NEMATHELMINTHES 239 



the larva of the last-named worm is prolonged into the form 

 of an awl, whereas the adult worm possesses a conspicuously 

 blunt posterior portion. 



Post-embryonic Development. 



The post-embryonic development in the parasitic Nematoda 

 is very diverse. It appears to be simplest in those cases 

 where the eggs of the Nematode reach the outside world 

 from the place where the parasite lives— for example, from 

 the intestine of the host with its faeces — and then are taken 

 up by another animal of the same species along with its 

 food. The eggs may be more or less developed at the differ- 

 ent stages of this migration, but in any event their envelopes 

 are first destroyed in the intestinal canal of the new host, 

 and the embryo here finds at once the conditions of life suit- 

 able to it. Leuckaet has observed such a direct conveyance 

 of the eggs into the intestine of the host in Trichocephalus 

 affinis and Heterakis vesicularis. 



The conditions are somewhat less simple when the eggs 

 are enveloped by only a thin shell, from which the embryos 

 hatch as larvae. These then live and take food in damp 

 earth or water, like those Nematodes which always lead a 

 free existence. In general they resemble the members of 

 the genus Bhabditis so closely that they are not distinguish- 

 able from them (Leuckart). During its free existence the 

 worm attains a certain size and development. Only when 

 it arrives in its host do the organs needed for a free exist- 

 ence degenerate ; it now adapts itself to the parasitic mode 

 of life. Such is the case, for example, in Bochmius tri- 

 gonocephalus and D. duodenalis. The RhabdUis-like larvae of 

 these worms undergo several moultings during their free 

 existence, are then swallowed by the dog along with its 

 drinking-water or by man, and, as the result of a gradual 

 metamorphosis, acquire the sharp mouth armature which is 

 peculiar to them in the adult condition. The process of 

 development is somewhat different in the Mermithidce, which 

 are found as sexually immature forms in the larvae of 

 insects. After prolonged periods of residence, they abandon 

 this place of habitation by breaking through the body- wall, 



