14 ANIMAL PHYSIOLOGY. 
Inasmuch as any part of the body may serve for the admission, 
and possibly the digestion, of food and the ejection of the use- 
_ less remains, we are not able to define the functions of special 
parts. Amceba exercises, however, some degree of choice as to 
what it accepts or rejects. 
The movements of the pseudopodia cease when the tempera- 
ture 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 Amoeba 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. 
Ameeba reproduces by fission, in which the nucleus takes a 
prominent if not a directive part, as seems likely it does in re- 
gard to all the functions of unicellular organisms. 
Conclusions.—It is evident that. Amoeba is, in much of its be- 
havior, 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, Amoeba 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 ex- 
ception, and forms a broad line of distinction between the ani- 
mal and vegetable kingdoms. 
Amoeba will receive further consideration later; in the 
