Page:Popular Science Monthly Volume 35.djvu/269

 pseudopodium, which has grown to be permanent, and which has a certain function to exercise, namely, to make motions of rowing and feeling. Starting with the amœba, the flagellate infusoria represent the next higher phase of morphological differentiation—that is to say, they represent the division of the homogeneous substance of the amœba into distinct parts, to which different functions are assigned. In social science one would allude to this as the commencement of a division of labor.

Fig. 5 represents an individual belonging to the genus Cothurnia, which is very frequently found in the depths of town and country wells. It possesses the power to withdraw with lightning speed into the transparent envelope which surrounds it whenever the cilia which are attached to its front come in contact with anything hard. K denotes the nucleus which no infusoria lack, and v represents the vacuole, which, however, at times may disappear.

Fig. 6 pictures a small creature, the Stenostonia leucops, which attains a length of about one millimetre, and which appears to the naked eye like a minute white thread. This kind of worm is of frequent occurrence, and has received its name from the rotary motion which the cilia that are on the surface of its body impart to the water when the animal moves or swims: g g, is the nerve-center (brain-ganglion), which is very considerable in proportion to the size of the worm. The mouth is not shown in