202 



ROTIFERA 



and the postero-lateral pair contain part of the nephridia and 

 bear the lateral antennae, they are true outgrowths of the body, 

 and are not homologous with the spines of Triarthridae. 



The front of the body constitutes the head, which is scarcely 

 distinct, though usually separated by a slight neck-like con- 

 striction. The disc, which terminates the head, varies greatly 

 in shape and in the arrangement of its parts. Imagine a 

 circular funnel, finely ciliated within, and with the mouth at 

 the bottom, the prominent rim bearing two zones of cilia, the 

 inner or anterior being the coarser, and termed the " trochus " or 

 hoop ; the outer finer, and termed the " cingulum " or girdle, 

 while a very finely ciliated groove lies between the two zones. 



Fig. 108. Diagram- 

 matic views of disc 

 of Rotifers. Cin- 

 gulum represented 

 by a black line, 

 groove shaded ; tro- 

 chus dotted ; the 

 black spot repre- 

 sents the mouth. 1, 

 Simple disc of Mi- 

 crocodon; 2, Bdelloid 

 disc of Rotifer or 

 Callidina, the star 

 represents the cili- 

 ated proboscis ; 3, 

 disc of Hydatina> 

 groove represented 

 by lobes bearing cili- 

 ated styles ; 4, disc of Melicerta, the star represents the ciliated ventral cup with open- 

 ings into it from the groove ; 5, disc of Conochilus ; 6, disc of Stephanoceros, cingulum (?) 

 of setose lobes, trochus horseshoe-shaped, mouth central. 



Either or both of these zones may be interrupted on the dorsal 

 or ventral median line, or both ; and the funnel-shaped mouth 

 may be shifted usually ventrally, so that it forms only a dila- 

 tation of the ciliated groove. Again, the wreath as a whole may 

 be festooned or lobed ; or the lobing may be confined to the area 

 between the cingulum and trochus, as in most Ploima (Figs. 106 

 and 108, 3). Very frequently on these lobes adjacent cilia are 

 fused together during life, producing " vibratile styles," whose 

 true nature is only revealed after death. In Microcodonidae the 

 structure of the disc (Fig. 108, 1) nearly conforms to the primi- 

 tive type ; but the ciliated groove is absent, and the " trochus " 

 is in two separate half-elliptical bands. In the Flosculariaceae 

 (Fig. 108, 6) the mouth is also central, the disc is funnel-shaped, 



