THE PROTOZOA 17 
are growing together they appear like a delicate growth 
of mold upon the water weed. The stalk is peculiar in 
being traversed by a muscle fiber arranged in a loose spiral, 
which upon any unusual disturbance contracts together 
with the body into the form shown in Fig. 8, 0. 
These few examples serve to show the general plan of 
organization and the method of locomotion of the Infuso- 
‘ria; but,as upward of a thousand species exist, with widely 
differing habits, many interesting modifications are present. 
Some have been driven in past time to adopt a parasitic 
mode of life within the bodies of other animals. At pres- 
ent they are devoid of locomotor organs, and as they absorb 
nutritive fluids through the surface of the body all traces 
of a mouth are also absent. The reproductive processes 
also are peculiar, but they do not concern us now. 
20. Characteristics common to the Protozoa.—We have 
now studied the principal structures which serve in loco- 
motion among these simple one-celled forms, also the means 
by which they catch their food, and we shall now glance at 
the internal processes, which are much the same in all. 
After the food has been taken into the cell, it is prob- 
ably acted upon by some digestive fluid, for it soon assumes 
a granular appearance and finally undergoes complete solu- 
tion. In every case the oxygen is absorbed through the 
general surface of the body, and uniting with the living 
substance, as in the squirrel, liberates the energy necessary 
for the performance of the animal’s life work. The wastes 
thus produced in a large number of forms simply filter out 
from the body without the agency of anything comparable 
to a kidney, but in several species they are borne to a 
‘definite spot, the pulsating vacuole (Figs. 5,7, 8, c.v.), where 
they gradually accumulate into a drop about the size of the 
nucleus. The wall between it and the exterior now gives 
way and the excretions are passed out. In active indi- 
viduals this process may be repeated two or three times a 
minute, but it is usually of less frequent occurrence. 
