PROTOZOA 293 



which carry the animal along swiftly by beating the 

 water. These, and still more the higher protozoa, the 

 "ciliated infusoria," need a special motor apparatus, 

 because their cell -body is surrounded by a delicate 

 membrane ; it gives more firmness to the animal, but 

 prevents the flowing movement of its substance. While 

 the flagellates have their lashes or whips, the whole 

 skin of the ciliated infusoria, or special parts of it, is 

 covered with countless hairs or " cilia," which strike the 

 water in a uniform direction, like a row of oars, and 

 force the animal forward. Further, the clothing of 

 the skin of these animals prevents them from feeding 

 as the amoebae do, and so the infusoria have a special 

 opening in the skin, through which the food can enter 

 the protoplasm and be ejected again. Round this 

 aperture, especially in the sedentary vorticellse, there 

 are long lashes that cause a small whirlpool in the 

 water and so bring the food down into the animal. 

 Naturally, unwelcome foreign bodies are often brought 

 into it in this way, and have to be ejected again. 



Thus we see that the single cell is capable of very 

 diflerent adaptations, and so we cannot be surprised if 

 the cells that compose the higher animals assume such 

 enormously different forms. But while the cells of the 

 latter are usually selected only in one direction, the cell 

 of the protozoa has to advance in every possible direc- 

 tion. In the protozoa the one cell discharges all the 

 vital functions, locomotion, nutrition, respiration, and 

 reproduction. 



The latter function is accomplished in a very simple 

 way in the unicellulars. Let us take the amoeba, for 



