362 ZOOLOGY. 



New generations of rediae are developed in the same way in 

 the generative cavity of a parent redia, so that a single 

 snail becomes infested with large numbers of the parasites 

 and is ultimately killed by them. 



9. The Cercaria. Eventually, usually on the approach 

 of autumn, the germ- cells of the rediae, instead of develop- 

 ing into new rediae, give rise to individuals of a different 

 form and structure, called cercariae. (" Redia " is derived 

 from " Eedi," the name of an Italian zoologist ; " cercaria" 

 from kerkos, the Greek word for " tail.") The cercaria 

 may be described as an immature stage of the adult fluke, 

 of cordate shape, and provided with a long muscular tail. 

 Like the redia it consists of external body- wall and an 

 internal digestive sac, but it already possesses many of the 

 characters of the adult fluke. Around the mouth at the 

 anterior end is a small oral sucker, and in the middle of 

 the ventral surface is a large dermal sucker. The enteron 

 bifurcates immediately behind the pharynx, and the two 

 halves extend back, one on each side of the body, as simple 

 tubes without branches. In the tissue of the body is an 

 excretory system which is paired and symmetrical, as in 

 the adult, consisting of longitudinal ducts into which open 

 numerous branches, the ends of the branches arising from 

 flame- cells. The generative organs of the adult are repre- 

 sented only by a germinal tissue between the wall of the 

 enteron and the body-wall. On each side of the body near 

 the surface are numerous large cells loaded with granules. 

 These are called the cystogenous cells, because they sub- 

 sequently secrete a substance which forms a spherical 

 envelope, or "cyst," around the cercaria. 



10. Encystment. The cercaria, after escaping from the 

 birth-aperture of the parent redia, does not remain in the 

 tissues of the snail in which it was produced, but very 

 shortly passes out and becomes free. It then, by means of 

 the movements of its tail and of its suckers, for a while 

 moves about in the water or among the wet grass ; but be- 

 fore long, attaching itself to a blade of grass or to a water 

 plant, ceases to move and becomes encysted that is, 



