Muced´ines.—Many of the Fungi belonging to this family are extremely common on decaying vegetable substances, and some are found upon living plants, to which they are very injurious. To the naked eye they usually appear as mouldy or cottony masses, either white, black, or coloured blue, yellow, &c. The spores are attached singly or in rows to branchlets arising from the ends of the filaments, so as to form little heads.

Bot´rytis parasit´ica (Pl. VII. fig. 19) is common upon the flower-stalks of the Shepherd’s Purse, forming white mealy patches. The fruit-stalks are comparatively large and thin-walled, the branchlets being slender, mostly curved, and terminated each by a large, spherical, smooth, simple, white spore.

Botrytis vulgáris is also common on various decaying plants. Its filaments are grey, and the branchlets lobe-like; the spores being minute, spherical, either white or greenish, and placed simply at the tips.

Botrytis infes´tans is the potato-Fungus. It forms white spots upon the under side of the leaves of the potato-plant, and by some authors is considered to be the cause of the potato-disease. The filaments are branched at the ends, and terminated by single oval spores, which are apiculate at the free end, and contain minute little spores or sporidia.

Oid´ium Tuck´eri is the well-known destructive grape-Fungus. It forms white cottony masses upon the vine and its grapes, the fruit-stalks being short and terminated by one or two end-to-end oblong spores. It appears to be the Coniomycetous form of another Fungus (Erysiphe).

Trichothécium róseum (Pl. VII. fig. 33) is found upon rotting sticks; very frequently upon willow-baskets kept in a damp place. It forms little rounded, slightly raised, pinkish spots, less than the size of a pin’s head. The branched and septate foot-stalks (figs. 34, 35) are terminated each by a little group of obovate spores, divided by a transverse partition (uniseptate). Sometimes this little Fungus is quite white, at others greenish; when perfectly ripe, the spores become oblong.

Penicil´lium glaúcum (Pl. VII. fig. 25) is the common Blue Mould found upon decaying substances, as cheese, &c., the interwoven mycelial filaments often forming large cakes or crusts upon the surface. The septate fruit-stalks (fig. 26) are fork-branched at the ends, the branchlets being terminated each by a row of very minute spherical smooth spores. On some decaying substances, as apples, gum, &c., the fruit-stalks are found aggregated into a thick stalk, the branchlets and spores forming a rounded head, so that the whole resembles a little blue mushroom (fig. 27). In this form the Fungus has been placed in a distinct genus, and called Corémium leúcopus. In other species the spores are pink and white.

This little Fungus is of special interest, on account of one form of it constituting the yeast-plant, or yeast as it is commonly called. This consists of rounded or oblong cells, which grow very rapidly in fermenting liquids by budding—the large quantity of sugar and gluten present favouring the vegetative or simple growing process, at the expense of the fructifying process. But this is only an instance of what we constantly find in flowering plants, the use of very rich soil rendering flowers double, which is really reducing their organs to the state of leaves. When the sugar has become exhausted, the cells of the yeast become longer and thinner, as if starved; they then form a more recognizable mycelium, which extends to the surface of the liquid, and produces finally the fruit-stalks and the Penicillium fruit. Aspergil´lus glaúcus (Pl. VIII. fig. 3) is an extremely common mould upon cheese, jams, &c. It resembles the last in appearance to the naked eye, except that it has rather a green tinge, the heads of fruit being much more compact and rounded. The fruit-stalks (fig. 4) are large, bulbous or inflated at the ends (fig. 5), and from the inflations arise the crowded rows of spores. The spores are rounded, and rough (scabrous) on the surface. On removing most of the spores from the head of fruit, each row of spores is found to arise from a very short stalk.

Plate VII. fig. 21 represents a beautiful species of Rhinot´richum, which is found upon decaying and sickly plants, and upon rotting sticks, forming a minute grey mould. The fruit-stalks (fig. 22) are large, sparingly branched, septate or jointed, appearing brownish under the microscope. Their ends are branched, mostly biternate (fig. 23), i. e. each branch dividing into three branchlets, and these again into three still finer ones. The ends of the branchlets are inflated, and coated with little points, upon each of which a smooth white spore (fig. 24) is placed.

Ascomycétes (ἁσκὀς, a bottle, μὑκης, fungus). The Fungi belonging to this Order are found upon the stems and leaves of plants, and upon decaying substances, as dung, &c. They are usually evident to the naked eye, some even equalling the Hymenomycetous Fungi in size; and many of them are brilliantly coloured. They are in general distinguishable with facility from the Fungi of other Orders, by the arrangement of the spores in colourless sacs or asci (Pl. VIII. fig. 9), resembling those noticed in the case of the Lichens. These asci are usually enclosed in a capsule or perithécium. The mycelium is usually buried in the matrix, so as not to be conspicuous.

Helvellácei.—To this family belongs the large genus Pezíza, some of the species of which are beautifully coloured, yet scarcely microscopic. Among these may be mentioned Pezíza omphalódes (Pl. VIII. fig. 6), which forms little red masses upon damp ceilings. It does not possess the ordinary form of a Peziza, which is that of a cup fixed at the end of a stalk, like a mushroom with the cup turned inside out, the asci lining its interior.

Peziza coccin´ea is not uncommon in woods. It is whitish outside, the interior of the cup being of a brilliant scarlet colour. It is from half an inch to an inch in height.

Peziza stercor´ea (Pl. VIII. fig. 7) is often found upon dung. The surface of the cup of this Fungus is granular and covered with bristles (figs. 8 & 11). The cup is concave (fig. 10), and lined with the asci (fig. 9), among which are simple paraphyses.

The Pezizæ are excellent Ascomycetous Fungi for exhibiting the asci, as they are more or less soft, and thus sections of them may be easily prepared, or they may readily be picked to pieces with the mounted needles.

Tuberacei.—In this family is contained the Truffle (Túber cibárium). The asci are situated upon the inner surfaces of the winding canals traversing the substance of the fleshy fruit (peridium) of which the truffle consists.

Phacidia´cei.—To this family belongs Hystérium frax´ini (Pl. VIII. fig. 30), which is found upon ash-twigs. The drawn-out capsules or perithecia (fig. 31) are black and elliptical, with a longitudinal fissure or orifice, and contain the asci (fig. 32) with the spores.

Sphæriácei.—Dothid´ea typhína (Pl. VIII. fig. 12) is a common Fungus upon the stems of living grasses. It forms an orange-coloured patch or layer encircling the stem, and covered with little dots. On making a section (fig. 14), it appears composed of a row of oblong or obovate closely placed capsules (perithecia) immersed in and continuous with a finely fibrous receptacular mass (stróma). The asci (fig. 15) are very slender, arising in a tuft from the bottom of the capsules, and containing eight still more slender spores. Except under a very high power, the spores appear as interrupted lines running down the interior of the asci. The little dots visible to the naked eye are the slightly projecting mouths of the capsules, which are more distinctly seen in the magnified portion of the Fungus (fig. 13). In the young state, this Fungus is whitish.

This Fungus cannot be mistaken for a Uredo, two species of which occur upon grasses—Uredo lineáris forming yellowish-brown spots, and Uredo rubígo yellow spots.

Dothidea ul´mi (Pl. VIII. fig. 24) forms black, slightly raised, and somewhat star-shaped spots upon the upper surface of the leaves of the elm. In a section (fig. 26) the cavities are seen, containing the very delicate asci (fig. 25). The spores (fig. 27) are oval, with a minute septum at one end.

Sphæ´ria rubel´la (Pl. VIII. fig. 16) is extremely common on the dead stems of the nettle, &c. In this Fungus the black bottle-like perithecia (fig. 18), containing the asci and paraphyses (fig. 17), are at first situated beneath the epidermis, through which they at length burst. The spores (fig. 19 a) are spindle-shaped, and from four-to seven-septate. When ripe, they escape by a hole or pore in the neck.

Sphæ´ria complanáta (Pl. VIII. fig. 22) is another common species, found in hedges, on dead sticks of the softer (herbaceous) plants, as the parsley-order (Umbelliferæ). Here the minute capsules, which are scattered over the stems, are at first rounded, then flattened on the top (depressed), the neck being very minute (fig. 23). The spores in this species are exceedingly minute, oblong, and not contained in asci.

Sphæ´ria bulláta also belongs to this family. It occurs upon decaying birch-sticks, presenting to the naked eye the appearance represented in Pl. VIII. fig. 20. The black, raised tubercles (receptacles) in their growth burst through the bark, splitting the epidermis. They consist of a white stroma (fig. 20 a), in which the bottle-shaped capsules (perithecia) are immersed, the necks projecting slightly above the surface as little points (papillæ). The tufted spore-sacs or asci (fig. 21), with the thread-like paraphyses, are contained within the capsules; and within the asci are the densely packed, very numerous and minute curved spores.

Another species, Sphæria discifor´mis, is also common on birch-sticks. It differs from the last in the tubercles being perfectly flat; the spores are also longer, straight, and spindle-shaped (fusiform).

Perisporácei.—Erys´iphe guttáta (Pl. VIII. fig. 33) is a member of this family. It appears on the under side of the leaves of the common hazel as a pale spot; and on closely examining it with the naked eye, little black dots are seen scattered on the surface. These are the capsules (conceptacles), which are seated upon straight white filaments. The filaments (fulcra) are six or seven in number, and are placed under the capsule, like the legs of a stool (fig. 34); they are rigid, and swollen or inflated at the base (fig. 35). The asci are broad and short, and contain only two spores.

Erysiphe maculáris is the very destructive hop-mildew; and other species are common on various plants.

Chætómium elátum (Pl. VIII. fig. 29) resembles little tufts of brown hairs, occurring upon decaying herbaceous stems. The capsule (fig. 28) is crustaceous, and covered with interlaced, rough, branched hairs (fig. 28 b). The spores (fig. 28 a) are oval, with a little point at one end (apiculate).

Physomycétes (φῦσα, bladder, μὑκης, fungus).—The Fungi belonging to this order include some of the commonest moulds growing upon decaying vegetable substances; while others are found upon leaves, &c. The flocci are generally very evident; and the spores are contained in little naked, bladder-like capsules (peridíola) at the ends of free filaments.

Mucorini.—In this family we have the common mould of paste, Múcor mucédo (Pl. VIII. fig. 36). It is easily recognized by the little spherical capsules terminating the long and tufted fruit-stalks (pedicels), which are perceptible to the naked eye. Each capsule consists of a simple enlarged cell, the cavity of which is separated from that of the stalk by a septum. They are white at first, subsequently becoming brown and black. The minute crowded spores (fig. 36 s) are at first oblong, afterwards spherical. In the centre of the capsule is a club-shaped body, or columel´la (fig. 36 a), formed by the elevation and inflation of the septum.

A beautiful little Fungus of this family, apparently referable to the genus Acrostalag´mus (Pl. VIII. fig. 37), is sometimes found upon soft decaying stems. The main filaments are soft, smooth, and not septate. The pedicels are very brittle, whorled, dichotomously branched, scabrous, and terminated each by a little scabrous spherical vesicle (fig. 37 a), containing two or three oblong spores.

Antennariéi.—In this family is Racódium (or Antennária) celláre, the Wine-cellar Fungus, forming the well-known cobweb-like masses hanging from the walls, &c. The little black capsules are seated upon slender septate filaments, and contain numerous round spores.

In examining leaves with the view of procuring Fungi, the reader will most likely meet with the two kinds of bodies represented in Plate VIII. figs. 38 & 39. These are not Fungi, but galls. They arise from an abnormal growth of the leaf-structures, produced by the deposition of the eggs of insects (Cynipidæ). The well-known oak-apple, and the red hairy-looking body found upon hedge-roses, are both galls produced in the same way.

Examination and Preservation.—The examination of the Fungi scarcely requires any special remarks. They should be viewed first as opake objects under a low power; and then sections should be made, or the textures separated with the mounted needles.

There is some difficulty in moistening the smaller filamentous Fungi with water, which is requisite in the determination of the arrangement of the spores upon the branches. Hence the best plan is to lay the Fungus upon a slide, apply a cover, then to add a drop of spirit of wine and afterwards a little water to the edge of the cover. When thus wetted, the spores may be more or less removed with a wet hair-pencil, when the ends of the branches will become perfectly distinct. In examination of the dried smaller Fungi as the Sphæriæ, the capsules should be macerated for a time in water.

The softer Fungi are very difficult of preservation in the entire state; but the sections or minute structures may be mounted in chloride of calcium or glycerine.

The harder and drier Fungi may be preserved by drying and gentle pressure between coarse absorbent paper. They may then be glued to pieces of paper and labelled, in the same manner as the flowering plants. Specimens of the capsules, as of the Sphæriæ, &c., may also be mounted in the dry state, the asci being preserved in the chloride of calcium or glycerine, in which liquids most of the smaller Fungi will keep extremely well.

CHAPTER X.

ANIMAL ELEMENTS AND TISSUES.

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 (κὑκλος, circle); but sometimes they are toothed at one end (fig. 23 a), forming cténoid (κτεἱς, 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 (βρὑον, moss, ξῶον, 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 δῶμα, 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 (πολὐς, many, ξῶον, animal), and that the name of polypidom has been altered to polyzóary.

CHAPTER XI.

ARTICULATA (ARTIC´ULUS, A JOINT).

Entomostraca.—The animals contained in this Order are met with in every pool or pond, some of them inhabiting the sea. They are mostly minute, yet visible to the naked eye, forming specks swimming actively or leaping through the water; hence some of them have been called water-fleas. The body of the animal is protected by a shell or car´apace, which in some consists of a single piece (Pl. IX. fig. 30), while in others it consists of two similar parts or valves (fig. 31), in the latter case the joints of the body being indistinctly visible. The head is furnished with usually two projecting feelers or antennæ (anten´na, a sail-yard), one of which is uppermost or superior (Pl. IX. figs. 30, 31, 34 a), the other lowermost or inferior (figs. 30 and 34 b); and these are often used for swimming. The antennæ are jointed, and sometimes beautifully plúmose (pluma, a feather) or feathery, i. e. furnished with rows of long and very slender filaments. There are several pairs of jointed legs, some of which serve as jaws (foot-jaws), while others are finely filamentous to serve for swimming and as respiratory organs (branchial feet). The four species figured are very common.

Cypris tristriáta (Pl. IX. fig. 34) is found in ponds and ditches. The carapace is bivalve, or has two valves, which are convex and oval; and it is of a greenish colour, with three irregular dark stripes behind. The superior antennæ (a) are jointed and finely feathery, the inferior antennæ (b) having a tuft or pencil of fine filaments arising from their anterior margin. The eye is single. The animal swims steadily and freely through the water.

The eggs of Cypris (Pl. IX. fig. 35) are often found in glasses of water containing the animals. They are rounded or oblong, of a red colour, glued together by an amorphous jelly, and adherent to pieces of stick or the sides of the glass. They are enclosed in a thick shell, which exhibits a cellular appearance in the surface view, and is striated in the side view; so that the structure of the shell is prismatic, as in that of the oyster. When the eggs escape from the shell, they present the appearance represented in fig. 35 a, the body of the young animal being enclosed in a transparent envelope, one end of which forms a blunt protrusion; there is also a separate slender process enclosing the superior antennæ. After a time, the envelope is cast off (fig. 35 b), when the animal begins its active stage of life. The cast-off envelopes (fig. 35 c), with the protruded portions wrinkled, are often found in the sediment of water containing the animals. The structure of these ova is that of what are called winter ova, which agree with the resting-spores of the lower plants or the Algæ.

Cy´clops quadricor´nis (Pl. IX. fig. 30) is another common species. In this the body is closely surrounded by the jointed shell, as in a lobster. The superior antennæ (a) are very long and many-jointed, each joint having short bristles arising from it, while the inferior antennæ are short and four-jointed. There is no separate head, this being united to or consolidated with the first joint of the thor´ax or chest, the head and thorax together comprising four joints. The remaining joints enclose the belly, or abdómen, which has the appearance of a tail; but the tail is constituted by the two last parallel pieces, which are furnished with fine feathery filaments.

The female is most commonly met with, and is easily known by having the egg-pouch, or ovary (o) external on each side, and filled with eggs or ova. The little Cyclops is readily recognized by its form and jerking motion through the water.

Daph´nia púlex (Pl. IX. fig. 31) is a very common Entomostracan, and is very well adapted to illustrate the structure, on account of its size and transparence. In this animal the body is loosely connected with a bivalve shell, which, on careful examination, is seen to be reticulated or marked with net-like lines. The superior antennæ (a) are very small, placed under a small beak, and have at the end a minute tuft of hairs. The inferior antennæ (b) resemble arms, being large and branched; and by means of them the animal rows itself through the water. The structure of the eye is curious, consisting of a number of round lenses aggregated together, the fine muscular threads by which it is moved being easily distinguished with a high power. The legs are flattened, and furnished with elegant feathery sétæ (seta, a bristle), serving as gills or branchiæ. They are constantly in motion, fanning the water so as to change incessantly the portion with which they are in contact. About the middle of the back is placed the little transparent heart, with its colourless blood, which may be distinctly seen beating, or contracting and dilating, in the living animal; and between the back of the animal and the shell are seen the ova, which remain there until they are hatched.

The genera and species of the Entomostraca are very numerous. Those mentioned above will serve to illustrate the general structure of the order. To distinguish the man´dibles (mandib´ula, a jaw) or proper jaws, the foot-jaws, and the branchial legs, the animals must be dissected in water with the mounted needles. The very delicate feathery filaments of the branchiæ may be best observed when these organs are dried on a slide.

The Entromostaca may be kept alive in a jar of water with water-plants for a long period. They may be removed from the water for examination by the dipping-tube, and are best observed in a live-box.

Arach´nida (ἁρἁχνη, spider) is the Class of spiders, scorpions, and mites.

Aranéida.—This Order contains the more highly developed forms of the Class, among which are the common spiders of houses and gardens; and some of their structures are very curious and interesting.

The head of spiders is united or fused with the thorax, forming one piece, which is called the ceph´alothorax (κεφαλἠ, head, θὡραξ, chest).

The claw-jaws, or man´dibles, are terminated by a curved and pointed claw, with which the spiders hold their prey. It is traversed by a slender canal, containing a slender tube or duct leading from a poison-gland, and opening near its point; and when the insect prey is transfixed by the mandible, the poison is pressed out and enters the wound.

Near the root or base of the mandibles on each side is a jointed feeler, or pal´pus; but spiders have no anten´næ. The eyes are simple, forming separate round shining dots, and are called ocel´li (ocel´lus, a little eye); they are usually placed on the top of the head, and are often arranged in a geometrical form, as a triangle, &c.

The legs are four pairs; they are hairy, and terminated by two or three claws, which are fringed with minute teeth, or pec´tinate. These claws serve to comb the fibres of the web, just as we comb our hair with a common comb.

The spinnerets, with which spiders form their web, are very curious organs. They are situated at the under and hind part of the body, and consist of two or three cones, or papillæ, on each side. On the summits of these papillæ are very numerous bristle-like tubes, through which the secretion of certain glands passes; this secretion, when hardened by exposure to the air, forms the fibres of the web.

On carefully examining a spider’s web, the radial fibres, or those which pass from the centre to the circumference, will be found to be smooth, these fibres serve to fix the web; while the cross fibres are covered with numerous viscid globules, which serve to attach flies or other prey to them. This difference of the fibres is best observed with a hand-lens.

Acarína, or the Order of Mites.—Here belongs cheese-mite, Ac´arus domes´ticus (Pl. IX. fig. 36). Its body is somewhat milky white, oval, and furnished with feathery hairs. When viewed from beneath, there is seen a transverse line, indicating the separation of the thorax from the abdomen; and another line in front of this, with four minute tubercles, from each of which arises a hair. The head is pointed and beak-like, forming a ros´trum (rostrum, a beak), consisting of two mandibles pressed together; these can only be seen to be separate when dissected apart with the mounted needles. Each mandible is chélate (χηλἠ, forceps), or has the form of a lobster’s claw; and beneath the two mandibles is a flat membranous under lip or labium, consolidated on each side with a palp. The legs are four pairs, as in all the Arachnida; they are pinkish, 6-jointed, and terminated by a leaflike sucker and a minute claw.

The males (fig. 38) are smaller than the females, the fore legs being much stouter, and furnished with a blunt tooth (fig. 38 a). The eggs can often be distinguished within the body of the female (fig. 36); they are oval and granular.

Another species of Acarus, A. sac´chari, is found abundantly in ordinary moist sugar. If a little of the sugar be placed in a wine-glass, some water added, and the mixture be stirred until the sugar is dissolved, the Acari will be found both in the sediment and floating on the surface.

A somewhat larger member of the order occurs as a parasite upon a species of Dung-beetle (Geotrúpes stercorárius) which is vulgarly known as the Lousy Watchman. The beetle is black, shaded with purple, about three-quarters of an inch long, and is found under cow-dung. The mites cling pertinaciously to the under parts of the beetle, and can easily be seen with the naked eye. They are whitish, with the mandibles, the sucker, and two claws very distinct; and the palpi are unattached to the labium, or free. These mites form the species Gam´asus coleoptrator´um. Another species, Gamasus telárius, is the red spider of the greenhouse.

Trombid´ium fuliginósum (Pl. IX. fig. 37 d) is a common red spider of gardens. It is of a scarlet colour, appearing velvety from the presence of a dense coat of feathery hairs (fig. 37 f). The palpi of this mite are large, free, the last joint but one (Pl. IX. fig. 37 a) being furnished with a claw, while the last joint is obtuse, and resembles a lateral appendage. The mandibles (fig. 37 b) are furnished with a sharp curved claw. The legs are long, especially the anterior pair, and terminated by two claws, with a delicate sucker-like appendage (fig. 37 c).

Another species of Trombidium, T. holoseric´eum, greatly resembling the last, is also found in gardens. It may be easily distinguished from the last by the club-shaped hairs (fig. 37 e) existing upon the body. The harvest-bug, which causes such irritation of the legs of persons who frequent corn-fields in the autumn, is also a species of Trombidium—T. autumnále.