THREAD-WORMS. 



125 



being smaller and shorter than the females. With the ex- 



ception of the skin and lemnisci, all the parts of the adult 



worm, the nervous and reproductive systems as well as the 



beak, originate in the primitive 



rudimentary digestive cavity, 



appearing as rounded masses of 



cells of like size, but differing in 



structure histologically. With 



the growth of the beak begins 



the development of the repro- 



ductive apparatus, and the hooks 



are simply modified cells, with 



the outer surface chitinized. 



Order 2. Nematodes. The first 

 suborder of this group, compos- 

 ing the true round worms, is re- 

 presented by Ascaris, Oxyuris, 

 Trichina, etc. The human 

 round worm, Ascaris lumbri- 

 coides Linn. (Fig. 83), is re- 

 markable for its large size, and 

 may be recognized by its milk- 

 white color, as well as by the 

 three papillae around the mouth. 

 It inhabits the intestines, some- 

 times the stomach and oesopha- 

 gus, and has been known to per- 

 forate the walls of the intestine. 

 The species of Ascaris are very 

 numerous, infesting mammals, 

 and especially fish, often occur- 

 ring encysted in the flesh of the 



cod and Other edible Salt and 



fresh water fish, but are as a 

 rule harmless. Ascaris mystax 

 lives in the intestines of the 



Fig. 83. Ascaris lumbncmdes. 



The common pin-worm lives in the rectum of children. 

 It is the Oxyuris vermicularis Linn. (Figs. 84, 85). The 



