222 THE INVERTEBRATA 



the liver of Anodoriy the cercaria encysts in the roof of the mouth of 

 the roach and only reaches sexual maturity when the roach is 

 swallowed by a perch. 



In Distomum macrostomum^ which is parasitic in the gut of thrushes, 

 there is no free-living stage in the life history. The eggs, passed out 

 with the faeces of the bird, are eaten by a snail, inside which the sporo- 

 cyst develops. The sporocyst finds its way into one of the tentacles. 

 It there develops pigment, being brightly coloured in bands of green 

 and red, while its presence stops the snail from withdrawing this 

 tentacle. Presumably this brightly coloured object attracts the bird 

 which devours the snail and infects itself by setting free the cercariae 

 from the sporocyst. 



-e.po. 



Fig. i6o. Schistosoma: the male (cJ) is clasping the female (?) in the gynae- 

 cophoral groove (gr.). e.po. excretory pore; M. mouth; v.suc. ventral sucker. 

 After Fritsch. 



Schistosoma ( = Bilharzia) is a parasite of man, living as an adult 

 in the abdominal veins (Fig. i6o). It is long and thin and well 

 adapted for this habitat. It is one of the rare examples of dioecious 

 trematodes. The male, however, does not lose touch with the female 

 once he has found her, but carries her permanently in a fold of the 

 ventral body wall. The eggs are laid in the blood vessels and, being 

 provided with a sharp spike, they lacerate the walls of the capillaries 

 and pass into the bladder. Immediately the urine is diluted the 

 miracidia hatch, but they wait for dilution before hatching. The 

 second host is a water snail. The cercariae swim freely in the 



