FLATWORMS AND THREADWORMS 



299 



This phylum is likely to prove confusing to students as there are 

 various systematists who classify thread-worms under different phyla 

 and under groups which they call uncertain. 



Nematodes form the single class of Nemathelminthes, and the two 

 best know r n forms used in the laboratory are Ascaris lumbricoides (Figs. 



Fig. 186. Tuberculous Cavity in Oesophageal 

 Wall of Man Containing an Ascaris 

 Lumbricoides. (From a photo- 

 graph lent the author by 

 Dr. E. L. Miloslavich.) 



Fig. 187. Trichinella Spiralia. 

 A. Encysted Trichina Embryo. 

 B. Adult female from Intestinal wall. 1, 

 parasite ; 2, membrane of cyst ; 3, muscle-fiber 

 of pig. (After Leuckart.) 



185 and 186), a parasitic worm found in the digestive tract of pigs, horses, 

 and man, belonging to the family Ascaridae; and Trichinella spiralis 

 (Fig. 187), of the family Trichinellidae, which causes a very dangerous 

 disease called trichinosis in rats, pigs, and man. 



The female Ascaris is the larger of the sexes ; in fact, it may grow 

 to a length of from five to eleven inches and a fourth of an inch in diam- 

 eter. The body is of a light brown color with a narrow white stripe 

 along the dorsal and ventral surface, and a broader white line lying on 

 each side of the dorsal and ventral stripe. 



The mouth-opening (which is surrounded by one dorsal and two 

 ventral lips) lies at the anterior end of the animal. The anal opening 

 lies at the posterior end. The tail-end of the female is straight, while 

 in the male it is slightly bent. In the male also there are penial setae, 

 which extend through the anal opening and which are used for copula- 

 tion. 



