280 



ZOOLOGY 



SECT. 



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 cmlri/o, 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 ei/sticcrcus, 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 scolcx 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 



eac 



FIG. 221. A Cysticercoid (Polycercus) with 

 the head and rostellum enclosed l>y the 

 cnudal vesicle, a. aperture through which 

 evagination takes place ; M. body ; r. 

 cavity of cyst ; caud. caudal vesicle ; ex. 

 aperture of excretory system ; ros. rostel- 

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



