276 



ZOOLOGY 



SECT. 



cases the invertebrate intermediate host actually forms the food 

 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 



res 



\ 



eac 



FIG. 225. A Cysticercoid (Polynerms) 

 with the head and rostellum enclosed by 

 the caudal vesicle, a. aperture through 

 which evagination takes place ; >bd. body ; 

 c. cavity of cyst ; caud. caudal vesicle ; r. 

 aperture of excretory system ; ros. rostel- 

 lum ; s. sucker. (After Haswell and Hill.) 



FIG. 226. A Cysticercoid with the 

 rostellum evaginated. ros. rostellum; 

 s , s. suckers ; caud. caudal vesicle. 

 (After Haswell and Hill.) 



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 

 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 



