2GO 



ZOOLOGY 



SECT. 



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 Cysti- 

 I eercus, or bladder-worm. The cysticercoid (Figs. 207 and 208) 

 ( form is to be regarded as the more primitive and less modified. 

 Cysticercoids of various tape-worms occur in a great variety of 

 different invertebrates, e.g. Insects of all kinds, Water-fleas, Cen- 

 tipedes, Earthworms. The hooked embryo loses its hooks and 

 develops into the cysticercoid in some part of the invertebrate 

 intermediate host. The cysticercoid consists of three parts 

 \ I a tape-worm head or scolex, with the hooks and suckers of the 

 U mature worm, a so-called body, and a caudal vesicle. Sometimes 

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

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



from the first, or as a result 



<3^ of later changes, encloses the. 



head as well as the body after 

 the manner of a cyst. While 

 undergoing these changes the 

 cysticercoid is usually en- 

 closed 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 con- 

 taining the cysticercoid, being 

 taken into the alimentary 

 canal of the final host. Some- 

 times, 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 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 



FIG. 207. A Cysticercoid (Polyccrcus) wfth 

 the head and rostellum enclosed by the 

 caudal vesicle, a. aperture through which 

 evagination takes place ; Id. body ; c. cavity 

 of cyst ; caml. caudal vesicle ; ex. aperture 

 of excretory system ; ros. rostellum ; s. 

 sucker. 



