THREAD. WORMS. 



167 



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. 



Orders. Nematodes. — The first 

 suborder of this group, compos- 

 ing the true round worms, is re- 

 presented by Ascaris, Oxt/uris, 

 Trichina, etc. The human 

 round worm, Ascaris luvibri- 

 coides Linn. (Fig. 113), 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 Fig. ns.—Asmrisiumincoides. <; 



„ I , n T \ J. female, natural size. b. head-end en- 



iresh water nsh, but are as a larged; c, the same, front view show- 



rule harmless. Ascaris mystax 'Znl\^XlX't\ Tit'r^^', 

 lives in the intestines of the ■'^tX^^^iit-^ftr^i^^^^n'' 

 cat. 



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

 It is the Oxyuris vermicularis Linn. (Figs. 114, 115). The 



