TREMATODES I BIOLOGY 



145 



contain granules. The tail is always cast off when encystment 

 occurs, and organs peculiar to the cercaria stage (boring papilla, 

 eyes) disappear almost entirely. On the 

 other hand, the genitals appear or become 

 more or less highly developed ; in extreme 

 cases to such an extent that they become 

 functional, and that after autocopulation the 

 creatures produce ova. 



The cycle of development of the dige- 

 netic Trematodes has hitherto been gene- 

 rally explained as a typical alternation of 

 GENERATIONS, one sexual generation regu- 

 larly alternating with one or two asexually 

 reproducing generations. Recently, however, 

 a few authors regard the cells from which 

 the sporocysts or rediae produce cercariae as 

 parthenogenetically developing ova, and 

 the sporocysts as well as the rediae as genera- 

 tions propagating parthenogenetically ; we 

 therefore speak of HETEROGENY, or rather 

 of ALLOIOGENESIS, because the former ex- 

 pression should be confined to the change of 

 generation caused by sexual intermediate 

 generations, such as occur in Rhabdonema 

 nigrovenosa. 



nified). (After Leuckart.) 



FIG. 80. The Cercaria 

 of the liver fluke : the cu- 

 taneous glands at the sides 



Other authors, again, only regard the of the anterior body (mag- 

 development of the digenea as a compli- 

 cated metamorphosis, which is distributed 

 over several generations before it is con- 

 cluded. 



BIOLOGY. 



Endoparasitic Trematodes, as fully-devel- FIG. 81. Encysted 

 oped creatures, occur in vertebrate animals 

 only, with very few exceptions ; they inhabit 

 almost all the organs (with the exception of the nervous and 

 osseous systems and the male genitals), by preference the intestine 

 in all its parts from the oral cavity to the anus, and in such a 

 manner that certain species or groups inhabit each only certain 

 parts of the intestine. Next to the intestine other species live 

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