1 68 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



continuing its life-history unless it encounters a water-snail — usually one 

 of the various species of Isidora ( = Bulinus). In the liver of the snail 

 the Schistosoma takes the form of a long more or less tubular sporo- 

 cyst, and the germ-cells in the interior of this, which were already 



a.s. 



Fig. 77. 



Schistosoma. A, Ventral view of adult male with the female in position between the infolded 

 edges of its body ; B, eggs of (i) S. haematobium, (2) S. mansoni and {3) S. japonicum ; C, mira- 

 cidium ; D, sporocyst ; E, cercaria. B and C are more highly magnified than the other stages. 

 fl.c. Alimentary canal (vestigial) ; a.s, anterior sucker ; g, gland-cell ; g.c, gerra-cells ; n, nephridium ; 

 n.g, nerve-ganglion ; v.s, ventral sucker. 



recognizable in the miracidium stage (Fig. 77^ C, g.c), develop into 

 cercariae (Fig. 77, D and E) with forked tails. 



These cercariae are not capable of remaining alive in the free state 

 for more than about 40 hours, but if within this period they come in 

 contact with the warm skin of a mammalian host they cast off their tails 



