318 



CaSLIIELMINTBES. 



Bub Class III. Hirudinei {Discophori). 



Three points of external structure clearly distinguish the leeches 

 from the chffitopods. First, the absence of bristles (except in 

 Acanllwbdella) and the presence of two suckers, one of which occurs 

 on the posterior ventral surface and is used only for attachment 

 and locomotion, the other, sometimes scarcely differentiated, 

 A 



Fio. 289.—Bonellia viridis. A, female (after Huxley); B, male (after Spengel). c, 

 cloaca; (i, rudimentary Intestine; y, excretory organ; i, intestine; m, muscles sup- 

 porting intestine; s, balls of spermatozoa in B, in A, proboscis (preoral lobe>; h, 

 single segmented organ, functioning as oviduct; rrf, nephridium with ciliated 

 funnel serving as vas deferens. 



surrounds the mouth and is used in sucking the food. In locomo- 

 tion anterior and posterior suckers are alternately attached, the 

 body being looped up and extended like that of a 'span worm.' 

 The animals can also swim well by a snake-like motion of the 

 whole body. 



A second point is the fine ringing of the body, there l)eing 

 usitally many more rings than somites, the primitive segment rings 

 being divided by secondary constructions, there being three, five, 

 or even eleven rings to a segment. The middle or one of the 

 anterior rings is often distinguished by bearing strongly developed 

 sense organs. As in the earthworms, certain of the somites at the 

 time of reproductio'U may develop into a clitellum which secretes the 

 ecru- cocoons. 



