PHYLUM PLATYHELMINTHES 



241 



end, which is provided with a triangular head-lobe (pap.). There 

 is an imperfectly developed intestine, and a pair of flame-cells, 

 each with a fine canal opening on the surface. The rest of 

 the interior is filled with a mass of germ-cells. The ciliated 

 larva swims about in water, or moves over damp herbage 

 for a time, and perishes unless it happens to reach a water-snail, 

 (species of Lymnceus, or of Bulinus or Planorbis), as a parasite of 

 which it is alone able to enter upon the next phase in its life-history. 

 When it meets with the Snail, the embryo bores into it by means of 



eye 



ffasf 



I: 



tyrac 



-ph. 



FIG. 193. A D. Development of Fasciola hepatica. A, ciliated 

 larva ; B, sporocyst, containing redise in various stages of develop- 

 ment ; C, redia, containing a daughter redia, and cercariee ; D, fully 

 developed cercaria. b. op. birth opening ; ent. enteron of redia ; eye, 

 eye-spots ; gast. gastrula stage of redia ; germ, early stages in the 

 formation of cercarise ; int. intestine of cercaria ; mar. morula stage 

 in the development of cere-arise ; ces. oesophagus ; or. su. oral sucker ; 

 pap. head-papilla of ciliated embryo ; ph. pharynx ; proc. processes 

 of redia ; vent. sit. ventral sucker. (After Thomas.) 



the head-lobe, corning to rest in the pulmonary sac or some other 

 organ of the mollusc. Established in the interior of the Snail, it 

 loses its ectoderm and grows rapidly into the form of an elongated 

 sac, the sporocyst (Fig. 193, B), with an internal cavity containing 

 germ-cells and lined by a layer of cells, with remnants of the eye- 

 spots, and with flame-cells. The sporocyst may divide into two 

 similar bodies by a process of transverse fission, but this is excep- 

 tional. Eventually cells are budded off from the layer that lines the 

 internal cavity of the sporocyst or from the germ-cells, and these 

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