148 THE SIMPLEST ANIMALS — PROTOZOA 
of the gullet is the mouth (Figure 135). The food thus col¬ 
lected passes into the protoplasm in the form of food vacuoles. 
Digestion is accomplished by the aid of 
enzymes which put into solution the avail¬ 
able parts of the food it eats, and the indi¬ 
gestible parts are cast off from the body. 
121. Respiration. —-As in amoebae, the 
Figure 138 . — Para- , . , . , . 
mecium Reproduc- ox yg en which is necessary to respiration 
ing by Fission. is obtained directly from the water and 
Compare the shape passes into the protoplasm at all points, 
of the nucleus with the 122. Excretion. — Excretory wastes are 
first collected in each of the two contrac¬ 
tile vacuoles and then cast from the body. Gases escape 
from the entire surface. 
123. Irritability. — Both the amoeba and paramecium re¬ 
spond to jars, food, and 
their enemies in a definite 
manner. In each of these 
simple cells there is no 
structure which can be 
compared to the nerve 
cells or brain of higher 
animals. The ability to 
respond to stimuli in Pro¬ 
tozoa seems to be a condi¬ 
tion that is present in the 
whole protoplasm of the 
cell. 
124. Reproduction.— 
Paramecia reproduce by 
fission, i.e. an animal di¬ 
vides, producing two ; 
these divide and produce two more. The process of fission goes 
on indefinitely (Figure 138). Unlike the amoeba these forms 
cannot encyst when conditions of life become unfavorable. 
This protozoon is supported on a stalk 
which can contract and expand. Of what 
use would this be ? 
