v PHYLUM PLATYHELMINTHES 265 



almost always an inhabitant of the alimentary canal of a verte- 

 brate. The intermediate host is frequently also a vertebrate 

 commonly of a kind which is liable to become the prey of the final 

 host. In the case of Twnia crassicollis of the intestine of the 

 domestic Cat, for example, the Cysticercus stage occurs in the 

 livers of Rats and Mice ; the Cysticercus of Tcenia scrrata of the 

 Dog is found in Hares and Rabbits ; and so with other tape- worms. 

 But in many cases the intermediate host is an invertebrate. In 

 either case the passage from one host to another is a passive 

 translation, not an active in iy ration as in the Trematodes. 



A few human parasites belong to theTrematoda; but none that are 

 of very common occurrence among Europeans. Distomum hepaticum 

 has occasionally been found in the human liver. D. rathousii 

 is a common intestinal parasite in China. D. sincnse occurs in 

 the liver of Man in China and Japan. Distomum lanceolatum 

 and various other species of the genus occasionally occur in the 

 human subject. Bilharzia luvnudobium, which differs from most 

 other Trematodes in being unisexual, is found in the portal system 

 of veins among the natives of Abyssinia. 



The commonest human Cestode parasites among Europeans are 

 Tcenia solium and T. saginata (otherwise called T. mediocanellata). 

 The Cysticercus stage of T. solium Cysticercus celluloscc occurs, 

 as already stated, chiefly in the muscles of the Pig ; that of 

 T. saginata in the muscles of the Ox ; and the relative prevalence 

 in different countries of these two Tape- Worms varies with the 

 habits of the people with regard to flesh-eating : where more 

 swine's flesh is eaten in an imperfectly cooked state Tcenia solium 

 is the more prevalent ; where more beef, T. saginata. BotJirioceplialus 

 latus, a very large tape-worm without hooks, and with a pair of 

 longitudinal sucking-grooves on the head, instead of ordinary 

 suckers, is a common human parasite in eastern countries. Its 

 Oysticerciis, which is elongated and solid, occurs in the Pike and 

 certain other fresh-water Fishes. 



Of all the Cestode parasites of man, however, the most formid- 

 able is one which occurs in the human body, not in the sexually 

 mature or strobila condition, but in that of the Cysticercus. This 

 is Tcenia echinococcus, the presence of which produces what is termed 

 hydatid disease. The adult Tcenia cchinococcus is a very small tape- 

 worm with only three or four proglottides, occurring in the intes- 

 tine of the domestic Dog. The eggs passing out with a liberated 

 proglottis in the faeces, may reach the alimentary canal of Man 

 uninjured in drinking-water, on the surface of salad vegetables, and 

 the like ; and, the egg-shells becoming dissolved, the contained 

 hooked embryos bore their way to the liver or the lungs or some 

 other organ. Arrived at its final destination, the embryo develops 

 into a cyst, which may become of enormous size. In the interior 

 of the primary or mother-cyst are developed a number of second '/</// 



