528 EVOLUTION OF ANIMALS Part V 



can survive. Otherwise, they die in the cysts and become calcified. The trichi- 

 nae in the human body constitute great populations of suicides since human 

 cannibaHsm is almost extinct. The survival of trichinae is kept up only by the 

 eating of infected scraps of meat, mainly by pigs and rats. 



Trichinae differ from other intestinal parasites in that the young do not leave 

 their native host and take their chances for a new one. The majority of young 

 trichinae stay within their home hosts, although this means destruction for so 

 many. How well the species can afford this is shown by the prevalence of 

 trichinosis. The invasion of the muscle is a critical step for the larvae and in 

 heavy infections highly dangerous for the host. The symptoms include intense 

 pain in particular muscles, great difficulty in breathing, and in movements of 

 the eyes and jaws. The surrounding muscle fibers become inflamed and dis- 

 integrate. About six weeks after the original infection walls form about the 

 larvae then curled up among the muscle fibers. Gradually one, sometimes two 

 or more larvae are walled into the capsule that at first is delicate but after a 

 year or more becomes hard and chalky. This encystment phase is the second 

 dangerous one for the host. Other symptoms continue and pneumonia is often 

 a complication. The host does not recover until eight weeks to several months 

 after the infection. Even after that there is a period of a year or longer when 

 the jarring and stretching of the muscles is made painful by the cysts. 



Filariae. With infections of trichinae the immature young are the chief cause 

 of disturbance; with infections of filariae the adults are the main trouble 

 makers. The adults, living in the human passages, produce young called micro- 

 filariae. The embryonic microfilariae must go through a stage of development 

 in a blood-sucking insect before they become infective to man (Fig. 26.7). 



Filarial parasites {Wuchereria bancrojti) are widely distributed in tropical 

 and subtropical countries, especially in coastal regions and on islands (Fig. 

 26.8). In the western hemisphere they occur throughout the West Indies, 

 Panama, and northern South America. The adults are the cause of elephanti- 

 asis. They live in the lymph passages, tangled together like snarls of coarse 

 white threads, the females about three inches in length, the males half as long. 



Life Cycle. Within the lymph passages the females give birth to the micro- 

 filariae. These are microscopic (about 0.2 mm.) slender squirmers that at 

 once bore into the blood and lymph capillaries, and are carried over the body 

 by the circulating blood (Fig. 26.7). Their further development depends on 

 their being sucked up with the blood by a biting mosquito (female) that may 

 belong to one of several genera. Anopheles, Culex, and others. Experiments 

 have shown that there must be at least 15 microfilariae per drop of blood in 

 order to infect the mosquito. Evidently they must be numerous enough to 

 condition their surroundings by their metabolic by-products. Blood containing 

 100 or more microfilariae per drop will kill a mosquito, even one of the trans- 

 mitting species. Yet, the blood of heavily infected persons commonly contains 



