280 



ZOOLOGY 



CO 



ccucct 



must in most cases reach the interior of a second or intermediate 

 host. This is a passive migration, since the embryo of the Cestode is 

 still confined within the egg-shell, and the transference has to 

 take place in the water or food. The digestive fluids of this inter- 

 mediate host dissolve the egg-shell and set free the contained six- 

 hooked or hexacanth embryo, which bores its way by means of its 

 hooks to some part of the body in which it is destined to pass 

 through the next phase in its life- history, and there becomes 

 encysted. 



The phase which follows presents two main varieties. In 

 cases in which the intermediate host is an invertebrate animal 

 the hooked embryo develops into a form to which the name of 



cysticercoid is given ; when, on 

 the other hand, the intermediate 

 host is a vertebrate, the form 

 assumed is nearly always that 

 termed ci/stircrcus, or bladder- 

 worm. The cysticercoid form 

 (Figs. 221 and 222) is to be re- 

 garded as the more primitive 

 and less modified. Cysticercoids 

 of various tape- worms occur in 

 a great variety of different in- 

 vertebrates — e.g., Insects of all 

 kinds, Water-fleas, Centipedes, 

 Earthworms. The hooked em- 

 bryo loses its hooks and de- 

 velops into the cysticercoid in 

 some part of the invertebrate 

 intermediate host. The cysti- 

 cercoid consists of three parts — 

 a tape- worm head or scolex with 

 the hooks and suckers of the 

 mature worm, a so-called body, 

 and a caudal vesicle. Some- 

 times there is a tail recalling to some extent the tail of a cercaria. 

 Sometimes the caudal vesicle is absent : when present, either from 

 the first, or as a result of later changes, it encloses the head as 

 well as the body after the manner of a cyst. While undergoing 

 these changes the cysticercoid is usually enclosed in an adventitious 

 cyst formed for it by the tissues of its host, but it often lies free 

 in the body-cavity. The transference to the final host is effected 

 by the intermediate host, or the part of it containing the cysti- 

 cercoid, being taken into the alimentary canal of the final host. 

 Sometimes, if the intermediate host is a relatively small animal, 

 such as a water-flea, this may take place " accidentally " ; in other 

 cases the invertebrate intermediate host actually forms the food 



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Fig. 221. —A Cysticercoid (Potyeercus) with 

 the head and rostellum enclosed by the 

 ciudal vesicle, a. aperture through which 

 evagination takes place ; bd. body ; c. 

 cavity of cyst ; caud. caudal vesicle ; ex. 

 aperture of excretory system ; ros. rostel- 

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



