I'HVLUM PLATYHELMINTHES 



247 



or proscolex (Fig. 199, A) enclosed within two membranes. If the 

 proglottides, or the eggs which have escaped from them, should 

 now be taken into the alimentary canal of the Pig, which forms the 

 ordinary second host of the parasite, the hooked embryos, becoming 

 freed from their coverings, bore their way with the aid of their 

 hooks through the wall of the alimentary canal, and reach the 

 voluntary muscles. Here they increase greatly in size, and develop 

 into rounded cysts with a large cavity filled with watery fluid 

 (B). On the wall of^the bladder, at one side, is formed a hollow 



Fit;. 199. Development of Tapeworm. A, hexacanth embryo ; B, early stage of bladder" 

 worm of Tcrnia saginata ; C E, stages in the formation of the scolex of the same ; C, the 

 invagination before the hooks and suckers have become developed ; D, after the appearance 

 of the hooks and suckers ; F,, partly evaginated ; F, fully evaginated scolex of T. solium 

 with caudal vesicle ; ft, scolex of T. serrnta with remains of the vesicle ; H, young tape- 

 worm of T. serrata. (After Leuckart.) 



ingrowth, or invagination (C) ; and on the inner surface of this 

 are developed the hooks and suckers characteristic of the head or 

 scolex of the adult (D). When these are fully formed the hollow 

 ingrowth becomes everted (E), the suckers and hooks thus coming 

 to be situated on the outer surface (F). The whole embryo has 

 now the form of a bladder or vesicle, with which is connected 

 at one point a process having all the characters of the head and 

 neck of the mature Tsenia solium ; this is the bladder-worm stage, 

 or cysticercus. If a portion of Pi.^'s muscle containing cysticercr 

 which have not been killed by cooking is taken into the stomach 



