THE SPONGE IN ACTION
The arrows show how the water enters by the small pores, to pass out by the large openings. Food is thus brought to the cells which line the channels. At the right the currents of water are seen passing from the outer openings, as seen under a powerful microscope.
THE BORDER-LINE BETWEEN ANIMAL AND PLANT LIFE
On the left are represented highly magnified animal skeletons of a class of Protozoa called Radiolarians. Most of them are under one-twenty-fifth of an inch in size. Millions upon millions of these little shells are found upon the floors of the ocean, and upon its shores. They are marvels of form and color—so wonderful, indeed, that man with all his skill cannot imitate them. When alive they consist of but a single cell, and live in colonies with the plant forms, called algæ, pictured on the right. The algæ are also single-celled, and of rare beauty of form, and the strange association of the simplest of animal forms with the simplest of plant forms has up to the present time proved the supreme enigma of science.
PROTOZOA OR SIMPLEST FORMS OF ANIMAL LIFE
ANIMALCULES, AMOEBA, RADIOLARIA, FLAGELLATES, CILIATES
ANIMALCULES (Protozoa)
In botany we find that the lowest plants are mostly of a microscopic size, and unicellular—that is, consisting of a single cell or structural unit, essentially a fragment of living matter (protoplasm), part of which is specialized into a nucleus. The lowest animals are also unicellular, and the popular term “animalcule”—a little animal—has reference to their diminutive size. One of the simplest known cases is afforded by the