GENERAL ZOOLOGY 



bate for years. It is now generally agreed that the production of rediae and 

 cercariae involves not the parthenogenetic development of eggs but rather a 

 process somewhat resembling polyembryony (cf. p. 209); it might be more 

 aptlv termed internal budding. It is not a sexual phenomenon but an asexual 

 one consisting of the fragmentation of a mass of germinal tissue, followed by 

 the organization and development of each fragment into an individual of the 

 next generation. The various intermediate stages may be interpreted as per- 

 sistent larval forms which mature only to the extent of carrying on these 

 reproductive processes. 



The life cycles of other digenetic trematodes differ from that of Clonorchis 

 in details onlv. In some the miracidium is a free-swimming larva which 

 activelv penetrates the body of the snail. In others there is more than one 

 redial generation within the liver of the snail, or the sporocyst may produce 

 cercariae directlv without the intervention of rediae. The place of encystment 

 of the metacercaria \aries also between species; some encyst upon vegetation, 

 where they may be readily ingested by final hosts which are herbivorous. In 

 the blood flukes, such as those of the genus Schistosoma, the cercaria actually 

 penetrates the skin of the final host and thus reaches its definitive situation 

 in the blood vessels; there is no second intermediate host, and no meta- 

 cercaria. Finally, the most complicated of the trematode life cycles is that 

 of the genus Alarm, found as adults in various carnivorous mammals. This 

 fluke requires three intermediate hosts for its successful transfer; a snail, an 

 amphibian, and a small carnivorous mammal are all involved in this cycle 

 before the fluke can reach the large carnivore which is its final host. 



Certain genera of flukes, such as Paragonimus, a lung fluke of carnivorous 

 vertebrates, and Schistosoma, the blood fluke, with species in Egypt, the West 

 Indies, and the Japan-Philippines area, are dangerous human parasites. The 

 United States has no widespread human infestations of this sort, although 

 there is always the danger that such parasites may be introduced by persons 

 infested in other parts of the world. The establishment of such a parasite 

 in a new country, however, would also require the establishment of its inter- 

 mediate hosts in the same area, or the adaptation of the life cycle to involve 

 a local group of hosts. The complicated life cycles of the digenetic trema- 

 todes, together with their host specificity, thus make it diflficult for them to 

 become widespread in the absence of the proper intermediate hosts. Knowl- 

 edge of the host relationships of flukes has made it posssible to take intelligent 

 steps toward their control, including the introduction of sanitary measures and 

 of measures for the control of the intermediate hosts. 



THE CLASS CESTODA 



The tapeworms, members of the class Cestoda, are familiar parasites, al- 

 though infestations of man and domestic animals are now less common than 

 they were before the life cycles of tapeworms became known and preventive 

 measures could be taken. A representative tapeworm with its segments and 



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