ral simplicity of structure. In the absence of distinct organs for the various purposes of the economy, they correspond with the lower Radiata, but they differ in the extreme activity of their movements. A separate division of the animal kingdom might almost be formed for them alone, so difficult is it to assign them any place in the ordinary scale. Some of them exhibit a tendency to associate into compound structures, like the Polypifera. The PORIFERA, or Sponge tribe, are of all animals those which approach nearest to plants, in the absence of the characters peculiar to the kingdom in which they are placed, and in the want of definiteness of form. Certain movements exhibited by them, however, and their close affinity with some of the Polypifera, render it proper that they should be classed among animals.

CLASS XIX-POLYGASTRICA.

This class includes the greater part of those minute beings termed Animalcules, which have been, from the time of the discovery of the microscope, such fertile sources of wonder and delight, both to those who have themselves observed them, and to those who have heard from others of their marvels. Previously to that epoch, it was not suspected that beings existed of such minuteness as to be invisible to the eye, much less was any idea entertained of the extreme smallness of many species; and still more improbable and absurd would the statement have been deemed, that such beings are not of rare occurrence, but abound in every drop of stagnant water, and even exist in the whole mass of the ocean. Yet such has been shown to be the fact, and every improvement in the powers of the microscope has enlarged our ideas of the extent of animal life in the fluids of the globe; so that the philosopher is now ready to admit no limit to the possible minuteness of living beings, but looks to still further improvements in the microscope as a means of extending his acquaintance with them, and not as likely to set any bound to his inquiries.

Animalcules may be obtained without difficulty for microscopic examination during the warmer part of the year, by skimming the surface of ponds, especially those in which the water exhibits a red or green tinge, or in which it is covered with duckweed, or with the slimy film which may often be noticed. Many curious species frequent these situations; but the commoner ones may be obtained with even less difficulty, by placing soft vegetable matter, of almost any description, in vessels with water, and exposing the mixture to the sun and air for a few days. As soon as decomposition begins actively to take place, Animalcules may be detected in the fluid, and in a short time they often crowd it most densely. These are generally at first of a simple kind; but new species soon prevail, and those first seen disappear. Different kinds of vegetable matter seem to favour the development of the different species; and there are some Animalcules that can be produced in no other way than from an infusion of some particular substance. Asparagus stems that have been boiled will favour the production of Animalcules, with perhaps as much readiness as any vegetable matter; hay, chopped straw, the leaves of plants, and other common ingredients, may also be advantageously employed.

In the class Poly gastrica are included all the most minute species of true Animalcules, and some among the larger ones; but as a whole, the beings composing it are smaller than the Rotifera, and far smaller than those of any other class. The largest among them are but with difficulty seen by the naked eye, and of the dimensions of the smallest the mind can scarcely form an adequate conception, although they may be numerically stated. The class takes its name from the belief entertained by the celebrated Prussian naturalist, Ehrenberg (who has

devoted almost his whole life to the study of the microscopic forms of existence), that the animals composing it may be characterized by the possession of many distinct stomachs or digestive sacs. There is some doubt upon, this question, however; that which can actually be seen will be presently stated; and it will be preferable to enter no further into the question in this place.

The bodies of these Animalcules are of very soft con sistence, and very transparent; so that they resemble flakes of very thin jelly. Their forms are extremely variable; and, in some species, the same individual at different times alters its shape so completely, that it could scarcely be recognised. Indeed, many mistakes have occurred from this cause. The softness of the tissues of the Polygastrica is also seen when, in swimming, they encounter an obstacle, there seems scarcely any limit to the change of form to which many will submit, in orde. to pass the obstruction. They are not all so flexible, however; for in some species the body is enclosed in a siliceous sheath of very great delicacy, which gives support and protection to the still more delicate structures it contains. It is the accumulation of such sheaths that has given rise to the collections of Fossil Infusoria (as they have been termed), which will be hereafter noticed. Sometimes the whole body is contained within the sheath; whilst in other instances a sort of trunk or foot may be projected from its opening.

The bodies of the Polygastrica are usually fringed with cilia, by the vibrations of which they are assisted in their own movements, and also in the acquirement of their food. Sometimes these cilia are disposed along the whole extent of the edges of the body; in other instances they surround the mouth only, and from that part they are seldom absent.

For some time after the discovery of the Infusoria, it was supposed that they must obtain their nutriment by absorption through the substance of their bodies, for no mouth, stomach, or alimentary tube could then be discerned. But, by placing them in water, through which very small particles of colouring matter (such as indigo or carmine) were diffused, it was perceived that these particles are introduced into the interior of the body, and are collected in cavities hollowed from the general mass. And, subsequently, the improved powers of the microscope have enabled an entrance to the interior of the body or mouth to be discovered in almost every instance, and a second orifice in a considerable number of species. The mouth is commonly furnished with a border of cilia, and sometimes with a set of projecting bristle-like teeth, which are used in laying hold of smaller Animalcules, on which the possessor of this apparatus feeds. The introduction of food into the cavity of the body in those species which are destitute of this appendage, may be best watched by diffusing colouring particles through the water in which the Animalcules are swimming. They are seen to be drawn into the mouth by the vortex or whirlpool occasioned by the action of the cilia; and soon after entering it, they are observed to be united together into little round balls, as if they had been compressed in a small spherical cavity. These balls are sent one after

the other into the general cavity of the body, where they seem to lie in the midst of a soft gelatinous pulp, and in which they perform a slow revolution--the foreinost ones escaping at intervals from the second orifice, whilst new ones are being pushed in from the mouth behind.

This is all that can as yet be certainly stated in regard to the digestive apparatus of the Polygastrica; since the ,opinion of Ehrenberg, that the whole body is occupied by a series of small distinct globular cavities or stomachs, connected by an intestinal tube, is not adopted by other naturalists. Nothing will therefore be said of his classification of these Animalcules, which is principally based upon characters furnished (according to his idea) by the arrangement of the stomachs.

The largest species of the Polygastrica probably never exceed 1-20th of an inch in length; the smallest at present known are about 1-2000th of a line in diameter; but there is no reason to suppose that this is by any means the limit of minuteness. They usually multiply by spontaneous division, the body of the parent splitting into two or more parts, each of which soon becomes a perfect being, capable of going through the same pro

cess.

From observations which have been made upon the species in which these changes are most rapidly effected, it has been calculated that, under the most favourable circumstances as to food, temperature, &c., a hundred and forty million millions may be produced in four days-a degree of fertility which assists in explaining the almost universal diffusion of these Animalcules, and their sudden appearance in countless swarms.

Our ideas of the vast amount of animal life existing in this class have lately received a considerable extension by the discovery that their remains, minute as they are, not unfrequently accumulate into masses of great extent. It is only of those species in which the bodies are covered with an envelope containing earthy matter, that the remains can be thus preserved; and the substance formed by their aggregation seems to be an impalpable powder, such is the minuteness of each particle. Such substances have long been known under various names. One is the Tripoli, or Rottenstone, used in the arts for polishing metals. Another is the siliceous meal which has been used in Sweden, on account of its supposed

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Fossil Remains of Animalcules, forming Tripoli. nutritious qualities, mixed in bread with flour and the inner bark of trees, in times of scarcity. Both these, as well as many other substances, consist entirely of the siliceous shields or envelopes of Animalcules, closely allied to, if not identical with, species at present existing; and the quantity of animal matter which is dried up in the latter, and which may be determined by the effect of heat (this dissipating the animal portion and leaving the siliceous particies unchanged), is sufficient to account for its nutritious properties.

CLASS XX.-ECHINODERMATA.

The class of Echinodermata, comprehending those well-known animals, the Asterias (Star-Fish), and Echinus (Sea-Urchin), takes its name from the prickly skin with which most of the tribes it includes are provided. But

* Of the larger species more than 180 millions are estimated to weigh only a grain.

this is not an universal character; for some of the species, which border upon other groups, have a skin destitute of any appearance of spines. There is a little difficulty, however, in distinguishing the animals of this class from all others, for in nearly the whole of them the radiated structure, or the arrangement of parts in a circular form, is very evident; and they are the only animals among the Radiata which have the power of moving from place to place, and have at the same time an integument firm enough to resist pressure.

Although the character and degree of organization in the different subdivisions of this class may be regarded as about the same, the form of the organs, and the mode in which they are arranged, are very different, so that it will be better to describe each group separately. The class may be distributed into three orders; the STELLE. RIDA, including the Star-Fishes and their allies; the ECHINIDA, including the Echinus and its allies; and the HOLOTHURIDA, a group less commonly known, and differing much from the others.

Order I-Stellerida.

The common Asterias, or Star-Fish, which may be taken as a type of the order Stellerida, is covered with a tough leathery skin, beset with prickles. The animal has the form of a star, with five or more rays springing from a central disc. In the middle of one side of the disc is situated the mouth, and this side, according to the usual habits of the animal, must be considered the lower one. The mouth opens into a globular stomach, which sends out prolongations into the several rays, but there is n intestine in this animal, nor any second orifice to the digestive cavity, so that the indigestible parts are rejected by the mouth, as in the Sea-Anemone.

Asterias: upper surface.

If the tough prickly skin be removed, it is seen that it is supported by a series of bony plates, beautifully jointed together. Along the under side of each ray, the plates exhibit a series of perforations, through which there issue, in the living state, a large number of minute tubes, which may be occasionally seen projecting on the out side. These tubes are termed the feel, on account of the use to which they are subservient. Every one of them is connected, on the interior of the shell, with a small vesicle or bag, which is capable of being distended with water by a system of vessels adapted to the pur pose, and of contracting so as to force its contents into the tube. The tube consists of a delicate elastic mem brane, covered with two layers of muscular fibres, the one circular, the other longitudinal, and furnished with a sucker at its extremity. When distended with water, the tube projects from the body; and, if the sucker be then applied to any movable substance, it will be drawn towards the body by the elasticity of the membrane, when the distending force is relaxed. Although each sucker is small and weak, the combined efforts of many give the animal considerable power, not only of drawing prey towards the mouth, but of moving its own body from place to place.

The order Stellerida includes a large number of forms having a general resemblance to the Stor-Fish, but dif fering much in the relative proportion of the body and rays. Thus, in some species, the arms seem to make up the entire animal, no central disc being present, save that formed by their union. In others, the arms appear sim ply appendages to the central disc, to which the stomach. and other important organs are confined. instances, the arms send off lateral appendages; and these occasionally again subdivide, so that a branch-like structure is produced, such as we find in the Comatuli. A very remarkable tribe, included among the Stelle

In some

these. In some species, the spines are five or six inches long, whilst the diameter of the body is much less. The tubular feet often escape notice on account of their trans

spines, when it is merely resting upon them as fulcra, and drawing itself forwarde by these curious organs. It is to be remembered that the body will weigh much less in water than in air, and thus may be supported upon spines of great delicacy.

Order II.-Echinida.

In the Echinida we find the body usually of a somewhat globular shape, and enveloped in a firm shell, composed of a very regular series of plates jointed together. In the Echinus, the shell of which is commonly known as the Sea-Egg, we observe two orifices situated at the poles, as it were, of the globe. The larger of these orifices is the mouth: at the smaller one the intestinal tube terminates. The mouth, as in the star-fish, is generally directed downwards. It is furnished with a very curious apparatus of teeth, which are worked by powerful muscles, attached to projections of the shell, that may be seen on the inner margin of the mouth; and their points can even be protruded beyond the mouth, so as to lay hold of prey brought to them by the long tubular feet. By the action of the teeth, the food is ground down before it passes into the intestinal tube, which is here of considerable length, and takes a couple of turns round the shell before its termination. Round the second orifice of the shell are disposed the ovaria, which are very largely distended with eggs at some seasons, and are eaten under the name of the roe of the sen-egg.

It is the exterior organization of these animals, however, that presents us with the greatest sources of interest. On looking at the Echinida in their living state, we see that most of them are covered with spines of considerable size, instead of with such small prickles as the Asterias bears. Moreover, these spines are seen to be movable at their bases, and their power of motion is due to their peculiar connection with the shell. Each spine is spread, at its root, into a cuplike form, and the hollow of this cup fits upon a little knob or tubercle projecting from the surface of the shell, so that a complete ball-andsocket joint is formed. The spines are connected to each other, and held on the shell by the skin which covers the latter, and which is attached around their roots; and it is by the contractions of this skin that they are moved.

On looking at the exterior of the shell of an Echinus, it is seen that the tubercles are arranged with great regularity, and that the larger ones are confined to particular rows of plates, which are hence called tubercular plates. Between these are smaller plates, commonly bearing smaller tubercles, and perforated with a number of minute holes, for the passage of the tubular feet; these are Shell of Echinus; a. tucalled ambulacral plates. The bercular plates; b, amtubular feet, like the spines, are bulacral plates. much longer than in the star-fish. They are always capable of being projected beyond the spines; and, taking an attachment by the suckers at their extremities, they can cause the shell to roll, as it were, upon the points of

The structure of the shell itself, and the mode of its increase, are not among the least interesting parts of the history of this animal. The shell is composed, as already stated, of a large number of plates disposed with great regularity, and accurately fitted together. These plates are usually of an hexagonal shape, but where large and small ones join, there is of course some modification. Now, it is obvious that a shell of globular form can only be regularly increased in all its dimensions by the equal growth of every part of it. This addition is provided for by the interposition of a thin layer of membrane, from which the shelly substance may be deposited between the edges of all the plates; and this membrane also answers the purpose of forming a connection between the skin covering the shell and the organs of nutrition within.

These animals are generally found on sandy shores, and especially in little nooks secluded from the direct influence of the waves. Some of them excavate hollows in the sand by means of their spines, and one species even works its way into solid rock. Their food is of a mixed quality. Fragments of shells, Crustacea, and other marine animal products, are found in their stomachs, as well as portions of sea-weed. They obtain their prey whilst lurking in their hollows, by allowing their tubular feet to play loosely in the water around; and when any small animal touches the sucker at the end of one of them, it is soon secured by the assistance of others, and drawn within the range of the powerful teeth.

It is not in every species of Echinida that the globular form is so well marked as in the Sea-Egg. There are many in which the shell is more or less flattened, and in which one or both the apertures of the alimentary canal are out of the centre. In these the dental apparatus i either absent or comparatively feeble.

Order III-Holothurida.

In the last order of Echinodermata, the Holothurida, we find the characters of the class remarkably blended with those of Articulated animals. The body is not enveloped in a hard shell, but in an elastic skin, destitute of spines or prickles. It retains, in some species, the globular form, but in many it is very much prolonged, so as to be almost cylindrical, and thus to resemble that of the Worm tribes; and it is occasionally even marked by transverse bands, indicating a division into segments. Still, however, a distinctly radiated conformation may be seen around the mouth; and some of these animals look as if a star-fish were set as a head on the body of a large worm. In the general conformation of the internal organs they correspond with the Echinida; but they are in some respects more complex, and the respiratory organs are constructed upon the plan of those of the Articulata. They thus form a very interesting link of connection between the Radiated and Articulated subkingdoms.

The skin of most of the Holothurida is so very elastic that they can change their size and form in a remarkable degree. They are capable, too, of swimming with con siderable rapidity, and some of them crawl like slugs upon solid surfaces. Small species are occasionally found in British seas; but on some tropical shores they are very abundant, and grow to the length of eighteen or twenty inches. They are sometimes eaten by the poor on the Neapolitan coast; but in the Malay archi