468 



WORMS. 



perish. Some however, in all probability, make their way into water. When this 

 has taken place, the egg, after two or three weeks, gives birth to a free-swimming, 

 ciliated, conical embryo, provided with a double eye and rudiments of an excretory 



system. By means of its cilia, this embryo swims 

 rapidly about in search of a particular species of 

 pond-snail. If it fails in its search, it perishes in 

 about eight or ten hours ; but, if successful, it proceeds 

 to bore its way into the soft tissues of the mollusc. 

 As soon as it has effected an entrance, it loses its cilia 

 and turns into an oval sac, the sporocyst. The latter 

 may multiply by "fission, but in any case, in its 

 interior, another organism, called after Redi, its 

 discoverer, Redia, arises. This bores its way out of 

 the sporocyst, which closing up again forms another : 

 but if too many are developed they may cause the 

 death of the snail. The Redia is cylindrical in shape, 

 and has a distinct mouth and stomach, and in the 

 hinder half of its body there is a pair of bud -like 

 processes, serving as rudimentary feet. The larva 

 in this stage takes up its abode in the liver of the 

 snail, where, in turn, it proceeds to propagate. Its offspring may be a Redia like 

 itself but more often it has a different form, and has received the name Cercaria. 



DEVELOPMENT OP Distomum 

 echinatum. 



LAEVAL FORM OF LIVER-FLUKE (magnified). 



It escapes from the parent Redia by an aperture situated near the front end of 

 its body ; and presents a considerable resemblance to a tadpole, consisting of a long 

 vibratile tail, and a wide heart-shaped body with a forked intestine, two suckers, 



