PLAT YHELMINTHES 1 09 



are cast out from the body into the bile duct in such quanti- 

 ties that it has been computed that each fluke produces half a 

 million eggs. 



The further development of the embryo only takes place 

 outside the body, and at a low temperature. If these condi- 

 tions be present, at the end of two to three weeks the egg 

 gives birth to a free-swimming ciliated embryo. This is a 

 conical larva, provided with a double eye-spot, and rudiments of 

 an excretory system in the form of cihated funnels (Fig. 73, A). 

 If this embryo is fortunate enough to be born in a pond or 

 ditch, it swims about looking for a certain species of water snail, 

 lAmnaea truticatula. If it fails in its quest, it dies in eight 

 or ten hours ; on the ■other hand, if it succeeds, it immediately 

 sets to work to bore into the soft tissues of the snail. This it 

 effects by elongating its head papilla into a pointed structure, 

 and revolving on its axis by means of its cilia. When once 

 it has forced an entrance into the tissues of the snail, it loses 

 its ciha and becomes a Sporocyst. This is an oval sac of cells, 

 whose wall is covered with a cuticle and contains circular and 

 longitudinal muscle fibres, and is lined by an epithelium (Fig. 

 73, B). The Sporocyst may multiply by transverse division. 

 Within its body certain germinal cells arise, and these ultimately 

 form a Redia, which bores through the walls of the Sporocyst, 

 and makes its way to the liver of the snail (Fig. 73, C and D). 

 The walls of the Sporocyst close up, and the process is repeated ; 

 but if too many Eediae are produced they may cause the death 

 of the Snail. The Eedia is a cylindrical larva with a terminal 

 mouth, which leads through a pharynx into a blind stomach 

 lined with a single layer of cells. A little way behind the 

 mouth the surface of the body is raised into a circular ring, 

 and posteriorly there are two projections which assist the larva 

 in its movements. The excretory system is well developed, and 

 the cells lining the body-wall give rise to the germinal cells. 

 These latter may produce fresh Eediae, but as a rule they give 

 rise to Cercariae : organisms which differ from their parent by 

 the possession of a forked alimentary canal, two suckers, a 

 tail, and certain cystogenous cells. The Cercariae escape from 

 the Eediae through an opening just behind the collar ; they are 

 at first active, and make their way out of the body of the snail 



