PHYLUM PLATYHELMINTHES 



261 



young 



tape- 



suckers and hooks come into play and attach the 

 worm to the wall of the alimentary canal. 



The cysticercus or bladder-worm differs from the cysticercoid 

 mainly in its much greater size 

 and in the development of a re- 

 latively large caudal vesicle or 

 caudal bladder. When the hooked 

 embryo has reached that part of 

 the vertebrate host in which it 

 is destined to develop into the 

 cysticercus it undergoes a re- 

 markable change ; it becomes 

 greatly enlarged, and a cavity, 

 filled with fluid or with a very 

 loose form of connective-tissue, 

 appears in its interior, so that 

 it assumes the appearance of a 

 relatively large bladder. On one 

 side of this bladder appears a 

 small invagination with a cavity 

 opening freely on the exterior. 

 On the bottom of this is formed 

 an elevation projecting into its 

 interior ; this is the rudiment of 

 the rostellum on which the 

 hooks are borne ; at its base, on 

 the inner surface of the side 

 walls of the invagination, ap- 

 pear the suckers. When everted this invagination corresponds 

 closely with the head and body of the cysticercoid ; the bladder 

 corresponds to the caudal vesicle. The chief difference between a 

 cysticercus and a cysticercoid is that in the former the caudal 

 vesicle is relatively very large, and that the order of development 

 of the parts is somewhat modified. 



A very small number both of cysticercoids and cysticerci 

 multiply by proliferation by the formation of more than one 

 tape-worm head from one embryo. In the few instances in which 

 this occurs among the cysticercoids the hooked embryo gives rise, 

 not directly to a cysticercoid, but to a mass of cells, from which 

 are given off" a number of buds, each developing into a cysticercoid 

 with the three parts already described. One such form occurs in 

 certain Earth-worms; another in a Myriapod (Crlomeris liiiil_tns). 



Tcenia ccenurns of the Dog has a bladder-worm stage, in the 

 Sheep and Rabbit, which gives rise to several tape-worm heads. 

 But the best known instance of multiple production of scolices in a 

 cysticercus is Tcenia cchinococcus well known as cause of the 

 disease termed *hydatids, common in Man and in various domestic 



FIG. 208. A Cysticercoid, with the 

 head evaginated. ro*. rostellum; .3., s. 

 suckers ; cai'.<i. caudal vesicle. 



