77. NEMATHELMINTIIES: NEMATODA. 



301 



Family 2. ASCARHLE. Mouth surrounded by three lips, one dorsal, 

 two ventral ; males with two spicules. Besides numerous species in lower 

 vertebrates two of the most common parasites of man, the human round- 

 worm and the pin worm, belong here. The former, Ascaris lumbricoides, * 



FIG. 265. Dorsal, end, and ventral views of head and hinder end of male Ascaris 

 lumbricoides. (From Hatschek.) 



inhabits the small intestine, often in enormous numbers. The females 

 average about 5-6 inches, the males 4 inches, in length. The ani- 

 mals are enormously prolific, a female containing about 60,000,000 eggs. 

 The eggs (fig. 246, a) are easily recognized. Shortly after fertilization 

 the eggs pass out of the intestine with the faeces, but develop without 

 intermediate host if in the course of two or three months, when the embryo 

 has developed, they are taken into the human intestine. The development 

 of the pinworm, Oxyuris vermicular is,* is somewhat similar except that 

 the embryos are developed in the egg at the time of oviposition, and hence 

 after a shorter stay outside the body are capable of infection. The white 

 worm, not half an inch long, lives in the rectum, especially of children, 

 and in crawling outside the anus causes intolerable itching. Ascaris 

 mystax* occurs in dogs and cats (occasionally in man). A. megalo- 

 cephala * (a favorite animal for cy tological researches) and Oxyuris equi in 

 the horse. These do little harm. On the other hand Heterakis maculosa 

 often destroys whole flocks of pigeons. 



Family 3. STRONGYLID.E. These are readily recognized by the bursa 

 of the male, a broadening of the hinder end of the body by two ring-like 



FIG, 266. Anterior end of AnTcylostomum duodenale, dorsal and side views, o, inner, 

 h, outer ventral teeth ; c, dorsal tooth of m, mouth capsule ; d, stylet ; e, ventral 

 ridge ; oe, oesophagus. 



processes, which contains two spicula. Frequent but not constant is a 

 widened capsule surrounded by papillae at the mouth. A number of 

 species of Strongyltts occur in domestic animals. Syngamus trachea- 



