350 NEMATHELMINTHES. 



enlargement of the oesophagus and with a pharynx armed with three 

 teeth. 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 

 trigonocephalus 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 Nematodes may, however, attain sexual 

 maturity in damp earth, as free Rhabditis 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 Rhabdonema 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 ovarian 

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

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

 the rectum, but finally pass to the exterior in the fa?ces, and so 

 reach the damp earth or muddy water, where they develop in a short 

 time into the jRhabditis-like forms, which have separate sexes and 

 are barely 1 mm. in length (fig. 282, a and b). The impregnated 

 females of the latter produce only from two to four embryos, which 

 become free inside the body of the mother, pass into her body cavity, 

 and. there feed on her organs, which disintegrate to form a granular 

 detritus. They finally migrate as slender, already tolerably large 

 Nematodes into the lungs of the Batrachia, passing through the 

 buccal cavity and glottis. The Leptodera appendicidata, which lives 

 in the slug Arion empiricomm, also presents in its development a 

 like alternation of heteromorphic generations, which, however, are 

 not strictly alternating, inasmuch as numerous generations of the 

 Rhabditis form may succeed one another. 



The Leptodera are peculiar in that the form parasitic in the 

 snail is a larva characterised by the absence of a mouth, and by 

 the possession of two long band-shaped caudal appendages; it 

 quickly attains maturity, but only after a migration into clamp earth 

 and after losing the caudal appendages and casting the skin. 



The Nemntoda feed on organic juices, some of them also on blood, 

 and are enabled by their armed mouth to inflict wounds and to gnaw 



o 



tissues. They move by bending their body with a rapid undulatory 

 movement towards the ventral and dorsal surfaces, which thus seem 



