EOTIFEEA. 



401 



defined and very dissimilar regions, but the internal organs show no 

 trace of any corresponding segmentation. There is therefore no true 

 segmentation, i.e., division of the body into metameres. It is usually 

 possible to distinguish an anterior region of the body, in which the 

 whol3 of the viscera are situated, and a posterior movable foot-like 

 region, which terminates in two opposed pincer-like styles and is 

 used both in locomotion and for attachment. The broad anterior 

 portion of the body, as well as the narrow posterior region, is often 

 divided by transverse constrictions into several rings, which can be 

 drawn into one another like the rings of a telescope and can be bent 

 more or less freely 

 upon one another. 



The anterior cili- 

 ated and usually re- 

 tractile apparatus 

 whieli projects at the 

 anterior end, and is 

 termed the trochal 

 disc, or from its like- 

 ness to a rotating 

 wheel, the wheel or- 

 gan, is an important 

 characteristic of the 

 Rotifera. Very fre- 

 quently, especially in 

 the parasitic forms, 

 this trochal disc is re- 

 duced, and in certain 

 cases entirely aborted 

 (Apsilus). In Notom- 

 mata tardigrada the 

 trochal disc is reduced 

 to a small ciliated 

 lip round the mouth ; in Hydatina (fig. 324) to the margin of the 

 head, the whole circumference of which is ciliated. In other 

 cases the ciliated edge projects over the head and forms the so- 

 called double wheel, e.g., Philodina, Brachionus, or becomes a 

 ciliated cephalic shield, e.g., Megcdotrocha, Tubicolaria. Finally, it 

 may be produced into ciliated processes of various form (Floscularia, 

 Steplianoceros). As a rule, the cilia form a continuous border, 

 starting from the mouth and returning to it. The cilia are chiefly 



26 



CBl 



FIG 324. Hydatina senta (after F. Cohn). a. Female; I, 

 male. Wpr, Trochal disc; CBl; contractile vesicle; Wtr t 

 ciliated funnel of the excretory apparatus (Ex) ; K, jaws ; 

 Dr, salvary glands ; Md, stomach, Ov, ovary ; T, testis ; 

 P, penis. 



