Chap, xiv.] DEVELOPMENT OF FLUKE. 



547 



it may pass to the neighbourhood of its next host ; 

 this has been shown by Thomas to be the small air- 

 breathing snail which is known as r.ymiiseiis 

 trimcatuliis. Provided at its anterior end with a 

 papilla which acts as a most effi- 

 cient boring-organ, the larva forces 

 its way between the cells of the 

 wall of the lung of the Lymnseus, 

 and makes its way into the lung 

 cavity. In this position it loses its 

 elongated and acquires a rounded 

 form, giving rise to the so-called 

 sporocyst stage. The cells with- 

 in the body which have not yet 

 been used up in the formation of 

 any tissue, arrange themselves in 

 definite groups, each of which gives 

 rise to an elongated larval form 

 not unlike a gastrula (Fig. 228), 

 save that it is provided with a 

 definite pharynx, has an " annular 

 ridge," and two short blunt pro- 

 cesses behind. We have now the 

 Redia stage. The R6dia, be- 

 coming free, may make its way 

 into other organs of the snail's 

 body ; within this Redia fresh 

 Redise may be again developed, or 

 the germinal cells within it may, in- Fig._228. Redia of D. 

 stead, give rise to yet another form. 

 At any rate, the final product 

 of redise, or daughter-rediae, is a 

 body of rounded form with a long tail (Fig. 229), 

 to which the name of cercaria has been long since 

 applied. The parasite in this stage makes its way 

 to the exterior, and, becoming enclosed in a firm cyst, 

 loses its tail; these cercarian cysts take up their 



Hepaticum. 

 Thomas.) 



(After 



n, Pharynx ; m, contained 

 germs ; r, posterior pro- 

 cesses. 



