Phylum Nemathelminthes and Related Phyla 431 



and can release 200,000 eggs in a single day. The individual e:gg has 

 a clear, inner shell surrounded by a heavy warty coat. This outer 

 coat is stained brown by the intestinal contents of the host. 



The male Ascaris lumhricoides is somewhat smaller than the fe- 

 male, being about 6 to 12 inches in length. Except for the character- 

 istic, hooked posterior end and smaller size, the male is similar in ap- 

 pearance to the female. Both have a single dorsal and two ventral 

 lips surrounding the mouth. These are provided with small sensory 

 papillae. Like most parasites, these worms have no pigmentation. 



^v-r- 



^m. 



r G, 



Fig. 141. — Some nematodes parasitic in human beings. A, Ascaris lumhrico- 

 ides; B, Trichuris trichiura, the whipworm; C, Enterobius vermicularis, the pin- 

 worm; D, Dracunculus medinensis, the Guinea worm, the infective larva; E, Loa 

 loa, the eyeworm; F, Trichinella spiralis, the trichina worm encysted in muscle; G, 

 Necator americanus, the hookworm, showing buccal cavity and mouth. 



There are no intermediate hosts in the life cycle of this w'orm. In- 

 fection occurs when the eggs are accidentally ingested by a new host. 

 As the eggs pass from the host, they are unsegmented, and require a 

 period of development in the soil. Under favorable conditions of 

 heat and moisture, it takes up to three weeks for the larva to develop. 

 The eggs are destroyed by excessive heat or dryness. When the eggs 

 are ingested after the developmental period, the shell is digested ofif 



