146 MANUAL OF ZOOLOGY. 



many ways, it is easy to understand how one of them may be 

 swallowed by a pig. When this occurs a l proscolex' is libe- 

 rated from the ovum, and bores its way through the walls of 

 the stomach, to become a ' scolex.' It now takes up its abode, 

 generally in the muscles, in which position it was originally 

 described as a cystic worm under the name of Cysticercus cellu- 

 losce, constituting what is commonly known as the ' measles ' 

 of the pig. In this state, the scolex will continue for an in- 

 definite period ; but if a portion of ' measly' pork be eaten by 

 a man, then the scolex will develop itself into a tape- worm. 

 The scolex fixes itself to the mucous membrane of the intes- 

 tine, throws off its caudal vesicle, and commences to produce 

 ' proglottides ' instead, becoming, thus, the ' strobila ' of the 

 Tcenia solium, with which we originally started. The other 

 common tape-worm of man, viz. the Tcenia mediocanellata, is 

 derived in an exactly similar manner from the ' measles ' of 

 the ox. In like manner, the tape- worm of the cat (Tcenia 

 crassicollis) is the mature form of the cystic worm of the mouse 

 (Cysticercus fasciolaris) ; the tape- worm of the fox (Tcenia 

 pisiformis) is derived from the cystic worms of hares and 

 rabbits (Cysticercus pisiformis') ; and the tape- worm of the dbg 

 (Tcenia serrata) is the developed form of the Ccenurus cerebralis 

 of the sheep, the cystic worm which causes the * staggers ' in 

 the latter animal. 



Besides tape- worms, however, man is liable to be affected 

 with ' scolices,' which are the Iarva3 of the tape- worms of other 

 animals. Thus, what are professionally called ' hydatids ' in 

 the human subject, are really the scolices of the tape-worm of 

 the dog. The disease is indicated by the presence of the so- 

 called ' hydatid- tumour,' which consists of a strong membra- 

 nous cyst the ' hydatid ' proper situated in some solid organ, 

 most commonly the liver, and filled with a watery fluid. To 

 the interior of the cyst are attached numerous minute scolices, 

 many others also floating freely in the contained fluid. These 

 ' Echinococci,' as they are called, do not differ in structure 

 from other scolices, consisting of a head, provided with four 

 suckers and a circlet of recurved hooklets, a vesicular body, 

 and an intermediate contracted portion or neck. The Echi- 

 nococci multiply within the hydatid cyst by gemmation, but 

 they develop no reproductive organs. If, however, an Echi- 

 nococcus should gain access to the alimentary canal of a dog, 

 it then becomes the tape- worm peculiar to that animal the 

 Tcenia echinococcus. 



