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 eave the body, the 
embryo then developes into a bisexual form known as the Rhab- 
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, ete. 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 
Fic. 83.— Trichina encysted 
. ioe - amongst muscular fibres. 
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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. 
