'36 



MANUAL OF ZOOLOGY 



tegument. The leaf- like form exemplified in the liver-fluke 

 prevails in most (Fig. 71), but a more elongated form some- 

 times occurs. The anterior end is distinguished from the 

 posterior by its shape, by the arrangement of the suckers, 

 and, in many of those Trematodes that are external para- 

 sites, by the presence of eyes. Suckers are universal in 

 their occurrence. They are always ventrally placed, their 

 chief function being to fix the parasite to the surface of its 



Fig. 71. — Trematodes. 

 aperture; m, mouth; 

 (After M. Braun.) 



A, Amfihistomum ; B, Ho7nalogaster. gp t genital 

 s, posterior sucker; te, testes; vit, vitelline glands. 



host in such a way as to facilitate the taking in by the 

 mouth of animal juices and epithelial debris. Their number 

 and arrangement vary considerably. There are nearly 

 always present an anterior set (or, as in the liver-fluke, 

 a single anterior sucker surrounding the mouth), and 

 a posterior set or a single large posterior sucker. There 



