A SURVEY OF THE GROUP 29 



certain molluscs. It is found through most of Europe and Asia. 

 Theromyzon is a worldwide genus ; members of most species enter 

 the nostrils of wading and swimming birds and feed from the 

 mucous membrane. Placobdella (= Haementeria) is another widely 

 distributed genus and includes parasites of crocodiles, water 

 turtles, frogs and fishes. Helohdella, on the other hand sucks the 

 body fluids of invertebrates such as snails and insect larvae. It is 

 really a specialized predator rather than a parasite, as it normally 

 kills the host. 



3. PiSCICOLIDAE 



The characters of a typical piscicolid leech are illustrated 

 by Branchellion torpedinis. Both anterior and posterior suckers are 

 clearly marked off from the body, which is divided into two 

 distinct regions. These are the short, narrow anterior portion 

 which includes the clitellum and the genital pores, and the longer 

 and wider posterior portion which bears gills on the lateral margins. 

 As in Glossiphonia, a complete mid-body segment has three annuli 

 and the middle one bears, in addition to sensillae, a pair of lateral 

 pulsatile vesicles which help to circulate the coelomic fluid through 

 the gills. The anterior sucker bears six eyes arranged in an arc 

 across the dorsal surface of segment IV. In a mature specimen 

 segment XII is lifted into a fold which lies over the preceding two 

 segments and covers the genital pores. It has been likened to the 

 prepuce which covers the glans penis of a mammal and is often 

 given that name. 



The arrangement of the internal organs is shown in Fig. 18. The 

 proboscis is followed by an oesophagus that bears a pair of oeso- 

 phageal diverticula. The crop has six pairs of diverticula, of which 

 all but the first pair are bilobed. The last pair of diverticula are 

 continued back to the hind end of the body. They lie ventral to 

 the intestine and so are almost completely hidden in dorsal view. 

 They anastomose at five points, as shown in Fig. 18c. The intes- 

 tine has four large pouches on either side and leads to the rectum 

 by way of a ciliated chamber bearing a pair of highly folded 

 diverticula. 



There are five pairs of testes lying between the crop diverticula ; 

 the general arrangement of the reproductive organs is of the 



