PHYLUM PLATYHELMINTHES 



281 



of the final host Thus a cysticercoid having as an intermediate 

 host an Earthworm is taken with the latter into the alimentary 

 canal of a Sea-Gull — its final host. In this way the cysticercoid 

 is set free in the alimentary canal of the final host, the head 

 becomes pushed out from the enclosing caudal vesicle and body 

 (probably owing to the stimulus of the higher temperature), so 

 that the suckers and hooks come into play and attach the young 

 tape-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 

 relatively 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 remark- 

 able 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 in- 

 vagination with a cavity opening 

 freely on the exterior. On the 

 bottom of this is formed an 

 elevation projecting into its in- 

 terior; 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, appear the 

 suckers. When inverted this in- 

 vagination corresponds closely 



with the head and body of the cysticercoid ; the bladder corre- 

 sponds to the caudal vesicle. Thus 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 cysticcrcoid 

 with the three parts already described. One such form occurs 



ccuui 



FlO. 228. — A Cysticercoid with the rostel- 

 lum evaginated. ros. rostellum ; s. , x. 

 suckers ; raixl. caudal vesicle. (After 

 llnswell and Hill.) 



