NEMATODA 



129 



Ascaris nigrovenosa has a curious history. It is a common 

 parasite in the lungs of frogs and toads, and in these hosts the 

 parasites are hermaphrodite. Their eggs pass into the ali- 

 mentary canal of the Amphibian, and leave the body, the 

 embryo then developes into a bisexual form known as the JRhab- 

 ditis generation ; in this form the ova develope in the uteri, 

 and the young embryos, making their way through its walls, 

 devour the whole interior of their mother until only the 

 cuticle remains, they then emerge and live in mud or water 

 until swallowed by a frog, when they resume the first form. 



Oxyuris vermicularis inhabits the human intestine, and 

 is particularly common in the caecum ; its ova when laid con- 

 tain embryos already mature, hence it spreads with great 

 rapidity. The ova are swallowed, and the solvent action of 

 the gastric juice sets free the young embryos in the stomach, 

 whence they pass into the intestine. 



Filaria sanguinis hominis passes its larval life in the body 

 of mosquitos, but the sexual female inhabits the lymphatic 

 glands of man in Australia, India, China, and Egypt, giving rise 

 to elephantiasis, etc. The embryos circulate in the blood and 

 give rise to further disease ; they are readily sucked up by a 

 biting mosquito, and in this way the parasites are doubtless 

 disseminated. 



Trichina spiralis (Fig. 83) is a very minute Nematode which 

 encysts in or between the muscle fibres. The adult worm lives 



Fia. 83. — Trichina encysted 

 amongst muscular fibres. 

 After Leuckart. 



in the alimentary canal of man and of other carnivorous mam- 

 mals ; it is viviparous. The young bore their way through the 

 wall of the intestine of their host and encyst in the muscles. 

 They do not become sexually mature unless eaten by some 

 animal, — often a rat, and sometimes a pig, — in which case its 

 flesh is liable to become " trichinised," and may carry the 

 disease Trichinosis to man. 



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