SUB-KINGDOM I.
Protozoa (first animals). These earliest formed animals are distinguished for the simplicity of their structure. In some cases their animal nature was long ago in doubt, and they were, for a time, put down as probably belonging to the vegetable kingdom. The border line between the two has never been very definitely located. Biologists may fail to tell just what special quality distinguishes the minute animal from the microscopic plant. This is not wonderful, when it is remembered that myriads of animals, known to be such, are so small that it requires a lens of strong magnifying power to discover them. Three thousand of them, placed side by side, would make a line but little over an inch long.
Class I.—Monera (single). These are the simplest forms of microscopic aquatic animals. They are entirely homogeneous, and without any developed organs; mere particles, of a jelly-like, but living, or life-supporting substance, called protoplasm, or more properly, bioplasm. This, all admit, is the physical basis of life, and the medium of its manifestation, just as the conductor is a medium of manifestation of electricity. But is it not a stupid blunder to confound the mere medium of its manifestation with the life itself? In neither case is the recognized physical basis of the manifestation necessary to the existence of that manifested by its means. Electricity exists without the conductor, and life may exist without the bioplasm.
Class II.—Gregarinida (living in herds). Minute animals which are found in the intestines of the lobster, clam, and cockroach. They are worm-like in form, and of a very simple, cell-like structure, the only organ being a nucleus.
Class III.—Rhizopoda (root footed). The representative forms of this class are the Amœba[1] and Foraminifera. The amœba is an indefinite little bit of bioplasm, as structureless as the monera, only that it is made up of two layers of the substance, has an apparent nucleus, and a contractile cavity within. These first animals vindicate their right to be recognized as such, by moving, receiving food, growing, and reproducing their kind. They move by a contractile force, throwing out at will processes of their soft bodies, to serve them as feet and arms. True, these are blunt, and without digits, but they answer the purpose. They eat either by simple absorption, or by wrapping their soft bodies around the food, and holding it in the extemporized stomach till it is, in some way, assimilated.
The work of reproduction is carried on principally by self-division, and budding. The animal rends itself into two or more parts, each having all the elements of the whole, or it throws out buds that mature and drop from the parent mass, having the vital element, and a portion of the bioplasm, or medium necessary to its development.
FORAMINIFERA, SHOWING ROOT-LIKE FEET.
The Foraminifera (perforated animals), of this class, have several peculiarities. The body is even more simple, being apparently without layers or cavity. The processes thrown out as arms and legs are not blunt or massive, but long and slender. And, moreover, small as it is, it has the wonderful property of secreting about itself an envelope, whose thin walls are built of the carbonate of lime. The delicate little structures are often singularly beautiful. Some are monads, having but a single shell; others, by a process of budding add new cells or chambers, often in a spiral coil. These are marine shells, and their numbers in many parts of the ocean are astounding. The bottom of the sea, for many degrees on both sides of the equator, wherever examined by dredging, is found covered with them, either still in their organic state, or ground by attrition to a fine lime dust. It is estimated that a single pound of the sea bottom in some localities contains millions of them, and they are the principal material of the chalk hills.
Class IV.—Infusoria. This class includes Vorticella (wheel animals), Flagellata (whip-shaped animals), Tentaculata (having tentacles), and others. Their general characteristics do not differ widely from those already mentioned. As the name imports, they are mostly found in vegetable infusions that have been exposed for some time, and are directly the product of invisible cells or animal germs that were floating in the atmosphere till a lodgment was found favorable to their development. Those called Vorticella, to the eye seem simply mould on the plant to which they are attached, but under the glass their animal qualities appear; and they multiply with amazing rapidity. Every drop of water from a stagnant pool is full of these animalculæ, of various shapes and dimensions, some of them constantly in motion, propelled by numerous cilia, or slender, hair-like appendages that fringe the circumference, and are used as oars. Their whole organism, though delicate, and having a thin membranous covering, is imperfect. There are two contractile openings, with a slight depression at the mouth, leading to a funnel-shaped throat, into which the nutritive substances descend.