Zoologist to distribute into groups, distinguished by some well-defined characters; so that in his classification those may be placed together which are alike, and those may be separated, more or less remotely, which are different. The task is one that no single individual could accomplish by his own efforts; but it has been effected by the combined and successive labours of many; and in this way the entire animal kingdom has been divided into four great groups. These are distinguished by their peculiarities of structure; not so much by those which are external as by those which are internal, and are connected with the nerves.

The first of the groups contains those animals which have a skull, and a backbone composed of a number of joints or vertebræ, as in our own bodies. All such animals are termed "vertebrate," and all in which the skull and backbone are wanting are called "invertebrate."

The following are the four groups or sub-kingdoms established by Baron Cuvier:

I. Vertebrated animals (Vertebrata).

INVERTEBRATE.

II. Soft-bodied animals (Mollusca).

III. Articulated animals (Articulata).

IV. Radiated animals (Radiata).

Let us begin with those at the foot of the scale, and gradually ascend to those beings which occupy a higher rank:

RADIATED ANIMALS.

"O Lord, how manifold are thy works! in wisdom hast thou made them all: the earth is full of thy riches: so is this great and wide sea, wherein are things creeping innumerable, both small and great beasts." -PSALMS.

IF we pick up a Star-fish which has been left on the beach by the retiring tide, we notice that the limbs or arms of the animal are like rays spreading out from a common centre. The Star-fish furnishes therefore a good illustration of what is meant by a "rayed" or "radiated" animal. There are, however, many others belonging to the group in which the rayed appearance is not exhibited in the outline of the body. It is easily discernible in the Sea-urchin, although the figure is rounded; but in others it must be sought for in the radiated arrangement of the parts surrounding the mouth; and in some it can only be detected by an examination of the internal structure.

The Radiated animals are arranged in four great groups or "Classes."

I. Infusory Animacules (Infusoria).

II. Internal Parasites (Entozoa).

III. Polypes (Zoophyta).

IV. Rayed Animals (Radiaria).

B

CLASS I.-INFUSORY ANIMALCULES.

INFUSORIA.

"Where the pool

Stands mantled o'er with green, invisible
Amid the floating verdure millions stray."-THOMSON.

Fig. 1.-INFUSORIA.

Ir any vegetable substance be allowed to remain for about ten days in a glass of water, exposed in a window to the rays of the sun, the water will appear to the naked eye to have undergone little change. But if a drop be taken from the surface and placed under the microscope, it will exhibit such a multitude of living beings swimming about, that the spectacle cannot be looked upon for the first time without surprise, and even astonishment. If a drop of the water containing them be placed between two pieces of glass, they will be seen swimming about with perfect ease in that little film of liquid, and passing and repassing without ever touching. Nor is the feeling of wonder diminished when we endeavour to calculate their size, and form

some estimate of their numbers. Some are so very minute, that Professor Ehrenberg has calculated that two thousand of them placed together would measure but one line, or the twelfth part of an inch. According to this estimate, a drop of water might contain 500 millions of them, a number nearly equalling that of the whole human race at present living on the earth.

These animalcules or little animals, abound in infusions of animal or vegetable matter, and from this circumstance derive their name. But they are not limited to such situations; they are met with not only in stagnant pools, but in lakes and rivers, in the purest water taken from the open sea, and even in that of the arctic ocean.

There are many species of these animalcules, but they are all included in one or other of two great divisions.

or

The first of these is named the "many stomached" (Polygastrica), because the body contains a number of sacs stomachs in which the food is received (Fig. 2). It may naturally be asked, how could these be discovered in creatures so inconceivably minute. The plan adopted by Ehrenberg was very ingenious. He removed some of them from the water in which they were found, and placed them in water of the purest and most

Fig. 2.- ANIMALCULE, one of the Polygastrica.