PLAT VHELMINTHES 109 
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 ege 
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 ciliated 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, 
Limnaca truncatula. 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 cilia 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 (Fie. 
73, B). The Sporocyst may multiply by transverse division. 
Within its body certain germinal cells arise, and these ultimately 
form a Ledia, 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 Rediae are produced they may cause the death 
of the Snail. The Redia 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 Rediae, but as a rule they give 
rise to Cercaviae: 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 Rediae through an opening just behind the collar; they are 
at first active, and make their way out of the body of the snail 
