230 



ZOOLOGY 



SECT. 



into the intestine, reaches the exterior with the fyeces. A por- 

 tion of the egg-shell at one end then becomes separated off as 

 a sort of lid or opcrculum, and gives exit to the contained embryo. 

 This, the ciliated embryo (Fig. 179, A], is a somewhat conical body 

 covered all over with vibratile cilia, and with two spots of pig- 

 ment, the eye-spots (eye], near the broader or anterior end, which is 

 provided with a triangular head-lobe (pap.). There is no vestige of 



FIG. 179. J D. Development of Distomum hepaticutn. A, ciliated 

 larva; 5, sporocyst, containing redise in vaiious stages of development; 

 C, redia, containing a daughter redia, and cercariai ; D, fully developed 

 cercaria. />. op. birth opening ; c/^.enteron of redia ; eye. eye -spots ; /.*/. 

 gastrula stage of redia ; gei-m. early stages in the formation of cercariic : 

 int. intestine of cercaria; mo/: morula stage in the development cf 

 cercarise ; a-n. oesophagus ; or. aw. oral sucker ; /><i,>. head-papilla of ciliated 

 embryo ; j>li. pharynx ; proc. processes of redia ; t -vnt. *;<. ventral sucker. 

 (After Thomas.) 



internal organs, with the exception of a pair of flame-cells. The 

 ciliated larva swims about in water, or moves over damp herbage 

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

 (Lyrnnaxi') or ;i Land-snail ( J-falios), 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 the head-lobe. Established in the interior of the 



