LIFE-HISTORIES. 



179 



as long. After impregnation the female brings forth numerous embryos 

 viviparously, sixty to eighty at a time, and altogether about 1500. 

 Most of these find their way through the wall of the intestine into 

 blood vessels, and are swept along in the blood stream to the muscles ; 

 occasionally some seem to migrate actively, boring their way, especially 

 through connective tissue, to the muscle fibres. There they grow, coil 

 themselves spirally, and become encysted within a sheath, at first 

 membranous and afterwards calcareous (Figs. 79 and 80). The cyst is 

 partly due to the muscle, and partly to the parasite. In these cysts, 

 which may be sometimes counted in millions, the young Trichina? 

 remain passive, unless the flesh of their host be eaten by another, — 

 pig eating rat, man eating pig. In the alimentary canal of the new 

 host the capsule is dissolved, the embryos are set free, and become 

 rapidly reproductive. 



Among the numerous other parasitic Nematodes the following may be 

 noted : — The giant palisade worm ( Eustrongylus gigas) occurs in the renal 

 region of domestic animals, etc. ; the female may be 3 ft. long. The 



Fig. 79. — Trichinae in muscle, 

 about to be encapsuled. — 

 After Leuckart. 



Fig. 80. — TrichiiiEe in muscle, 

 encapsuled. — After Leuckart. 



armed palisade worm (Strongylus armatus) occurs in the intestine and 

 intestinal arteries of horse, causing aneurisms, colic, etc. The young 

 forms are swallowed from stagnant water, bore from gut into arteries, 

 become adult, return to gut, copulate and multiply. Various other 

 species of Strongylus occur in sheep, cattle, etc. Of the genus Ascaris 

 alone, over 200 species have been found in all types of Vertebrates ; 

 — A. megalocephala in horses, A. lumbricoides in man, A. mystax in 

 cats and dogs. Syngamus trachealis occurs in the trachea of birds, 

 causing "gapes." Various species of Tylemhtis, especially T. devas- 



