SCOLECIDA. 117 



from the alimentary canal of the host. The joints thus expelled die 

 and decompose, and their contained eggs are thus set free. Each 

 egg is covered with a little leathery capsule which protects it from 

 injury, and contains a minute embryo in its interior. If this micro- 

 scopically small egg be swallowed — as in many ways it easily may 

 be — by another warm-blooded animal (in this particular case by the 

 pig), then a fresh series of changes ensues. The leathery case of the 

 ovum is dissolved in the stomach of the new host, and the embryo, 

 or "pro-scolex" (iig. 73,/), is set free, when it bores its way through 

 the walls of the stomach by means of little hooks or spines with 

 which it is provided. Having reached a suitable locality, the young 

 tapeworm proceeds to surround itself with a kind of cyst, and it 

 develops from its hinder end a kind of bladder or vesicle filled with 

 a clear watery fluid. The young tapeworm is now what is techni- 

 cally called a " scolex," and it constitutes what used to be termed 

 a " Cystic Worm " (fig. 73, g). It consists not only of the caudal 

 bladder, but also of a short neck carrying an oval " head," which 

 is furnished with four suckers and a little crown of booklets. The 

 " scolex " of the Tcenia solium is generally about a fourth of an inch 

 in length, and is found embedded in the muscles of the pig, when 

 the pork is said to be " measly." When first discovered, the young 

 tapeworm was regarded as a distinct species of worm, and was 

 described under the name of Cysticercus cellnlosce (fig. 73, g). In 

 this cystic stage the young tapeworm may remain for an apparently 

 indefinite period, being quite incapable of developing eggs. Tor its 

 further development it is necessary that it should now be introduced 

 into the alimentary canal of man. If a portion of measly pork be 

 eaten, with these cystic worms embedded in it, then the young tape- 

 worm is liberated from its cyst : it fixes itself by means of its 

 suckers and booklets to the mucous membrane of the intestine, and 

 its caudal bladder drops ofi'. It is now converted into the head of 

 the adult tapeworm. It finally commences to throw out buds from 

 its hinder extremity, and in these buds or joints the reproductive 

 elements are produced, so that ultimately we get the long flattened 

 jointed colony with which we started. 



A like extraordinary series of phenomena is now Known to occur 

 in other cases, but these can be merely alluded to here. Thus, 

 another of the common tapeworms of man — viz., the Beef-tapeworm 

 (Taenia mediocanellata) — lives as a " scolex " or " cystic worm " in the 

 muscles of the ox. The tapeworm of the cat (Tcenia crassicollis) 

 spends its " cystic " stage in the tissues of the mouse. One of the 

 tapeworms of the dog (viz., Taenia serrata) inhabits in its early life 

 the bodies of hares and rabbits. Another tapeworm of the dog (viz., 

 Tcenia ccenwws) is found as a " cystic worm " inhabiting the brain of 



