking in turpentine and mounting in balsam, as transparent objects.

It may be remarked that the name Bryozoa for this class of Mollusca, which was thoroughly established, has recently been changed in this country to PolyzÓa (p????, many, ????, animal), and that the name of polypidom has been altered to polyzÓary.