180 



METAZOAN PHYLA 



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 parasites feed upon 

 the blood of the host, which is obtained by puncturing the intestinal wall. 

 When this puncture is made, a poison is introduced which prevents 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 preven- 

 tive 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 very 



largely eradicated. Through education of 

 the people further infection has been 

 prevented. 



212. 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 animal which acquires this 

 parasite does so by eating the meat of another 

 animal which has the disease and in the flesh 

 of which 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 intes- 

 tine, where they live upon the food in the canal and become mature in as 

 short a period, perhaps, as 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 viUi (Fig. 8) . The 

 larvae follow the lymphatics and blood vessels to the voluntary muscles 

 in various parts of the body where they encyst (Fig. 87). After a rapid 

 development in the cysts they are ready to be transferred to another host. 

 If this does not occur soon, lime becomes deposited in the walls of the cysts 

 and the larvae may remain alive for many years. The length of life 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 another 

 and to give rise to generation after generation. Man comes into this 

 cycle of parasitism by eating inadequately cooked pork and thus taking 

 in the larvae, which pass the rest of their life history within his body. 

 The disease is very serious, being usually fatal. It can, of course, be 

 prevented by the inspection of meat and by the destruction of infected 



Fig. 87. — Section of pork 

 containing encysted larvae of 

 Trichinella. 



