THE ART OF PARASITISM 22:, 



the lung passages over the "saddle" to the esophagus, and there 

 are swallowed, reaching the stomach and eventually the intestine. 

 Within the next fortnight two more molts occur, after which the 

 parasites reach maturity, copulate, produce eggs, and continue the 

 cycle. 



The large roundworm, Ascaris lumhricoides, lays eggs which de- 

 velop into infective embryos within three weeks under proper con- 

 ditions of temperature and moisture. After reaching the digestive 

 tract of the host together with food or drink, the newly hatched 

 larva burrows through the mucous layer and starts on a "10-day 

 tour" following essentially the same itinerary as that of the hook- 

 worm. 



Among the protozoa the Ameba, Endamcha histohjtica, the cause 

 of amebic dysentery, is transmitted from one human host to another 

 and thence to the outside world, and back again to the human large 

 intestine by means of resistant cysts carried in contaminated food 

 and drink. 



Parasites Requiring Two Hosts. The dread pork roundworm, 

 Trichinclla spiralis, while a permanent parasite having a relatively 

 simple life cycle, nevertheless requires two hosts to complete its cycle. 

 The encysted larvae occur in a variety of hosts, but are normally 

 secured by man through eating insufficiently cooked pork. The 

 parasites mature rapidly in the small intestine and reproduce within 

 twenty-four to forty-eight hours of their arrival. Each viviparous 

 female produces between 10,000 and 15,000 larvae, which are depos- 

 ited directly in the lymph or capillaries lining the intestine, and are 

 thus circulated by the blood until they reach the voluntary muscles of 

 the body. There, these minute roundworms leave the blood stream, 

 enter the muscle fibers, where within a month a lemon-shaped cyst is 

 deposited about them. Since man is not cannibalistic, the introduc- 

 tion of these parasites into his body becomes a blind alley so far as 

 completing the life cycle is concerned. Unfortunately, when these 

 parasites are once established in the body, there is no way of getting 

 rid of them. In due course of time, calcium carbonate is deposited 

 about the cyst and eventually the parasite dies, but the obnoxious 

 cyst remains to remind the infected person of his injudicious meal 

 by frequent muscular pains which may accompany this infection 

 for years. The normal hosts of TrichineUa seem to be the rat, 

 mouse, and pig. The former are commonly found in numbers about 

 slaughter houses and the percentage of their infection is usually 



