174 



COLLEGE ZOOLOGY 



FIG. 113. Trichinella spiralis encysted 

 among muscle fibers. (From Shipley and 

 MacBride, after Leuckart.) 



when inadequately cooked meat from an infected pig is eaten. 

 The larvae soon become mature in the human intestine, and each 

 mature worm deposits probably about 10,000 young. These 

 young are either placed directly into the lymphatics by the female 



worms or burrow through 

 the intestinal wall; they 

 encyst in muscular tissue 

 in various parts of the 

 body. As many as 15,000 

 encysted parasites have 

 been counted in a single 

 gram of muscle. Pigs 

 acquire the disease by 

 eating offal or infested 



rats. In a few countries pork is inspected for this and other 

 parasites by government agents. 



The family FILARIID^E is also important because of the human 

 diseases caused by certain of its members. The most injurious 

 species is Filaria bancrofti, a parasite in the blood of man. The 

 larvae of this species are about yg-Q inch long. During the day- 

 time they live in the lungs and larger arteries, but at night they 

 migrate to the blood-vessels in the skin. Mosquitoes, which 

 are active at night, suck up these larvae with the blood of the in- 

 fected person. The larvae develop in the mosquito's body, be- 

 coming about one twentieth of an inch long; make their way 

 into the mouth parts of the insect; and enter the blood of the 

 mosquito's next victim. From the blood they enter the lym- 

 phatics and may cause serious disturbances, probably by ob- 

 structing the lymph passages. This results in a disease called 

 elephantiasis. The limbs or other regions of the body swell up 

 to an enormous size, but there is very little pain. No successful 

 treatment has yet been discovered, and the results are often fatal. 

 It is said that from 30 per cent to 40 per cent of the natives of 

 certain South Sea Islands are more or less seriously afflicted. 

 One of the most recent discoveries with regard to parasitic 



