214 



THE FLUKES 



worked on the life history of this species, chiefly at El Marg, 

 near Cairo, Egypt. He found that Schistosoma embryos are 

 attracted by several species of fresh-water snails and that they 

 penetrate the bodies of three species, Bullinus contortus (Fig. 

 66A), B. dybowskii and Planorbis boissyi (Fig. 66B). Here 

 they undergo transformation into sporocysts, from which daugh- 

 ter sporocysts bud off (Fig. 67). After leaving the mother 

 cyst the daughter sporocysts migrate into the tissue of the liver 



Fig. 67. Larval forms of blood flukes teased from liver of Planorbis; A, 

 sporocyst containing daughter sporocysts; B, daughter sporocysts in liver tissue; 

 C, cercaria. Note forked tail, characteristic of Schistosoma cercarise. (After 

 Leiper.) 



and grow rapidly. They become greatly elongated and eventu- 

 ally ramify throughout the organ, so increasing its bulk and color 

 that an infected snail can be detected at a glance. The sporo- 

 cysts move by wriggling movements, and absorb nourishment 

 directly through the body wall. When they become over- 

 distended with the cercariae developing within them the wall 

 ruptures and the cercariae are set free in the snail. The cercariae 

 are discharged from the mollusc in " puffs," a number being 

 periodically shot into the water. 



