v PHYLUM PLATYHELMINTHER 275 



(ectoderm) and a central mass, all enclosed in a membrane composed 

 of a single layer of cells thrown off when the embryo escapes from 

 the egg. The ectodermal cells become ciliated, so lar as is known, 

 only in Bothriocephalus ; in the others they are thrown off or 

 ultimately absorbed without developing cilia. The central mass of 

 cells alone forms the embryo. The embryo, while still consisting 

 of a small number of cells, develops a series of six chitinous hooks. 

 These early changes all take place in the majority of Cestodes while 

 the egg is still in the uterus of one of the most posterior of the 

 proglottides of the parent worm. When the proglottis in question 

 becomes separated off, and has passed out from the body of the 

 final host, the eggs are discharged. 



In order that development may proceed further, the embryo 

 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 cysticercus, or bladder-worm. The cysticercoid form 

 (Figs. 225 and 226) 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, Centipedes, 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 a tape- worm head or scolex with the hooks and suckers of the 

 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, 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 



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