TREMATODA 



219 



to their hosts, but the numbers infesting one host is seldom very 

 considerable and they have no economic importance as parasites. The 

 eggs when laid are normally attached to the body of the host, Poly- 

 stomum being exceptional in laying the eggs in the bladder whence they 



vas def. 



Fig. 158. Polystomum integerrimum, ventral view, showing the reproductive 

 system. After Zeller. g.i. genito-intestinal canal; g.o. common genital open- 

 ing; ho. booklet; M. mouth; oot. ootype; ov. ovary; p. penis; sue. sucker; 

 t. testis; ut. uterus with eggs inside it; vag. vagina; vag.po. vaginal pore; 

 vas def. vas deferens ; vit. vitellarium ; vit.d. vitelline duct. 



pass out to the exterior into water. The egg hatches as a larva with eye- 

 spots and a large ventral posterior sucker. It swims by means of cilia 

 which are arranged in bands round the body. These larvae make their 

 way to some particular spot on the host after being free-swimming 

 for a time. As soon as they attach themselves the ciliary covering is 

 cast off and the generative organs develop. The larva of Polystomum 



