122 BOAED OF AGRICULTURE. 



then a second constriction separates the stomach or gizzard 

 from another somewhat enlarged cavity, which tapers grad 

 ually into the slender intestine. The anal orifice is near the 

 posterior end of the body, and in the male also gives exit to 

 the intromittent organ, which is a single, sharp, grooved 

 spicnlum, with a very small accessory one. The female 



Fifrure 79. 



orifice is situated near the anterior third of the body. This, 

 like the other species of Oxi/uris, produces eggs having firm 

 shells. These probably hatch in water, and the embryos may 

 thus be swallowed in drinking. The intestines of these para- 

 sites are generally filled with vegetable matters derived from 

 the food of the animals in which they live, and the structure 

 of the digestive organ seems to be adapted to vegetable food. 

 For this reason it is probable that they ordinarily cause little 

 or no inconvenience to the horses that they inhabit. 



The Pin-worm of Man (^Oxyuris vermicularis Bremser). 



This is a much smaller species than the preceding, but is 

 similar in form. It is very common, often in great numbers, in 

 children, and occurs also in adults, especially in aged and 

 debilitated persons, and inhabits chiefly the lower part of the 

 rectum and the vicinity of the anus. 



The male is about one-eighth of an inch long, with the tail 

 curved in a spiral form, terminating in a very short point. 

 The female is about four-tenths of an inch long and one- 

 fiftieth in diameter, with the posterior part of the body taper- 

 ing to a very slender tail. 



The digestive system and structure of the mouth is similar 

 to that of the preceding species. But the head sometimes 

 shows inflated lateral lobes. 



Reproduction. 



This species is very prolific ; each female produces thou- 



Figure 79. — Oocyuris curvula, female, twice natural size. From Guerin, 



