Page:A Text-book of Animal Physiology.djvu/44

 Inasmuch as any part of the body may serve for the admission, and possibly the digestion, of food and the ejection of the useless remains, we are not able to define the functions of special parts. Amœba exercises, however, some degree of choice as to what it accepts or rejects.

The movements of the pseudopodia cease when the temperature of the surrounding medium is raised or lowered beyond a certain point. It can, however, survive in a quiescent form greater depression than elevation of the temperature. Thus, at 35°C., heat-rigor is induced; at 40° to 45°C., death results; but though all movement is arrested at the freezing-point of water, recovery ensues if the temperature be gradually raised. Its form is modified by electric shocks and chemical agents, as well as by variations in the temperature. At the present time it is not possible to define accurately the functions of the vacuoles found in any of the organisms thus far considered. It is worthy of note that Amœba may spontaneously assume a spherical form, secrete a structureless covering, and remain in this condition for a variable period, reminding us of the similar behavior of Torula.

Amœba reproduces by fission, in which the nucleus takes a prominent if not a directive part, as seems likely it does in regard to all the functions of unicellular organisms.

Conclusions.—It is evident that Amœba is, in much of its behavior, closely related to both colored and colorless one-celled plants. All of the three classes of organisms are composed of protoplasm; each can construct protoplasm out of that which is very different from it; each builds up the inanimate inorganic world into itself by virtue of that force which we call vital, but which in its essence we do not understand; each multiplies by division of itself, and all can only live, move, and have their being under certain definite limitations. But even among forms of life so lowly as those we have been considering, the differences between the animal and vegetable worlds appear. Thus, Amœba never has a cellulose wall, and can not subsist on inorganic food alone. The cellulose wall is not, however, invariably present in plants, though this is generally the case; and there are animals (Ascidians) with a cellulose investment. Such are very exceptional cases. But the law that animals must have organized material (protein) as food is without exception, and forms a broad line of distinction between the animal and vegetable kingdoms.

Amœba will receive further consideration later; in the