PH YL UM TROCHELMINTHES 



139 



istically associated with vegetation, while still others live on 

 muddy bottoms of ponds and streams. 



Gross Morphology. — The body, which is extremely variable in 

 shape, usually consists of a trunk and a tail. In many genera a 

 flexible cuticula covers the trunk, but in others there is a firm. 



Siomach 



Ovar^ 



Urmaru 

 hhdder 



Moufh 



Phart/nx 



I l\ 



Gasfri'c 

 • » 1 gland 



Viklhn'um 



Infieshhe 



Foofgtand 



Fig. 72.- — A rotifer, Hydatina senta, viewed from ventral surface. (Orig.). 



shell-like covering called the lorica. The anterior extremity 

 of the trunk is usually modified to form a structure known as 

 the trochal disc (Fig. 72) which is one of the most char- 

 acteristic structures of the rotifers. This disc bears cilia in 

 highly variable arrangement and is capable of retraction within 

 the anterior region of the trunk. The mobile posterior extremity 

 is usually recognizable as a tail with some sort of adaptation for 



