PROGRESS OF PARASITISM IN THE DISTOMID^. 119 



intermediate hosts, but we may take it for granted that such was not 

 the case from its commencement. On the contrary, these second inter- 

 mediate hosts brought their Trematodes to sexual maturity in the same 

 way as was formerly the case, according to our supposition, with the 

 Eedise. Since the caudal appendage, by means of which the Cercarife 

 swim about, is lost when they force their way into a new host, so the 

 developmental condition of these sexual animals must in the main 

 have been like the present one. 



The entozootic Trematodes are accordingly Helminths, in which 

 the change of hosts had already come about at a time when the 

 vertebrates, which are now almost exclusively concerned in it, had 

 not yet come into existence. 



The supposition that the Cercarise originally attained sexual 

 maturity in their hosts, and only later developed retrogressively into 

 more intermediate forms, finds some support in the fact that these 

 animals even now, under certain circumstances, become sexually 

 mature, and produce ova in their intermediate hosts. On a former 

 occasion (p. 73, note) some cases of this kind were cited, and others 

 are continually forthcoming. These sexually mature Helminths are 

 not separate species, possessing no other sexual condition ; they are 

 rather nothing more than certain specially privileged individuals 

 belonging to species which, under other conditions, are accustomed to 

 attain their maturity only after transference into a vertebrate. 



It is, moreover, a common phenomenon that the Distomidse not 

 only commence the formation of their sexual organs in the interme- 

 diate hosts, but bring them to a state of complete functional capacity. 

 This phenomenon we meet also in other intestinal worms, although 

 individual species present great variations in this respect, so that 

 many are undifferentiated sexually even when passing into their 

 definitive host. "With respect to the latter, I may mention CucuUanus 

 and Spiroptera, whilst others, like Hedruris and all the EcTiinorhyncM, 

 assume all their external and internal peculiarities in their interme- 

 diate liosts, which is certainly a case of persistence of an earlier 

 state. Of course such differences are not without influence upon 

 the length of time occupied by the development ; instead, perhaps, of 

 weeks and months being necessary, as usual, the worm of the latter 

 kind requires only a few days, after leaving its temporary host, in 

 order to attain full maturity, and to acquire the ability to propagate 

 its species by sexual means. 



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