230 



PLATYHELMINTHES. 



and a passive one) the final host, where they become sexually mature. 

 They are furnished (Fig. 182, d) with an exceedingly motile caudal 

 appendage, frequently with a buccal spine, and occasionally with eyes, 

 and they present in the rest of their organization great resemblances 

 to the adult, excepting that the generative organs are not developed. 

 In this form they make their way out of the body of the redia 

 or sporocyst, and of their host, and move about in the water, 

 partly creeping and partly swimming. Here they either perish 

 or find a new host (snail, worm* insect larva, crustacean, fish, 



FIG. 182. Stages in the life-history of Distomwm liepaticum. a, Miracidium (ciliated embryo), 

 fc, Sporocyst with rediae R (after Leuckart). c, redia (after Thomas); D gut, C Cercaria, 

 R redia, K germ-cells, rf, Cercaria (after Thomas). 



batrachian), into which they penetrate aided by the powerful vibra- 

 tions of their tails ; they then lose the tail and encyst. 



The Cercariae thus become distributed amongst a number of hosts, 

 and in each case give rise to an encysted form, which only differs 

 from the adult in being without generative organs. This young 

 Trematode migrates passively with the flesh of its host into the 

 stomach of another animal, and thence, freed from its cyst, into 

 the organ (intestine, liver, etc.) in which it becomes sexually 

 mature. 



There are then, as a rule, three different hosts, in the organs of 



