306 ZOOLOGY SECT. 



undergoes more or less degeneration. This process gives rise to 

 various morbid symptoms in the host, but after some months the 

 cysts become calcified, and the danger to the infected individual is 

 over. The flesh of a " trichinised ' human subject has been 

 estimated to contain 100,000,000 encysted worms, and that of an 

 infected pig 85,000 to the ounce. In order that further develop- 

 ment of the encysted and sexless Trichinae should take place, it is 

 necessary for the infected flesh of the host to be eaten by another 

 animal in which the worm is capable of living, e.g., that of Man 

 by a Pig or Rat, or that of a Pig by Man. When this is done the 

 cysts are dissolved by the digestive juices, the worms escape, 

 develop reproductive organs, and copulate, the young migrating 

 into the muscles and producing the disease as before. The 

 result of eating an ounce of '' trichinised " or " measly ' pork, 

 improperly cooked, might be the liberation in the human 

 intestine of perhaps 80,000 worms ; and, if half of these were 

 females, each producing 1,000 embryos, some 40,000,000 worms 

 would shortly begin to migrate into the muscles, and produce the 

 various symptoms of " trichiniasis." 



It will be noted that in this case the parasite is able to exist in 

 various hosts, and that both sexual and asexual stages are passed 

 through in the same host, dispersal of the species taking place by 

 the flesh of an infected animal being eaten by another, either of 

 the same or of a different species. 



The female Guinea-worm (Filaria medinensis) attains a length 

 of 30-200 cm. (1-6 ft.), and lives in the subcutaneous connective- 

 tissue of Man. The eggs develop in the uterus, and the new-born 

 young pass out of the body of the host through abscesses caused 

 by the presence of the parasite. If, as must often be the case, 

 they escape into water, they make their way into the body of a 

 Water-flea (Cyclops), which is the intermediate host, and in this 

 condition probably reach their human host once more in his 

 unfiltered drinking-water. Filaria bancrofti and other species are, 

 in the larval condition, parasites in the blood of man. The adult 

 females of F. bancrofti live normally in the lymphatic vessels. 

 They are viviparous, and the young when they escape reach the 

 blood and are thus distributed. Normally they are to be found in 

 the peripheral vessels only at night-time, when the superficial 

 vessels are more dilated and thus permit of their passage. They 

 are transmitted from one human host to another by the agency 

 of mosquitoes, which act as intermediate hosts. F. bancrofti is 

 very widely distributed in tropical countries, and is the cause of a 

 disease called filariasis, with a variety of symptoms such as 

 anaemia, lymphatic tumours, elephantiasis. 



