the world around it; and it reproduces its like. All animals discharge these three sets of functions, but the higher animals have much more complicated relations with the world outside than the lower. The study of the way in which these functions are discharged by different animals, belongs to the science of Physiology, and will not be discussed here. In what follows, therefore, we shall simply consider the form or structure of the more important types of animal life.

CHAPTER II.

CLASS RHIZOPODA.

THE animals belonging to this class are mostly very minute, and none of them, except the Sponges, can be said to be familiarly known. Owing to their complexity of structure, comparatively speaking, it will not be advisable to take one of the Sponges as the type of this class. We shall therefore select as an example the little animalcule known as the Amœba.

The Amoeba, or "Proteus-Animalcule" (fig. 1), is an inhabitant of most collections of stagnant water, especially where decaying vegetable matter is present; and it is microscopic in its dimensions. It derives both of its names from its wonderful power of altering its shape (Greek, amoibos, changing; Latin, Proteus, a sea-god who had the faculty of assuming different shapes). This power it owes to two circumstances. In the first place, the entire body is composed of a soft gelatinous substance, which has very little cohesion, and which can readily flow in different directions. In the second place, the animal has the power of thrusting out thick, blunt, and finger-shaped processes of its body-substance, which may

be compared to little roots. Hence the name of the entire class Rhizopoda (Greek, rhiza, a root; podes, feet).

Fig. 1.-A, Amoebae developed in fluids containing organic matter, very greatly magnified (after Beale). B, Amaba princeps (after Carter), highly magnified: c Rudimentary circulatory organ or heart; e Outer, transparent layer of the body.

These root-like processes can not only be thrust out at any part of the surface of the body, but they can also be withdrawn at will, leaving no traces of their former existence. By means of these, as by temporary feet, the animal creeps about slowly, and by means of these it also obtains its food. Thus, the Amoeba possesses no mouth, but whenever it comes across any substance or small animal that is eatable, it wraps one of these temporary processes round it, and gradually withdrawing the hand thus extemporised, lodges its capture within the soft substance of its body. Instead, therefore, of possessing a single permanent mouth, the Amoeba can make a mouth for itself at any point of the surface of the body, and its process of feeding has been appropriately compared to thrusting a stone into a lump of dough.

As before remarked, the substance of the body in the Amoeba is entirely soft and gelatinous. It exhibits little that can be called "structure," except that the outer layer of the body is more transparent than the central portion,

and that this latter is thickly crowded with little rounded particles, and is somewhat more fluid than the exterior. Not only is there no mouth, but there is no trace of any stomach or other digestive organ, and the particles of food, engulfed by the temporary feet, merely pass into the central, semi-fluid substance of the body. There is an organ (fig. 1, B, n), which is perhaps concerned with the production of young Amoeba; and there is a little chamber or bladder (c), which dilates and contracts at intervals, and which may perhaps be regarded as a rudimentary heart. Beyond these no internal organs of any kind have hitherto been shown to exist.

The Amoeba is not only to be found in most ponds, but it is usually developed in all fluids which contain vegetable or animal matter, and are allowed to stand exposed to the air in a warm place. Thus, if we take a little hay and boil it in water, and then let the fluid thus prepared stand in any place where the air is allowed free access to it, we shall generally be able to detect Amabæ in it after a few days, by the use of the microscope.

RECAPITULATION OF ESSENTIAL CHARACTERS.-The body is soft and gelatinous, and has no very definite shape. From any part of the surface (or at least from some portion of the surface), processes of the body-substance can be thrust out, which act as temporary feet and hands, and which can be withdrawn when their purpose is fulfilled. There is no mouth, or digestive system, no nervous system, and no breathing-organs; and the circulatory system is at most represented by a rudimentary contractile sac or chamber. These characters distinguish the class of the Rhizopoda as a whole.

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CHAPTER III.

CLASS INFUSORIA.

THE animals known as "Infusorian Animalcules," are all microscopic in their dimensions, and, though of universal occurrence, they are for this reason only known to scientific observers. They derive their name from the fact that they occur very frequently in what are called "vegetable infusions." By this term is meant the fluid which is obtained by pouring hot water upon any vegetable substance (Latin, in, in; fundo, I pour), and then straining off the solid particles. The Infusoria, then, are so called because they are always, or almost always, to be found in fluids of this nature, which have been exposed for a longer or shorter time to the air. They are also of almost universal occurrence in all stationary collections of water, whether fresh or salt. As the type of this class we may select the animalcule known as Paramœcium, a very brief description of which will suffice to indicate the leading points of interest in these animals.

Paramecium (fig. 2) is one of the most common of animalcules in stagnant water, and is only visible under the microscope. Its body is somewhat oval or slippershaped, and is almost quite transparent. Externally it is covered by a delicate membrane, which supports a number of little vibrating hair-like processes, which are called "cilia" (Latin, cilium, an eyelash). These little filaments when in active motion cannot be detected by the eye, owing to their transparency, but they can readily be seen when nearly at rest; and they are still more easily recognised by seeing that all minute solid particles which may be floating in the water are forcibly driven away when they approach within a certain distance of the animal. By the vibrations of these "cilia," which lash to and fro like so many little whips, currents are set up in the sur

rounding water, and particles of food are brought to the mouth of the animal.

Fig. 2.-A, Paramecium, showing the "cilia" covering the surface, and the rudimentary organs of circulation (v v). B, The same, dividing transversely into two halves. C, The same, dividing longitudinally.

Immediately within the delicate external membrane which covers the body, is a layer of a firmer and more consistent character; and this, in turn, gradually melts into a soft, semi-fluid central mass, which constitutes the greater portion of the body, and which contains numerous small solid particles floating in it.

The animal is provided with a distinct mouth, and a short, funnel-shaped gullet, but the mouth does not open into a stomach, or even into a distinct body-cavity. Hence the food simply passes through the mouth into the semi-fluid central substance of the body, and the indigestible parts of it are expelled by a second opening placed near the mouth.

The most important internal organs, and indeed almost the only ones, are one or two little chambers (fig. 2, A, v v), which open and shut at regular intervals, perhaps five or six times in a minute, and which appear to drive the fluid which they contain through all parts of the body.