THE WHEEL ANIMALCULES (ROTATORIA) 557 



various systems of organs, to notice their variations and modifica- 

 tions, and the uses they serve. 



Perhaps the chief concern of all organisms is to provide material 

 for carrying on the complicated chemical processes that are going 

 on within, — that is, to get food and oxygen. How does the 

 rotifer accomplish these ends? 



This is done mainly by the aid of the ciliated surface at the 

 anterior end, — the corona. The cilia of this region are fine, hair- 

 like processes which are in constant motion. They strike back- 

 ward more strongly than forward, so that they cause a current to 

 pass backward from in front of the animal to its mouth, and thence 

 over the surface of the body (Fig. 858). In the simplest notommatids 



\ \ N 



N ^ 



*^ 



Fig. 858. Currents of water caused by the cilia of a rotifer. The dotted area shows how material lyin? 

 in front of the rotifer is drawn out in the form of a vortex to its mouth. (The rotifer is Proales sordida Gosse, 

 from a figure by Dixon-Nuttall.) 



the corona is a mere flattish disk on the ventral side of the anterior 

 end, covered uniformly with short ciha (Fig. 859). In other rotifers 

 there are great variations in the size and arrangement of the cilia; 

 these variations will be taken up later. The water current pro- 

 duced by the corona has a number of different uses: 



1. It continually renews the water that bathes the surface of the 

 animal, thus insuring a constant supply of fresh oxygen. The 

 oxygen thus supplied is absorbed by the entire surface of the ani- 

 mal, apparently, for there are no special respiratory organs. 



2. The current brings to the mouth any particles of food that 



