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PHYLUM PLATYHELMINTHES 259 



give rise to the development of rcdice in the interior of the 

 sporocyst, and these give rise, either directly or through the inter- 

 mediation of a second generation of recline, to tailed cercaricc. The 

 cercaria3 escape from the body of the intermediate host, and, in 

 many cases, instead of merely becoming attached to herbage, as 

 in the Liver-Fluke, make their way into the body of a second 

 intermediate host (usually an invertebrate), and there become 

 encysted. The encysted larva is eventually taken into the interior 

 of the final host (generally a vertebrate) by the second intermediate 

 host being swallowed by the latter : the cyst-wall becomes dis- 

 solved, and the young Trematode becomes free, and develops into 

 the sexually mature condition, either in the interior of the alimen- 

 tary canal itself, or in some related part. There is thus to be 

 recognised in the Digenetic Trematodes an alternation^of^eneratiGns 

 comparable to that which has been described as so general in the 

 Ccelenterata. In the Trematoda, however, it is 1xrbe-observed, it 

 is~~anT alternation of a sexual, not with art-asexual, but with a 

 parthenogenetic generation (the "sporocyst), the ova of which develop 

 into a second parthenogenetic generation (the redire) ; and these 

 finally produce larva? (the cercarise) capable of developing into the 

 sexually mature form. The term heterogeny is applied to a life- 

 history of this kind, in which several distinct generations succeed 

 one another in a regular series. 



The egg of a Cestode is similar in essential respects to that of 

 a Trematode : there is a tough, chitinoid membrane or egg-shell, 

 which encloses not only the ovum but a number of yolk-cells, the 

 latter becoming absorbed as the process of segmentation of the 

 ovum goes on. The result of segmentation is the formation of a 

 superficial layer of cells (ectoderm) and a central mass, all enclosed 

 in a membrane composed of a single layer of cells thrown off when 

 v the embryo escapes from the egg. The ectodermal cells become 

 ciliated, so far as is known, only in Botlirioceplialus ; in the others 

 they become thrown off or ultimately absorbed without develop- 

 ing 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 while the egg is still in the uterus of one of the 

 most posterior of the proglottides of the parent worm. When the 

 progiottis in question becomes separated off, and has passed out 

 from the body of the final host, the eggs become discharged. 

 In order that development may proceed further the embryo must 

 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 intermediate host 

 dissolve the egg-shell and set free the contained six-hooked 

 Kexacanth embryo, which bores its way by means of its hooks to 



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