TREMATODA 



193 



there are irregular commissures between the cords, while in the 

 Malacocotylea the commissures are few in number and regular. 



Life history of the Heterocotylea. The usual habitat of this order is 

 on the gills of fishes where they often live isolated. Self-fertilization 

 must therefore be practised, but copulation has been observed in 



,.vas def. 



Fig. 152. Polystomum integerrimum, ventral view, showing the reproductive 

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

 ing; ho. hooklet; 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. 



Polystomum and also in Diplosoon, where it is permanent. The eggs 

 when laid are attached to the body of the host, Polystomum being 

 exceptional in laying the eggs in the bladder whence they pass out to 

 the exterior into water. The egg hatches as a ciliated larva with eye- 

 spots and a large ventral posterior sucker. These larvae make their 



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