Preparation.—The organs of the mouth, &c., of the Spiders are easily prepared for examination, by carefully pulling them off with forceps or the mounted needles, then drying them under pressure between two glass slips, macerating in turpentine, and mounting in balsam. Those of the Acarina should be dissected out with the needles, after the body has been crushed in a drop of water on a slide, and the internal substance has been gently washed away with a hair pencil. They may then be dried on a slide, with a cover laid on, and turpentine applied to the edge of the cover, balsam being added when most of this has evaporated. The various parts may also be mounted in chloride of calcium or glycerine.

Insects.—The members of the class of Insects are extremely interesting to the microscopic observer, not only on account of the beautiful structures which they present, but also from these being comparatively large, usually coloured, and easily distinguished under the lower powers. Hence they form admirable objects for study to those who are but little accustomed to the use of the microscope.

Myriap´oda (μὑριος, myriad, ποῦς, foot).—This Order contains those insects which are popularly known as the hundred-legs and millepedes; by many zoologists they have been arranged in a distinct class.

The most common member of this Order is Lithóbius forcipátus (Pl. X. fig. 4), which is found under stones, in cellars, and among garden-rubbish. It is of a yellowish-brown colour, with long, many-jointed, gradually tapering or setáceous (séta, a bristle) antennæ (figs. 4 a, 5 a), and two large and powerful mandibles (fig. 5 b) resembling those of the spiders. It has also a broad, notched, and toothed lower lip, or labium (fig. 5 d), above which are two toothed jaws, or maxillæ, and two lip-feelers, or labial palpi (c). The eyes consist of a group of ocelli on each side (e). The body is protected by alternately larger and smaller dorsal plates, which are fifteen in number; and there are fifteen pairs of legs, which are terminated by a single claw. On the sides of the body will be found some oval dark-looking bodies, fringed with hairs; these are the spir´acles (spirac´ulum, a breathing-hole) or breathing-pores. They form the orifices of certain branched and transversely striated tubes, which are distributed throughout the body; the tubes are called trácheæ (trachea, the windpipe), and their walls contain an elastic spiral fibre which keeps them open. These parts of the insect can only be distinctly seen when the body has been slit up on the under side; and, after washing away the animal matter with water by the aid of a hair pencil, pressed between two slides with a clip, dried, soaked in turpentine, and mounted in balsam.

Thysanúra (θὑσανοι, fringe, οὑρἀ, tail).—The insects belonging to the genus Podúra, of this Order, are very minute and difficult to examine; but they are specially interesting, on account of the structure of their scales. They are common in gardens and cellars, under flower-pots, &c., and are about one-tenth of an inch long. They are of a brownish or silvery-leaden colour, wingless, with six legs, and when touched they leap like a flea. The leaping motion is produced by the action of the tail, which is forked and bent under the body.

The body is usually covered with minute scales (Pl. X. fig. 19, a, c), and these are used as test-objects. The structure of the scales varies in the different genera and species; those usually used (fig. 19 a) are stated to belong to Podúra plum´bea; it appears, however, that this is not correct. The scales sold as test-objects under this name are covered with minute and short raised lines (fig. 19 a), arranged in irregular but somewhat parallel wavy rows. It requires a good microscope and a high power to show them distinctly, and they should appear perfectly black and separate. The little lines are much coarser in some scales than in others; so that there are easy and difficult scales, as they are called.

The Poduræ may be caught by holding a sheet of paper near their haunts and disturbing them; and when they have jumped upon the paper, a slide laid upon them and gently pressed will remove some of the scales for examination.

The scales should be mounted as dry transparent objects; for if wetted, they become very transparent, and the markings appear removed, which however is not really the case.

The scales of Lepis´ma saccharína (Pl. X. fig. 19 d), a member of this Order, were formerly used as test-objects; but they are too easily made out to serve for this purpose with modern microscopes. The insect is not common. The scales (fig. 19 d) exhibit continuous nearly parallel longitudinal lines or ribs.

Anoplúra (ἄνοπλος, unarmed, οὑρἀ, tail).—This third Order of insects consists of the Lice of the Mammalia and birds. They are minute, resembling mites to the naked eye, but may be at once distinguished from them by the distinct head and thorax and the presence of six instead of eight legs.

Some of them are suctorial, i. e. have a short and slender tube, with which they suck the blood of the animals of which they are parasites; while others are mandibulate, or have mandibles, and also maxillæ, their food consisting of portions of feathers, hairs, and scurf. The legs are usually short and stout, and the claws large and powerful, to enable them to hold firmly to the hairs, &c. The Anoplura are most abundant on dirty and diseased animals.

Suctor´ia.—The fourth Order of insects consists of the genus Púlex,—Pulex irrítans being the human flea. Other species are found upon different animals, as upon the dog, the rat (Pl. X. fig. 22), the fowl, the pigeon, &c.

The head and the dark eye (fig. 21, head of the human flea) are very evident. The antennæ or head-feelers are very minute and difficult to find, being sunk in a little pit or fossa behind the eye; they may, however, generally be recognized by the detection of the last joint, which is pectinate or cut like a comb. The body, including the head, consists of thirteen joints, one for the head, three for the chest or thorax, and nine for the belly or abdómen,—it being understood that by “joint” is meant a segment, and not the line of junction of two segments. The indication of these joints is afforded by the horny integument, which consists of a corresponding number of rings, forming in fact the skeleton of the animal. This in the flea, as in all insects and other Articulata, is external or cutaneous (cútis, skin), and consists of the hardened skin, the peculiar animal substance of which it is composed being called chitine (χιτὠν, tunic). These chitinous rings overlap, and are composed of a dorsal or upper, and a ventral or lower half; and near the middle of each is a row of hairs directed backwards. Along the sides of the body of the insect may be seen a row of dots; these are the spiracles or orifices of the breathing-tubes (tracheæ).

The legs are many-jointed, long, furnished with numerous spines, and terminated by two slightly curved claws, each with a little blunt tooth at its base. The claws are not perfectly smooth on the inside, but are covered with slightly raised lines, like a file, so that a better hold can be taken of bodies.

But the most interesting parts of the flea are those of the mouth, with which it punctures the skin and sucks the blood. There are nine of these; and they are best seen when the head of the flea is pulled off with the mounted needles, and the parts spread out and mounted in balsam, a high power being used to examine them. One of them forms a long and slender bristle (seta) or tongue, furnished with distant minute teeth. On each side of this is a flattened seta, with two rows of teeth on the edges; these are the lancets, and when not in use these organs are inclosed in two jointed sheaths. Outside these are two representatives of jaws, or maxillæ (b), each having a jointed feeler or palp (a) arising from it; the probable use of the palps being to feel the position of the skin, so that the animal may be able to adjust the lancets at a proper distance for puncture.

The eggs of the flea are often visible within the body of the parent; and when this is crushed, they are more distinctly seen, of various sizes, and contained within a long tube, which is the ovary or egg-bag. The eggs are laid by the animals upon carpets, woollen garments, or in the cracks of dirty floorboards; they are just perceptible to the eye as white oblong specks, and they may always be found on the rug when a cat is kept in the house. When hatched, they give rise to a minute white worm-like maggot, or larva (fig. 30), having a 12-jointed body, with two rudimentary antennæ and two slightly curved hooks appended to the last joint. The mouth-organs of the larva are adapted for biting, and not for sucking, as in the perfect animal, the jaws or maxillæ being distinctly toothed. When they have acquired full growth, which takes place in about twelve days in warm weather, they spin around themselves a little silky cocoon, and become transformed into a chrysalis or pupa; and from this, in about a fortnight, the perfect insect escapes.

Dip´tera (δἰς, twice, πτερὀν, wing).—This, which forms the fifth Order of Insects, consists of the two-winged insects, or flies, as the house-fly, the blue-bottle, the gnats, &c.

Mus´cidæ.—The house-fly, and the blue-bottle or meat-fly, are both species of the genus Mus´ca, belonging to this family, the former being Musca domes´tica, the latter Musca vomitor´ia. Both these insects are seen to be wonderfully constructed when minutely examined, and they possess considerable resemblance in general structure.

On examining the head of the house-fly (Pl. X. fig. 17) under a low power, and as an opake object, the observer will be struck with the remarkable appearance presented by the two eyes, which are large, placed one on each side of the fore part of the head, and composed of very numerous little eyes closely packed together, or they are compound, as it is called. The use of this compound structure is evidently to enable the little animal to see in all directions without moving the head and eyes. Each little eye has a lens to bring the rays of light emanating from objects to a focus upon a nerve. The packing of the eyes together gives rise to their angular form or their straight sides, each of the little surfaces or facets being hexagonal, or bounded by six sides (fig. 33). In front of and between the eyes are seen the two small antennæ; these have three joints, the third of which is larger than the rest (fig. 17 a), and arising from near its base is a feathery bristle or seta; these structures are best seen when the antennæ are pulled off with a pair of forceps and mounted separately. Below the antennæ, and extending downwards and forwards is the proboscis, or tongue, as it is called, which can be entirely retracted within a pit in the fore part of the head, or protruded at the will of the animal. This is a very beautiful and complicated instrument, and is best examined when spread out and separately mounted (Pl. X. fig. 29). It consists of a fleshy tube, dilated at the end into two lobes, which are flattened beneath to form a sucking-disk. The end is furnished with two solid horny lateral branches to keep it expanded, and with two longitudinal tubes beneath, running parallel, from the outer sides of which arise numerous nearly parallel branches. These tubes and their branches are incomplete beneath, and consist of imperfect rings, otherwise greatly resembling tracheæ. On each side of the proboscis is a lip-feeler or labial palpus, for the organ represents the labium of other insects. All these parts are better seen in the proboscis of the blow-fly than in that of the house-fly, on account of their larger size. The head, the thorax, and the abdomen are very distinct in the fly, being separated from each other by well-marked constrictions.

The legs are composed of five parts, each having a separate name. The first piece or joint, which is that attached to the body, is called the hip, or coxa; the next is a very small, somewhat triangular piece, and is the trochan´ter; next comes the long and stout thigh, or fémur; this being succeeded by the tib´ia, which, as in most insects, is furnished with strong spines at the end; and lastly is the foot, or tar´sus, consisting of five joints, the three last of which are represented in the figure (Pl. X. fig. 32). At the end of the fifth or last joint of the tarsus (fig. 32) (for it must be noted that the joints of the limbs of insects are always numbered in order of distance from the body) are two soft little cushions, or pulvil´li, which are covered on the under surface with numerous hair-like bodies dilated at the end, acting as suckers in enabling the fly to adhere to smooth surfaces. In addition to these organs are two curved claws, and between them a sharp straight spine.

The eggs of the fly are deposited upon heaps of decaying animal and vegetable matters, as dungheaps, &c. The blow-fly deposits its eggs in the same situations, but especially those where the animal matters are most abundant: every one knows the eggs as deposited upon tainted meats, when they are called flyblows. When the eggs are hatched, the larvæ or maggots make their appearance. The larvæ of the blow-fly are well known to the angler, who uses them for bait, and calls them gentles.

These larvæ exhibit some interesting points of structure. The jointed or ringed condition of the body is distinct to the naked eye. The head is provided with two rudimentary palpi, placed each upon a rounded papilla; also with two brown curved and horny hooks or jaws. On the posterior end of the body are two brown spots, which consist of spiracles, and have three sieve-like oblong orifices; and at the anterior margin of each segment of the body are very numerous little short spines, with the points directed backwards. These answer the purpose, to some extent, of legs; for when the larva is moving, and has forced itself through the matter in which it burrows, the little spines prevent the body being forced backwards as the head is pushed forwards and meets any resistance.

To examine the structure of these larvæ, the gentles should be killed by immersion for a time in warm water, and then dried by touching them with blotting-paper. The hooks and palpi can be seen by holding the body in the forceps as an opake object. To observe the spiracles, the end of the body should be cut off, and the animal matter washed away in a watch-glass with water and a hair pencil, then spread out, dried between two slides, and mounted in balsam.

The larvæ of the house-fly and of the blow-fly very closely resemble each other, so much so that the former are generally overlooked; hence it is often wondered where flies come from, although they are so numerous in every house. When the larvæ of these flies are fully developed, they gradually assume a brown colour, the organs of the head are retracted, and the skin becomes dry and hard. This is the state of pupa, or chrysalis; and while remaining in this state of rest, the development of the wings, legs, &c., takes place; so that when the insect emerges from the shell of the chrysalis, it has attained its highest state of development, and forms the imágo or perfect insect. It may be remarked here, that when insects pass through the three states of larva, pupa, and imago, they are said to undergo complete metamorphosis. There is not a more curious object than that presented by the young fly contained in its case, as seen on carefully cutting away portions from one end of the case of the chrysalis. The body and head are quite white, with beautiful blood-red eyes.

Most persons must have noticed another kind of house-fly, having the wings more widely separated than in the common fly, and moving more slowly through the air. This is Stomox´ys cal´citrans. The proboscis of this fly (Pl. X. fig. 18) differs from that of the house-fly in being longer, more bent, and but little expanded at the end; also in being provided with two long setæ, one forming a slender sharp lancet, the other being somewhat stouter, and forming its sheath. The fly is thus enabled to pierce the flesh and suck the blood.

Culic´idæ.—We will now say a few words about the gnats, which are old favourites for microscopic examination. The three states of larva, pupa, and imago must be considered separately.

The imago or perfect form is well known. The males are easily distinguished from the females by the difference in structure of the antennæ, which in the males (Pl. X. fig. 10, head) are very beautifully plumose or feathery, whilst in the females (fig. 11) the hairs are very short—the long proboscis, or rostrum, or bundle of biting-organs forming the striking feature; this difference is immediately evident to the naked eye.

The antennæ of the males (fig. 10 a) consist of numerous small joints, from each of which arises a ring of long hairs, giving the appearance of a tuft on each side when flattened for view as a transparent object under the microscope; at the end are two longer joints, the first having a small ring of shorter hairs.

The proboscis of the female is a very complicated organ, consisting of six separate bristle-like pieces or setæ, all nearly of the same length. Two of these are somewhat curved near the end, and provided on the inside with fine teeth; another pair consists of very thin lancet-pointed instruments; then comes another lancet-pointed seta, very sharp at the end, and traversed by a canal; next, a stouter and darker-looking tube, slit up underneath, which serves to contain the two toothed setæ; and lastly, a stout and broader sheath, also slit up throughout its length beneath, in which all are packed. This sheath has two lobes at the end, and has some resemblance to the proboscis of the fly, of which organ it is the representative.

The gnat lays its eggs in water. The eggs are longish oval, with a kind of neck at the upper end; they are glued together side by side in large numbers, and form a boat-like mass, floating on the surface of the water. The larvæ (Pl. X. fig. 9) are very commonly found skipping through the water, or hanging, as it were, by the tail from the surface. The head is very broad; and arising from each joint of the body are tufts of hairs. Near the end of the body is a tube which communicates with the tracheæ; and when the animal is quiet, this tube is brought to the surface; so that when the animal appears hanging to the surface, it is breathing. In addition to this method of respiration, there are other respiratory organs attached to the last joint of the body, consisting of leaf-like plates; hence these larvæ have an aquatic as well as an aërial respiration: the respiratory or branchial plates also serve as a tail to aid in swimming. Running down the back of the larva will be seen a thin delicate vessel, dilated opposite the thorax, and beating or contracting at regular intervals. This is the dorsal vessel, the dilatation representing the heart of the higher animals; and it serves to propel the colourless blood throughout the body.

After several moultings or castings of the skin, to allow of the growth of the insect (for insects grow only in the larval state), the larva becomes transformed into the pupa (Pl. X. fig. 8). In this state the animals still move about in the water, and are often found suspended from its surface by two respiratory tubes, which, however, are not connected with the tail, but arise from the thorax; and the various parts of the perfect insect may be seen through the case or skin, within which they are closely packed.

When the pupa has attained its full development, the perfect insect emerges from it, leaving the water to fly about and seek food.

Tipu´lidæ.—In the water of ponds and pools a young larva (Pl. X. fig. 31) will often be met with, which is that of Chiron´omus plumósus, a largish gnat-like insect, belonging to the daddy-long-legs family (Tipulidæ). This larva exhibits the usual thirteen segments, including the head. In the young state the body is nearly colourless; but in the mature larva it is of a blood-colour, and about an inch in length. Beneath the first joint of the body are two foot-like processes covered with hair; and at the end of the body are also two processes, surmounted with hooks. The three last joints of the body are also furnished each with a pair of fleshy processes, those of the first pair being very short.

Hymenop´tera (ὑμἠν, membrane, πτερὀν, wing).—This, which forms the sixth Order of insects, contains the bee, the wasp, the ant, &c.

In the Hymenoptera there is a curious contrivance for linking the two wings on each side, so that they may form a single piece in the flight of the insect. It consists of a row of hooks, placed upon the anterior nerve of the hind wing, which play upon the folded-in corresponding edge of the fore wing; the hooks, sliding along this edge, allow of freedom of motion, although still holding the two wings together. This structure may be well seen in the wings of the Humble Bee when mounted in balsam.

The sting of the wasp and bee is also a singular organ. In both insects it is much alike, consisting of a sheath, slit up beneath, in which are contained two long setæ, or lancets, with bent-back (recurved) teeth near the end. These setæ are inserted into the flesh during the act of stinging, and at the same time the poisonous secretion from two glands is forced into the wound, which causes the severe pain resulting from the sting.

In the wingless neuters of the common ant, attached to the end of the tibia (Pl. X. fig. 34 c) is a beautiful pectinate process, somewhat resembling a comb (fig. 34 a).

Lepidop´tera (λεπἰς, scale, πτερὀν, wing).—This Order contains the butterflies and the moths, the entire bodies of which are covered with minute scales. When the insects are handled, these scales adhere to the fingers as a fine dust; and on pressing a slide against the insects, they may be removed for examination. They consist of a very slender and short quill, by which they are attached, and a flattened plate of various forms (Pl. X. fig. 19, b, f); it is, however, generally narrower near the quill, and expanded towards the free end, where it is often cut into lobes or toothlike segments. The scales are usually covered with continuous longitudinal lines or ridges, with granules of colouring-matter (pigment) situated between the two thin layers of which the scales consist. In some of them the form is that of a filament, either simple or branched at the end (fig. 19 e), when they resemble minute hairs. In the males of the large white Cabbage Butterfly (Pi´eris brass´icæ), certain of the scales of the wings are covered with longitudinal rows of very minute dots (fig. 19 f), and have little tassel-like bodies at the end. The males may be distinguished from the females by the front wings having no black spots, while those of the females have two upon each wing. When the scales are examined as they exist upon the wings of the Lepidoptera, they are found to be imbricated (Pl. X. fig. 3) or overlapping each other like the tiles on the roof of a house.

The Lepidoptera suck the honey of flowers by means of a spiral tongue (Pl. X. fig. 20 c) or ant´lia (antlia, a sucking-tube); this consists of two halves, which represent the maxillæ of other insects; and their margins are fringed with little tassel-like bodies, probably organs of taste. The antennæ (fig. 20 a) are many-jointed, clubbed at the ends (d) in the butterflies, and simple in the moths. The palpi (fig. 20 b) are short and densely covered with scales. In the large eyes the facets are very distinct and suitable for examination.

The larvæ are well known as caterpillars. They have six legs, as in the perfect insects, but rudimentary and with single claws; also some additional pairs of pro-legs, as they are called, with a crown of hooks, towards the hind part of the body. The spiracles of caterpillars are very favourable for observation.

Neurop´tera (νεῦρον, nerve, πτερὀν, wing).—This Order contains the Dragon-flies (Libellúlidæ), the Day-fly (Ephem´era), &c., in which the wings are usually so large and so beautifully netted. The species figured (Pl. X. fig. 1), which is wingless, is very common in old books and in collections of dried plants. It is whitish, mite-like, with setaceous many-jointed antennæ, 3-jointed tarsi, and very broad thighs (femora). Its name is At´ropos pulsator´ius.

Hemip´tera (ἤμισυς, half, πτερὀν, wing).—This Order contains the bugs and other noxious insects. Those which we shall notice are the species of A´phis, commonly known as Plant-lice and Green-fly, which are found too frequently upon unhealthy plants. The species figured (Pl. X. fig. 2) is that of the geranium (Pelargonium). The head is small and notched or emarginate. The body is oval and furnished behind with two prolonged tubercles, these being covered with scales, giving them a somewhat striated appearance (fig. 2 b). The antennæ are 6-jointed (c), the second joint being very small, the last joint long, excavated on one side, and ringed: these organs are reflexed over the body in the natural state. There are two compound eyes, and three simple eyes or ocelli, forming a triangle on the top of the head. The proboscis or rostrum is bent under the body, 4-jointed, and contains three setæ, two of them forming very slender lancets. The legs are long; the tarsi (fig. 2 a) 2-jointed, the first or basal joint being very minute, and the last furnished with two claws.

In a colony of these insects, some are winged and some wingless; those without wings being usually in the larva state, the pupæ having rudimentary wings, and the males and females usually perfect wings. Aphis brass´icæ is the destructive turnip-fly.

Coleop´tera (κολεὀς, sheath).—This, which is the last Order to be noticed, contains the Beetles, so easily recognized by their hard and horny fore wings or wing-cases. The parts of the mouth in these insects are exceedingly well adapted for examination; and as they are not fused or consolidated with each other, they serve to illustrate the typical constitution of the organs as existing in these animals.

Pteros´tichus (Ster´opus) mad´idus (Pl. X. fig. 23) is common in cellars and gardens among vegetable rubbish. The body of this beetle is shining black, the head projecting; the antennæ (fig. 23 a) are filiform, and compressed towards the end. The thorax is somewhat rounded, with a deep rough pit and a longitudinal stria at each posterior angle. The wing-cases, or elytra, are longitudinally striated; the wings, which in most beetles are concealed beneath the elytra when the insects are at rest, being absent. The tibiæ of the fore legs are notched on the inside (fig. 24 a), a fringe of hairs being situated in the notch; the tarsi are 5-jointed (fig. 24 b), the first four joints being triangular, the last elongate and terminated by two curved claws. In the male the three first joints of the tarsi of the anterior legs (fig. 24 b) are dilated and heart-shaped.

The parts of the mouth (which are named after an analogy with those of the higher animals) consist of the following pieces:—An upper lip, or lábrum (fig. 25 a), which is squarish (quad´rate) and slightly notched; a quadrate lower lip, or lábium (fig. 27), with a process on each side, and two 4-jointed lip-feelers or labial palpi (b); and below the labium is a chin, or mentum (fig. 27 a), with a projecting bifid tooth: these parts form the roof and the floor of the mouth. Next come the man´dibles (fig. 26), one on each side, which are stout, curved, and pointed; beneath which are the maxil´læ (fig. 28), also one on each side, and provided with a fixed claw (a), ciliated on the inside, and furnished with two pairs of jaw-feelers, or maxillary palpi, the inner (b) being 2-jointed, while the outer (c) are 4-jointed. It will be noticed that the jaws work laterally, or from side to side, and not perpendicularly as in the higher animals.

These parts may be found in most beetles which the observer may submit to examination, being however somewhat modified in different genera. We may consider those existing in the Lady-birds, or species of Coccinel´la, by way of comparison. The body in these insects is very convex, and the head sunk deeply in the thorax. The antennæ (fig. 15) are short, clubbed (clávate), and compressed. The thorax is short and lúnate, or half-moon shaped. The mandibles (fig. 13) are curved, bifid at the apex, and with a tooth on the inside near the base. The labrum (fig. 14) is transverse, or broader than long. The labium (fig. 12) is furnished with two palpi, which are 3-jointed. The maxillæ (fig. 16) are two-lobed, the lobes (b, c) being ciliated, and the 4-jointed palpi (a) have the last joint large and hatchet-shaped.

Coleopterous insects undergo complete metamorphosis, the larvæ being commonly known as grubs. The larvæ of the aquatic beetles will often be met with in the water of ponds or ditches, especially that of the common large water-beetle (Dytis´cus marginális), or water-boatman as it is called (Pl. X. fig. 7), and in various stages of growth. The structure of the mouth-organs (fig. 6), which are, however, imperfect or rudimentary in some parts, can be readily made out; and their names may easily be found by comparison with what has been stated in regard to the organs of the perfect beetle.

Examination, &c.—The means of catching insects will readily occur to the reader. A bag-net made of a curved piece of cane, to which is fitted a bag made of net, will serve to catch those which trust to flight for escape from their enemies, such as the Lepidoptera; and these may be killed by firm pressure of the thorax between the finger and the thumb. The running insects, as the beetles, may be caught in a spoon or with forceps; and they may be killed by immersion in boiling water or in camphorated spirit. In an excursion, most insects may be carried in a well-corked bottle containing a little wool and a lump of camphor, which stupifies them. When the insects are dead, the limbs should be extended into the natural position by means of pins, the insect being transfixed by a pin run through the thorax or one of the elytra and extending into a sheet of cork. To preserve them, they may be kept in a box, the bottom of which is covered with sheet cork, into which the pins are stuck.

The smaller beetles, &c., which cannot be transfixed with a pin, may be mounted as opake objects upon slips of card, the legs &c., being carefully spread out, and gummed in position with a strong solution of gum-tragacanth in boiling water. Many of the smaller Curculion´idæ or diamond-beetles, in which the labium forms a rostrum or beak, with elbowed or half-bent antennæ, form beautiful opake objects when thus mounted, on account of the brilliant scales with which they are covered.

There are two ways of examining insects—either in the entire state as opake objects or the separate parts mounted as transparent objects. In the former case the pin with which the insect is transfixed should be stuck into a slide made of cork, and this laid upon the stage, or the pin may be held by the forceps. In this way, with the use of the side condenser and a low power, the general form and arrangement of the parts of the insect can be made out. The more minute details must be searched for in the individual organs which have been picked off with forceps, and mounted in balsam.

If it be required to submit the parts of a dried insect to examination, this must be previously soaked in warm water for a time, as the legs, &c., become very brittle when dry, and are thus easily injured.

Rotator´ia (róta, a wheel) or Rotif´era (rota and fero, to bear).—The animals contained in this class are minute, being just distinguishable to the naked eye as white specks. They are common in long-kept infusions and among Confervæ in the water of pools and ditches. Their body is usually longer than broad, often presenting indications of rings; and at or near the posterior end is frequently found a prolongation resembling a tail, but terminated by two short moveable thumb-like processes, rarely a sucker, which enable the animals to cling to objects. The most characteristic organ, however, is a kind of rounded or oval disk, placed at the anterior end of the body, and furnished with cilia. When these are in active motion, the organ appears as a revolving wheel, whence the name of Wheel-animalcules, by which they are sometimes designated. The wheel-organ enables the animals to swim through the water, and also brings their food to the mouth by the currents which it produces. It is usually cleft into two or more lobes, and can be retracted, as is commonly the case when the animals are disturbed.

In many of these animals the body is more or less covered by a horny shell or carapace; and in some it is fixed at the bottom of a tube, within which it can be withdrawn. On the anterior part of the body are frequently seen two or more red spots, which represent eyes. The alimentary canal is mostly distinct, being indicated by the colour of its contents, and it is lined with cilia. Towards its front portion is a gizzard (Pl. XI. fig. 2 a) containing teeth, which are sometimes attached to a jointed jaw-like framework; these are usually in active motion. No heart or blood-vessels have been observed in the Rotatoria; but on each side of the body in many of them is a long wavy tube, containing at intervals minute ciliated bodies, the cilia propelling the water through the tubes, and so exerting an aërating or respiratory function. The reproduction of the Rotatoria takes place by the formation of ova, which may often be distinguished within the body of the parent.

Rot´ifer vulgáris (Pl. XI. fig. 2) is a common species. It has a spindle-shaped body, which is capable of contraction almost into a ball. The front or head end is sometimes protruded (fig. 2), at others retracted and obscured by the exserted disk (fig. 2*); and beneath it is a tentacle-like organ, supposed to represent an antenna (fig. 2 b). The position of the jaws is indicated at a. The alimentary canal is seen running down the body; and two ova exist, one on each side of it, these being often recognizable by the existence of the eyes and jaws. At the end of the body are two lateral processes, and a tail-like piece, which can be withdrawn or protruded and is furnished with two moveable portions or toes.

Pterodína pat´ina (Pl. XI. fig. 3).—This species has a shell or carapace on the back, a two-lobed rotatory organ, two eyes, and a slender wrinkled tail ciliated at the extremity. The curved alimentary canal, and the two strong muscles inclined at an angle, are easily distinguishable.

Flosculária ornáta (Pl. XI. fig. 4) is a very beautiful member of the Rotatoria, and is found adhering to Confervæ and other water-plants. The body is club-shaped, and contained in a transparent tube, the ringed narrower portion being fixed to its base. The rotatory organ is divided into five or six lobes, furnished with long, slender, radiating tentacular filaments; these are not vibratile like ordinary cilia, but can be slowly moved. In the contracted state, the filaments form a pencil-like bundle.

Examination, &c.—The Rotatoria are best examined in the living state, the drop of water in which they are viewed being very small, so that their movements may be impeded; and while they are struggling to escape, the various parts of the body will come into view. Their preservation has been attempted by drying on a slide; but when dead they become so contracted and altered, that it is difficult to make out their structure. Should the observer wish to record any observations on their reproduction or habits, it will be well to preserve a specimen of the jaws and teeth, as the species might be with certainty identified by careful examination of their minute structure.

Entozóa (ἑντὀς, within, ξῶον, animal).—This class consists of the parasitic worms, as the Tape-worm (Tænia), the Thread-worm and Round-worm (As´caris), which live within the bodies of man and animals. It also includes the microscopic eel-like animalcules (species of Anguil´lula) which are found in sour paste (A. glútinis), in vinegar (A. acéti), and in blighted wheat (A. trit´ici). Some of the species of allied genera are met with in damp moss and in the débris or fragments of vegetable substances decaying in water. The general appearance of the microscopic species is that of a minute colourless eel, writhing in the water (Pl. XI. fig. 1). Their internal organs are difficult to distinguish. The alimentary canal is usually evident, and dilated into a kind of stomach, containing near its commencement some rod-like or otherwise-formed teeth. In the species figured there are two apparently tubular lancets, which are capable of protrusion, and evidently serve to wound the prey.

CHAPTER XII.

RADIATA.

Polypi.—These animals are mostly marine. They are either single (Pl. XI. fig. 5), or compound (Pl. XI. fig. 15), i. e. the bodies are united; in the latter case the bodies being usually situated in horny cells upon a branched polypidom. But in many of them, which do not occur in this country, there is an internal solid calcareous skeleton, of which coral is an example. The animal bodies are soft, and furnished at the front end with a crown of tentacles (fig. 15 a); these are contractile, and serve to enable the animals to catch their prey. The horny, branched, and plant-like polypidoms are often found on the seashore, and are popularly confounded with sea-weeds.

Hy´dra vulgáris (Pl. XI. fig. 5) is a fresh-water species, which is commonly met with among collections of water-plants, and may generally be obtained by collecting some of these and placing them in a glass jar of fresh water. When the water has stood for some hours, the Polypes will be seen, on careful examination, adhering to the sides of the glass. The body of the animals is cylindrical, hollow, and furnished with from six to ten tentacles, arranged in a circle, in the centre of which is the mouth. The tentacles are hollow, and communicate with the cavity of the body. On examination with a high power, the tentacles will be found to exhibit minute oval sacs, containing a long fibre coiled up within them; and when the tentacles are touched by any foreign body, the fibres are suddenly discharged. These are the stinging or urticating organs. The Hydræ move very slowly; but the body is very contractile, and is often seen of various forms. When a minute animal, as an Entomostracan, happens to come into contact with the tentacles, these curve around it, holding it firmly, and finally bringing it to the mouth. It is then forced into the cavity of the body of the animal, where it is digested, the remains being discharged at the mouth. The movements of the Hydra, when devouring its prey, form a very curious and interesting spectacle. The Hydræ are propagated by budding or gemmation, also by the formation of capsules in the walls of the body, containing ova and spermatozoa. The young Polypes formed by budding are represented in the figure, adhering to the base of the parent.

Sertulária púmila (Pl. XI. fig. 15) is a marine species, the polypidom being frequently found adhering to Fuci and other sea-weeds; it is about half an inch long. The cells are opposite, pointed at the ends, and with an oblique orifice. The tentacles are fourteen in this species. In the summer large ovate cells are found, arising from the polypidom; these contain the eggs, and are called ovisacs or ovig´erous vesicles.

CHAPTER XIII.

PROTOZÓA (πρῶτος, FIRST, ξῶον, ANIMAL).

Amœ´ba dif´fluens (Pl. XI. fig. 10) is common in water in which portions of plants have been kept for some time. When first placed on the slide, the body appears as a minute, transparent, rounded mass of jelly; but if observed for some time, it will be seen slowly to protrude its root-like processes; and foreign bodies, as Diatomaceæ or other minute Algæ, will often be found imbedded in its substance.

Arcel´la vulgáris (Pl. XI. fig. 6) is found among Confervæ in ponds and ditch-water. It is contained in a hemispherical shell or carapace, from the round orifice of which the lobed processes are protruded. The shell is covered with minute pits.

Arcel´la aculeáta (Pl. XI. fig. 7) has the convex shell furnished with spines; fig. 8 represents the animal with its processes extended; while Arcella dentáta (Pl. XI. fig. 9) exhibits an angular or somewhat toothed membranous shell. Both the latter species are met with in the same localities as the first.

Actínophrys sol (Pl. XI. fig. 12) is a very beautiful and excessively delicate Rhizopod. The body is spherical, and covered with very delicate and slender cilia-like processes. Its movements are exceedingly slow, and can only be observed by prolonged watching. The body appears to be reticulated, from the presence of numerous vacuoles.

Two large groups of genera and species of Rhizopoda, the animal bodies possessing the above general characters, mostly with very slender processes, exist, in one of which (the Foraminifera) they are contained in calcareous shells, often of elegant forms; while in the other (the Polycystina) the shells are siliceous or composed of flint, both kinds of shells being perforated with holes. These shells, which occur in the fossil state in enormous numbers, sometimes forming mountain-masses, are extremely beautiful objects for the microscope.

Spon´giæ.—This class contains the Sponges, almost all of which are marine and foreign, and therefore not likely to come under observation in the perfect state. The substance commonly called sponge is the horny skeleton of the animal, consisting usually of rounded fibres (Pl. XI. fig. 13), irregularly netted and interlacing. The surface of a sponge exhibits minute pores and larger pouting orifices; the former of which admit currents of water, to be discharged at the latter, both being the mouths of continuous channels. The surfaces of the channels are lined with sarcodic matter, which takes the form of ciliated amœbiform bodies, by which the currents of liquid are produced.

The horny fibres of sponges are strengthened by little siliceous or flinty bodies of various forms (Pl. XI. fig. 13 a, b, c), which are imbedded in the substance of the fibres or attached to their surface, and form very curious microscopic objects. They are called spic´ula (spiculum, a dart), being often of a pointed form. In some sponges they are calcareous.

Infusor´ia.—The animals contained in this class are usually very minute, being rarely even perceptible to the naked eye, except when existing in very large numbers, so as to render the water milky, green, or red. They are found in all kinds of water, but especially in stagnant pools and in decomposing solutions or infusions of vegetable matters. The true structure of their bodies is a matter of doubt, some authors having considered them as being highly organized, while others have regarded them as consisting of simple cells; and whether they are correctly referred to the Protozoa must remain at present a matter of doubt. The body is of various forms, as represented in Plate XI. figs. 16-40. In some of them it consists of a simple sarcodic mass, evidently without any outer skin, as shown by its ready adhesion and laceration on accidental contact with foreign bodies; while in others the surface is regularly dotted with little depressions, or with nodules, so as to resemble a definitely organized structure.

The most striking character of the Infusoria is the presence of vibratile cilia, which are variously arranged; in some entirely covering the body, irregularly or in regular rows, in others being situated at definite parts only. By the action of the cilia they are enabled to swim freely in the water, also to obtain their food, which consists of minute Algæ or fragments of animal matter. In many of them there is a special row or set of cilia, which, by the currents it produces, urges the particles of food suspended in the water towards the mouth. The cilia also act as respiratory organs, by changing the water with which their bodies are in contact. In some of the species there are stout bristles or setæ, by which they are enabled to crawl upon water-plants.

On carefully examining the bodies of the Infusoria, rounded granular spots will be seen, frequently containing minute Algæ, &c. (fig. 28 b). These spots are the digestive cavities, and have been called gastric sac´culi; but whether they are definite sacs or mere excavations, formed by the particles of food having been forced into the softer internal substance of the body, has not been positively determined. The sacculi may be filled artificially by mixing very fine indigo, or carmine, on a slide with the water in which the Infusoria are contained. A definite food-tube or alimentary canal has been detected in a few of the Infusoria; but it cannot be shown to exist in the majority of them.

A mouth exists in most of them, and is sometimes indicated by a row or set of cilia somewhat larger than those existing upon other parts of the body, and leading to or placed near it. The particles of food which have entered the body are often seen to pass round it, as if circulating, descending on one side and ascending on the other.

In addition to the gastric sacculi, certain clear transparent spots may also be seen within the body, appearing light or dark according to the adjustment of the focus. If these are attentively watched, they will be seen to contract and finally disappear, becoming again distended and vanishing at tolerably regular intervals. These are the contractile vesicles (figs. 27 a, 28 a, 37 a), and they contain a clear liquid, the nature of which is uncertain.

In many of the Infusoria is a round or elongate granular body (figs. 31 a, 36 a), which is called the nucleus, the term having been applied to it from a notion that the Infusoria consisted of simple cells. A minute red spot is also often seen at the anterior end of the body, which is supposed to represent an eye, and is called an eye-spot. The Infusoria are propagated in several ways:—by budding or gemmation, new beings sprouting out in a bud-like form, usually from the base of the parent; by division, either transverse (fig. 26 a) or longitudinal (fig. 34 b), of the body gradually into two parts, each of which subsequently becomes a perfect animal; by encysting, the body contracting into a globular form, and forming a firm coat around it, the contents becoming resolved into a numerous progeny of young; and by conjugation and the agency of spermatozoa and ova. We will now proceed to the examination of a few species, arranging them in the order of the families to which they belong.

Monad´ina.—In this family the bodies of the Infusoria are very soft, and without a skin or integument; they are also exceedingly minute, and will not admit the particles of indigo.

Mon´as lens (Pl. XI. fig. 16) is very minute, and commonly found in old infusions. Its body is rounded and flattened, and granular on the surface. At the front end of the body is a whip-like or flagel´liform (flagel´lum, a whip) filament, differing from a cilium in being rigid at the base and moveable at the end only, by which it is enabled to row itself through the water with a wriggling motion.

Cercom´onas glob´ulus (fig. 17) has a spherical body, with two flagelliform filaments, one arising from the front, the other from the end of the body. In Cercomonas crassicau´da (fig. 18) the posterior filament is replaced by a tail-like narrowing of the body.

Heterom´ita ováta (fig. 19) has the body ovate, with two long anterior flagelliform filaments, one of which is directed forwards, while the other trails behind.

Anthophy´sa mül´leri (fig. 20) has the monad bodies arranged in little heads at the ends of an irregularly branched brown stalk. After a time they become detached and revolve freely in the water.

Dinobry´ina.—Dinobry´on sertulária (Pl. XI. fig. 21) forms a minute Sertularia-like polypidom, consisting of rows of cells, each containing an oval monad with a single anterior filament. The two last species are common in bog-water.

Thecamonad´ina.—In these Infusoria the body is inclosed in a firm and sometimes brittle shell or carapace.

Trachelom´onas volvoc´ina (Pl. XI. fig. 22) has a spherical red shell, the body being furnished with a single filament and a minute red eye-spot; while Chætogléna volvoc´ina (fig. 23) has an oblong shell, covered with little spines.

Euglénia.—In this family the form of the body is constantly changing, being at one time spherical, at another fusiform or ovate. It is covered with a contractile skin or firmer external portion, and has one or more flagelliform filaments for locomotion. The species are common in stagnant pools, often colouring the water green or red.

Eugléna vir´idis (Pl. XI. fig. 24) has a spindle-shaped body when fully expanded, the ends being pale; and at the front end is a red eye-spot.

Astásia hæmatódes (fig. 25), which is probably a form of the Euglena, is found in stagnant pools, which it renders red. It has no eye-spot.

Enchélia.—These Infusoria are found in stagnant water and in decomposing infusions. The body is covered with cilia variously arranged, but there is no integument nor mouth.

En´chelys nodulósa (Pl. XI. fig. 26) has a colourless, oblong, irregularly nodular body, coated with very slender radiating cilia, and often exhibits numerous vacuoles. It is frequently found undergoing transverse division (fig. 26 a), the body becoming gradually constricted until it separates into two parts, which become perfect animals.

Alys´cum sal´tans (fig. 39) has an ovoid-oblong, slightly furrowed body, surrounded with radiating cilia, and has a side bundle of long retractile cilia, by means of which it leaps from place to place in the water.

Kerónia.—In this family the body is soft, irregularly ciliated, without a special integument, but has an oblique row of vibratile cilia leading to the mouth, and stouter cilia or bristles (setæ) on certain parts of the body. The sacculi often contain Diatomaceæ, &c.

Oxyt´richa gib´ba (Pl. XI. fig. 27) has a colourless, oblong body, somewhat expanded in the middle, with setæ at the two ends. In the side view (fig. 27 a), the body is seen to be convex above and flattened beneath.

Paramec´ina.—The species belonging to this family have a soft, flexible body, which is usually oblong and flattened beneath, with an integument covered regularly with pits and rows of cilia.

Col´poda cucul´lus (Pl. XI. fig. 30) has a slightly compressed body, ciliated all over, and kidney-shaped or rounded on one side and notched on the other, the surface exhibiting rows of nodules. The mouth is situated at the bottom of the notch.

Paramécium aurélia (fig. 28) has the body oblong or oblong-ovate, the mouth being placed near the anterior third of its under part. This infusorium is of comparatively large size, and is often found in immense numbers in infusions, which it renders milky. It is admirably adapted for showing the sacculi, which are easily filled with indigo. The body exhibits two remarkable stellate organs, consisting of a central contractile vesicle, surrounded by several radiately placed oval vesicles, which may be seen to contract and dilate with great regularity. The body is coated with very fine cilia.

Amphilep´tus fascíola (fig. 29) is furnished with an elongate fusiform or lanceolate flattened body, with a lateral oblique mouth.

Chil´odon cucul´lulus (fig. 37) has an oblong thin body, irregularly wavy on the sides; the mouth being situated obliquely in front of the middle, and furnished with a cylinder of parallel rod-like teeth.

Nas´sula el´egans (fig. 31) has the body ovoid or oblong, becoming globular when contracted, the mouth being furnished with teeth as in Chilodon. It is often found among Oscillatoriæ.

Urceolarína.—Vorticel´la convallária (Pl. XI. fig. 34) is very commonly met with in decomposing infusions. The bell-shaped body is fixed at the end of a slender stalk, which is often seen to be extended and then suddenly contracted into a spiral (fig. 34 a). The cilia are arranged around a raised rim at the front of the body, and extend down a fissure leading to the mouth. The sacculi of this infusorium may be readily filled with indigo. The process of longitudinal division may also often be observed, taking about an hour for its completion; and when the new individual is about to separate from the parent, a ring of cilia may be noticed to have sprung up around the base (fig. 36). The encysting process is also often visible, the cilia disappearing, and the body becoming globular and secreting a cyst around it; after a time the contents become resolved into a number of embryos, which escape by the bursting of the cyst (fig. 35). In some cases the Vorticella assumes the form of a Podoph´rya (fig. 40), the surface becoming covered with tentacle-like processes. This Podophrya was formerly considered a distinct species.

Vaginic´ola crystal´lina (fig. 33) is contained in a crystalline tube, from which the body can be protruded. The body is of variable form, presenting when fully extended a trumpet shape. The cilia exist at the anterior end, and extend down a lateral fissure as in Vorticella. It is found attached to Confervæ in the water of ponds and bog-pools.

Sten´tor polymor´phus (fig. 38) is a very beautiful trumpet-shaped infusorium, the body being covered with spiral rows of cilia. The rim is furnished with stouter cilia, its margins at the notch being spirally turned inwards. This infusorium is often found in little groups attached to a gelatinous mass (fig. 38 c); and it is met with also in a free or unattached state.

Colep´ina.—Cóleps hir´tus (fig. 32) has a barrel-shaped carapace, transversely and longitudinally furrowed, the furrows being occupied by cilia. It has two or three spines behind, and ten or twelve at the front end of the carapace. It is common among Confervæ, and is very voracious, feeding upon dead Entomostraca, &c.; and if disturbed, at its meal by moving the cover, it will soon return and resume feeding as before.

Examination.—The Infusoria must be examined during life; for they are so altered by preservative liquids that they cannot be well preserved. The shells of those that are provided with them may be kept simply dried upon a slide, and in this way a few will retain their form, and the cilia of all may be more easily distinguished; the vacuoles may also then be seen very distinctly. When they are confined in a small quantity of water and are about to die, a curious phenomenon may be observed in them, a number of oil-like sarcodic globules exuding from the body, and within these, vacuoles may often be seen to form spontaneously.

The Infusoria may be collected in small phials; but it is difficult to keep them, as they form the food of the Entomostraca, the Rotatoria, and the larvæ of insects; so that their enemies are very numerous, and they soon disappear.

Classification.—Before leaving the subject of living bodies, it may be well to make a few remarks upon their systematic relation as defined by classification.

All natural bodies are referable to one of three great kingdoms, viz. the Animal, the Vegetable, or the Mineral Kingdom. The bodies belonging to the latter seldom come under the notice of the microscopic observer, as they are mostly visible to the naked eye, and their minute structure is the same as that of the larger masses. The general structure of the members of the vegetable and animal kingdoms has been illustrated in the preceding pages. These bodies are distinguished from those of the mineral kingdom by their vital power of appropriating surrounding matters to their own nutrition and growth—this power being exercised by their organs, or, in the lowest forms, by any portion of their substance. Hence animals and vegetables or plants are termed organic bodies, while minerals are termed inorganic bodies, as having no organs; and the material of which organic bodies consist is termed organic matter, that of minerals being inorganic matter. But in both animals and plants inorganic matter is mixed with the organic matter, having been taken up or absorbed from the inorganic kingdom, although it does not usually exist in its characteristic condition, which is that of crystals, i. e. angular solids, as crystals of Epsom salts, &c. The individual members of the animal and vegetable kingdoms are systematically divided into certain groups, and these into successively smaller groups until we arrive at the species. These groups are founded upon the possession of certain points of resemblance by their members, forming the distinguishing characters, and their names are significant and definite. They usually run as follows, the larger and higher groups standing first in order:—Kingdoms, Subkingdoms, Classes, Orders, Families, Tribes, Genera, and Species. But it must be observed that the term species has a different value from that of the other terms; for the individuals of which the species consist are not only related by resemblance of structure, but also by their origin—being supposed to have derived their origin from a parent of original creation; while the other groups have, as far as we know, no other relation than that of similarity of structure.

It is obvious that the characters of the various groups might be founded upon peculiarities of any kind. But on this point two methods must be specially distinguished, in one of which the groups are founded upon the sum of all the peculiarities, while in the other they are based upon the structure of single parts or organs. In the former case the system is called natural, in the latter artificial. And while the latter often brings together beings which have perhaps but one or two points of resemblance, and separates others which are closely related, the former associates those which are really and naturally similar.

All the groups have special names, so that they may be referred to and spoken of as in the case of common things; the names being composed of Greek or Latin words, so that they may be intelligible to all nations; and as these are dead languages, they will remain good for all time.

In mentioning the name of an animal or plant, the name of the genus is always used with that of the species; thus, the name of Chickweed is Stellaria media. Because there are mostly several species in a genus; so that if the name of the genus only were used, the species meant would be uncertain; and as there are often species of the same name in different genera, if the name of the species only were used, the genus meant would be doubtful.