196 THE INVERTEBRATA 



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

 in the abdominal veins (Fig. 154). It is long and thin and v^ell 

 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 water, 



-e.po. 



Fig. 154. Schistosoma: the male (o) is clasping the female (?) in the gynae- 

 cophoral groove {gr.). e.po. excretory pore; M. mouth; v. sue. ventral sucker. 

 After Fritsch. 



and in districts in China and Egypt where the disease is common they 

 swarm. Bathing, washing or drinking the infected water allows the 

 cercaria to enter the final host. The cercariae penetrate the skin with 

 great rapidity and, entering the blood system, make their way to the 

 abdominal veins where they become mature. The disease can be pre- 

 vented by strict sanitary measures in regard to water, and it can be 

 cured by the administration of compounds of antimony to infected 

 patients. That the disease is a very old one in Egypt is shown by the 

 discovery of Schistosoma eggs in the kidneys of mummies of the 

 twentieth dynasty (1250-1000 B.C.). 



The hatching of miracidia from the egg of Schistosoma is de- 

 pendent on the dilution of the urine by fresh water and this serves to 



