PARASITIC DISEASES 



171 



is cylindrical, with the greatest diameter toward the 

 anterior part. At the anterior extremity is a re- 

 tractile proboscis armed with six rows of hooks, by 

 means of which it attaches itself to the intestinal 

 wall. 



The average length of the female is about eleven 

 inches; that of the male, from two and a half to 

 three inches. 



The eggs average about one two-hundred-and- 



FW. 21 — MALE THORN-HEADED WORM ATTACHED TO WALL OF 

 INTESTINE 



fiftieth of an inch in length, and are smooth and 

 somewhat cylindrical in shape. 



Source of Infection. — The development of the 

 thorn-headed worm requires an intermediate host, 

 the larvae of the May beetle, or some other in- 

 vertebrate form, in which they encyst. The female 

 deposits her eggs in the hog's intestines, and they 

 pass out along with the excrement. In this way 



