154 PARASITOLOGY. 



and one-half percent of the hogs in the United States. 



Description. — The adult worm is capillary in size 

 and one-twelfth inch in length ; the larva is one- 

 twenty-fifth inch in length ; the female is ovovivipa- 

 rous ; the ova undergo complete development in the 

 uterus and hatch in the external genital organs and 

 then escape; the larvae become encysted in lemon 

 shaped cysts which appear as very minute white 

 specks, scarcely visible to the unaided eye ; a micro- 

 scopic examination is necessary to make a diagnosis. 



Life History. — When meat containing the live 

 encysted larva is eaten by animals, in from eighteen 

 to twenty hours the meat surrounding the cyst and 

 cyst capsule is digested ; the larva thus liberated 

 develops into mature form in from three to four days; 

 frequently on the third day the ovary shows ova in 

 process of segmentation ; ovulation commences 

 about the sixth or seventh day ; this period lasts five 

 to six weeks, during which time as many as two 

 thousand embryos may be produced by each female; 

 as soon as laid the embryos pass through the stomach 

 or intestinal wall and migrate to various parts of the 

 body; they may burrow directly through the tissues 

 or they may enter the blood and be carried by the 

 blood current, or the lymph vessel and be carried by 

 the lymph stream ; at the end of eight or ten days 

 the embryo reaches a resting place and coils in a 

 kind of loop, then in a figure three shape, finally in a 

 coil of about one and one-half turns. As a result of 

 the irritation in the elements of the surrounding tis- 

 sue there is produced a cyst enclosing the embryo; 

 the embryo soon arrives at the fully developed larval 

 state ; the cyst is usually lemon shaped, although it 



