372 THE ANIMAL PARASITES OF MAN 



lengths of time in the open, in moist earth, or in water. Thick-shelled 

 eggs can maintain their developmental capacity for a long time, even 

 after prolonged desiccation. 



Finally, a nematode-like embryo develops, which usually lies 

 somewhat coiled up within the shell, and varies in its further 

 development according to the species to which it belongs. 



In the simplest forms, as in the free-living Nematodes, the 

 embryos, apart from their size, resemble their parents, and grow up 

 into these after leaving the egg-shell. In many parasitical Nema- 

 todes, however, the young must be called larvcv, as they present 

 characters which are subsequently lost. 



The manner of conveyance of the eggs or the embryos contained 

 in them after they have left the body into the definite host is very 

 different in the various species. 



(i) Without Intermediate Host. (a) In many the conveyance into 

 the definite host is effected directly after the larvae have developed 

 within the eggs ; thus, for instance, the feeding of suitable animals 

 with the embryo-containing eggs of species of Trichocephalus and 

 Ascaris leads to an infection of the gut, for the young Trichocephali 

 or Ascarides only leave the egg-shell when they have attained the 

 intestine of the final host, in which they become adult. 



In other cases (b) Ancylostoma, Necator, the larvae hatch in the 

 open, and live for a time free, changing their form ; they grow, cast 

 their skin, and finally gain the intestine of the host by means of water 

 or through the skin, when they lose their larval characters and 

 assume the structure of the adult worm. 



(c) In a number of Nematodes, however, HETEROGONY occurs. 

 This terms signifies a mode of development in which two structurally 

 different sexual generations of the same species alternate with each 

 other. To these appertains, for instance, Angiostomum (syn. : 

 Rhabdonema) nigrovenosum, which lives in the lungs of frogs and 

 toads ; this Nematode measures about i cm. in length and is 

 hermaphrodite (protanrdic). The eggs are deposited in the pulmonary 

 cavity, and through the cilia of the same reach the oral cavity, where 

 they are swallowed and thus conveyed into the intestine. They pass 

 through the entire gut, and are finally evacuated with the faeces ; often, 

 indeed, the young themselves emerge from the egg-shell within the 

 hind-gut of the frogs. These young forms become sexually differ- 

 entiated, remain much smaller than the parent, their oesophagus is 

 differently constructed (rhabditis form), and they are non-parasitic 

 (fig. 266). After having grown in the open they copulate ; the males 

 die soon after copulation, and the females in their own bodies develop 

 a few young, which, given the opportunity to get into frogs, infect 

 them, and are transformed into the hermaphroditic Angiostomum. 



