THE SIMPLEST ANIMALS OR PROTOZOA 31 



bly 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. 7, 9, 10, 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. 



The loss in bodily waste is continually made good by 

 \he manufacture of the food into protoplasm, and if the in- 

 come be greater than the outgo, growth ensues. But, as in 

 all other forms, growth is limited, and ultimately the cell is 

 destined to divide, resulting in two new individuals. This 

 process may be repeated many times, but not indefinitely, 

 for sooner or later various members of the same species 

 unite in pairs temporarily or permanently, exchange nu- 

 clear material, and separate again with apparently renewed 

 energy and the ability to divide for many generations. 



31. Simple and complex animals. It is important to note 

 that these same processes of waste, repair, growth, feeling, 

 motion, and multiplication are the same as those of the 

 squirrel, and, furthermore, are common to all living crea- 

 tures, so that the difference between animals is not in their 

 activities, but in their bodily mechanisms ; and according 

 to the perfection of this, the animal is high or low in the 

 scale. Comparing, for example, the Amc&ba and Slipper 

 Animalcule, which are relatively low and high Protozoa, we 

 find in the former that any part of the body serves in loco- 

 motion and in the capture of food, while in the latter these 



