300 



ROTIFEBA. 



There is often a spur-like process, carrying a tuft of setae and 

 projecting from the dorsal middle line close to the trochal disc : 

 this is the calcar or antenna. There may be, in addition, a pair 

 of ventral setigerous processes or antennae of a similar character. 

 The single nerve-ganglion is placed on the dorsal side of the body, 

 close to the trochal disc. 



The velum varies in its arrangement. It may consist of a simple 

 circle of cilia at the margin of the trochal disc (Microcodon) : in 

 this case it surrounds the mouth, and is a circum-oral ring. In the 



Floscularidae (Fig. 



a 245, 3), this ring is 



reduced to a half-ring 

 on the ventral side of 

 the mouth, and the 

 edges of the disc may 

 be produced into long 

 processes (Stephano- 

 ceros). In other forms 

 the ring encircles the 

 disc twice by bending 

 on itself (Fig. 245, 1, 

 2) ; thus enclosing the 

 mouth, and having a 

 dorsal gap between the 

 points of flexure (Rhi- 

 zotd). In this form of 

 velum a fusion of the 

 two points of flexure 

 would result in the 

 formation of two rings, 

 one pre-oral (above the 

 mouth), and one post- 

 oral (circum-oral) 



below the mouth. In the Bdelloidea the ring is doubled on itself, 

 but there are two gaps, one dorsal, at the point of flexure, and the 

 other ventral in the upper ring just above the mouth (Fig. 245, 4}- 

 A T ery frequently, especially in the parasitic forms, the trochal disc is 

 reduced, and in certain cases entirely aborted (Apsilus). In Notoin- 

 inata tardiyrada the trochal disc is reduced to a ciliated disc round 

 the mouth. In some cases the ciliated edge of the disc projects over 

 the head and forms the double wheel above referred to (Philodina, 



CBl 



Fid. 244. Hii'latina scuta (after F. Colin), n, female. I, 

 male. Wj>r ciliated ring (velum) ; CBl contractile vesicle 

 of the excretory system ; Wtr ciliated lobule of the 

 excretory organs (Ex) ; A" mastax ; Dr salivary glands ; 

 Mil stomach ; Ov ovary ; T testis ; P penis. 



