The glasslike skeleton of a radiolarian, a protozoan. (From model at American Museum of Natural History.)
The Animal Series.—We have found that a one-celled animal can perform certain functions in a rather crude manner. Man can perform these same functions in an extremely efficient manner. Division of labor is well worked out, extreme complexity of structure is seen. Between these two extremes are a great many groups of animals which can be arranged more or less as a series, showing the gradual evolution or development of life on the earth. It will be the purpose of the following pages to show the chief characteristics of the great groups of the animal kingdom.
I. Protozoa.—Animals composed of a single cell, reproducing by cell division.
The following are the principal classes of Protozoa, examples of which we may have seen or read about:—
Class I. Rhizopoda (Greek for root-footed). Having no fixed form, with pseudopodia. Either naked as Amœba or building limy (Foraminifera) or glasslike skeletons (Radiolaria).
Class II. Infusoria (in infusions). Usually active ciliated Protozoa. Examples, Paramœcium, Vorticella.
Class III. Sporozoa (spore animals). Parasitic and usually nonactive. Example, Plasmodium malariæ.
II. Sponges.—Because the body contains many pores through which water bearing food particles enters, these animals are called Porifera. They are classed according to the skeleton they possess into limy, glasslike, and horny fiber sponges. The latter are the sponges of commerce. With but few exceptions sponges live in salt water and are never free swimming.
A horny fiber sponge. Notice that it is a colony. One fourth natural size.