ROTIFERA 



239 



Like the Nematoda they consist of a small number of cells and all 

 the tissues, except the cells of the velum, may lose their cell boundaries 

 and become syncytial. Not only is there a superficial resemblance 

 to heterotrichous ciliates in the Protozoa but the tendency to the 

 acellular condition carries this a step further. 



Hydatina senta may be taken as a type of the group (Fig. 170). 



jC.m.c- 



ecdu. 



ect.< 



Fig. 171. A, Side view, diagrammatic, from Shipley and MacBride, and B, 

 tansverse section of a female rotifer. An. anus (cloacal aperture); c.m.c. 

 circular muscle cell; cu. cuticle; D. dorsal; e. eye; ecdu. excretory duct 

 (nephridium) ; ect. ectoderm; int. intestine; l.m. longitudinal muscle; m. 

 muscle; od. oviduct; tc. trochus; V. ventral. Other letters as in Fig. 170. 



The female is pear-shaped, the posterior end being the stalk. The 

 anterior end is flattened and form^ the trochal disc. This is, in many 

 rotifers, bordered by a double ciliated ring, the velum, the outer part 

 of which (the cingulum) is the original velum and is composed of 

 strong cilia. The inner is called the trochus. Between the two rings, 

 which are thus preoral and postoral respectively, is a ciliated groove 

 in which is situated the mouth. The velum in life gives the impression 



