280 NEMATHELMINTHES. 



The development of the Nematode larvae often makes a consider- 

 able advance within the intermediate host into which they have 

 migrated. Thus, for instance, in Cucullanus eleyans, the embryos 

 migrate into the Cyclops, and in the body cavity of these small 

 Crustacea undergo two ecdyses and essential alterations of form, 

 obtaining at this early stage the characteristic oral capsule of the 

 sexually adult stage, to which they only develop in the intestine 

 of the perch. According to Fedschenko,* a similar mode of develop- 

 ment occurs in Filaria medinensis. The embryos pass into puddles 

 of water, and migrate thence into the body cavity of the Oyclopidae ; 

 and after casting their skin assume a form which, except for the 

 absence of the oral capsule, resembles that of the larva of Cucullanus. 

 After the expiration of two weeks there is another ecdysis, with 

 which is connected the loss of the long tail. The later history is 

 unknown. It lias not yet been discovered whether the migration 

 of the Filarian larva into the permanent host (man, see p. 289) takes 

 place within the body of the Cyclops, or independently after copulat- 

 ing in the free state. 



The embryos of some Nematoda develop in damp muddy earth, 

 after casting their skin, to small so-called Rhahditis forms with a 

 double enlargement of the oesophagus, and with a pharyngeal arm- 

 ature. They lead an independent life in this habitat, and finally- ' 

 migrate to lead a parasitic life within the permanent host, where, 

 after several ecdyses and alterations of form, they attain the sexually 

 mature condition. This mode of development occurs in Dochmius 

 trigonoceplialus from the intestine of the dog, and very probably 

 in the nearly allied D. (Ancylostomum) duodenalis of man, and also 

 in Sclerostomum. 



The offspring of parasitic JSTematodes may, however, attain sexual 

 maturity in damp earth, as free Rhdbditis forms, and represent a 

 special generation of forms whose offspring again migrate and become 

 parasites. Such a life-history is a case of heterogamy. It occurs 

 in Khaldonema nigrovenosum, a parasite in the lungs of Batrachians. 

 These parasites, which are about half to three-quarters of an inch 

 long, all have the structure of females, but contain spermatozoa, 

 which are produced (as in the viviparous Pelodytes) in the same tubes 

 as, but earlier than the ova. They are viviparous. The embryos 

 make their way into the intestine of their host, and accumulate in 



* Compare Fedschenko, " Ueber den Ban mid Entwicklung der Filaria 

 medinensis," in the Berichten der Freunde der Naturuyissenschaften in Moskau, 

 torn. 8 and 10. 



