PHYLUM NEMATHELMINTHES 



183 



the larvae pass either by the veins alone or by lymphatic vessels and 

 veins to the heart and from there, still following the stream of venous 

 blood, through the pulmonary artery to the lungs. Leaving the blood, 

 they pierce the walls of the lung cavities, enter those cavities and follow 

 the air passages to the pharynx, from which, by way of the alimentary 

 canal, they go to the intestine. Here they mature and another generation 

 is produced. 



In the passage through the lungs injury is done which predisposes 

 the host to lung diseases. While in the intestine the worms feed upon the 

 blood of the host. This blood is obtained by puncturing the intestinal 

 wall. When this puncture is made, a poison is introduced which pre- 

 vents the coagulation of the blood, and so the person loses blood from 

 hemorrhage in addition to that which is taken 

 by the parasite. Anemia and weakness result 

 which incapacitate the person for any effective 

 effort. By preventive measures, such as 

 putting floors in the houses, requiring the 

 people to wear shoes, providing sanitary 

 means for the disposal of fecal waste, and by 

 appropriate treatment of the patients, the 

 disease has been considerably reduced in the 

 United States. Hookworm disease is a very 

 serious problem in many tropical countries. 



214. Trichinella. — A third parasitic nema- 

 tode is that known as Trichinella spiralis 

 (Owen) , which is the cause of a disease known 

 as trichinosis in rats, pigs, and human beings. The host acquires 

 this parasite by eating the meat of another animal in whose flesh are 

 the encysted larvae in an advanced stage of development. When these 

 are taken into the alimentary canal they are freed from the cyst and 

 escape into the intestine, where they hve upon the food in the canal and 

 become mature in as short a period as, perhaps, two days. In the body 

 of the female, eggs are produced, and fertilization, development, and 

 hatching take place, the worm being viviparous. The female burrows 

 into the wall of the intestine and deposits the young larvae in the lymph 

 spaces of the vilh (Fig. 11). The larvae follow the lymphatics and blood 

 vessels to the voluntary muscles in various parts of the body where they 

 encyst (Fig. 93). After a rapid development in the cysts, they are ready 

 to be transferred to another host. If this transfer does not occur within 

 a few months, Hme becomes deposited in the walls of the cysts. Although 

 the larvae may remain alive for some years, they eventually become calci- 

 fied and die. The length of Hfe of the adults is usually only a few weeks. 



Since both rats and pigs eat the dead carcasses of other animals, 

 the parasite, when present, is likely to be passed from one animal to 



Fig. 93. — Section of pork 

 containing encysted larvae of 

 Trichinella. 



