V PHYH'M PLATYHBLMINTHES 285 



ciliated larvae, they again enter into a state of parasitism as sporo- 

 cysts or redise in a second host ; and, after a second free interval as 

 cercarise, may enter the body of a third host to become encysted. 

 The second host is, very generally, a Mollusc, and the cercaria 

 may become encysted in the same animal. 



The Cestodes are, of all the Platyhelminthes, those that are most 

 modified in accordance with the condition of internal parasitism 

 in which they remain throughout life. The adult Cestode is 

 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 2\cnia cvassicollis of the intestine of 

 the domestic Cat, for example, the cysticercus-stage occurs in the 

 livers of Rats and Mice ; the cysticercus of Tmnia serrata of the 

 Dog is found in Hares and Rabbits. But in many cases the inter- 

 mediate host is an invertebrate. In either case the passage from 

 one host to another is a passive translation, not an active 

 migration as in the Trematodes. 



A few human parasites belong to the Trematoda, but none 

 that are of very common occurrence among Europeans. Fasciola 

 hepatica has occasionally been found in the human liver; Dis- 

 tomum rathousii is a common intestinal parasite in China ; 

 Opisthorchis sinensis occurs in the liver of Man in China and Japan ; 

 Dicrcelium lanceolatnm and various other species of the genus 

 occasionally occur in the human subject. Schistosmnum hccma- 

 tohium and S.japoniciom, which differ from most other Trematodes 

 in being unisexual, are found in the human portal system of veins 

 in various parts of Africa, in Arabia, the Philippines, and Japan. 

 Eggs with contained larvae are voided with the urine, and if they 

 reach water, the larvae may gain access to the human host by 

 being swallowed in drinking water or by perforating the skin. 



The commonest human Cestode parasites among Europeans are 

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

 The cysticercus stage of T. solium (Cysticercus cellulosm) occurs, 

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

 T. saginala 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 Tcvnia solium 

 is the more prevalent; where more beef, T. saginata. Bothrio- 

 cephalus 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 

 cysticercus, 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 



