THE PROTOZOA OR THE SIMPLEST ANIMALS 151 



they prey upon other protozoa and even the smaller 

 many-celled animals, and they get rid of disintegrating 

 animal and vegetable matter of various kinds. In turn 

 they are eaten by larger, organisms. Together with the 

 unicellular plants they afford most of the food of that large 

 class of animals which, like the clams, sweep in their food 

 supply by the action of cilia. They are eaten by hordes of 

 smaller animals, such as copepods, free-swimming larvae, 

 etc. The smaller animals in turn supply food for larger 

 animals, such as fishes, and it is proverbial that the big 

 fishes eat up the little ones. The aquatic world would be- 

 come a vast grave yard were it not for the unicellular plants 

 and animals. 



The protozoa are of much interest on account of their 

 simplicity of structure and behavior. Animal life is here 

 reduced to its lowest terms. Within a single cell is con- 

 tained, as in germ, the power of performing most of the 

 functions which are discharged by the various special 

 organs of the bodies of higher animals. In the simple, 

 almost structureless body of an Amoeba, we have locomo- 

 tion without limbs or permanent organs, digestion without 

 stomach or intestine, respiration without lungs or gills, 

 circulation without heart or blood vessels, contraction with- 

 out muscles, and response to stimulation without sense 

 organs or nervous system. The living substance of the 

 body performs all these functions, not so readily as each 

 would be performed by organs devoted solely to one par- 

 ticular activity, but still sufficiently well to enable the 

 Amoeba to make its living and propagate its kind. As 

 we pass up the scale of life we find these various functions 

 taken over by different organs which become perfected 

 along one special line, at the same time losing the ability 

 to do other things. This process is called the physio- 

 logical division of labor, and it is quite analogous to the 



