142 PLAT VHELMINTAHES. 
its length into a large two-layered sac 
called the sfovocyst. ‘The inner layer 
buds cells intotheinternal cavity, which 
develop into organisms called vedie 
through a mora and gastrula stage. 
A vedia has an elongated body with 
mouth at the anterior end, a pharynx 
and simple intestine. Externally it has 
a collar or thickened ridge round the 
anterior end, behind which is a small 
pore into the body-cavity, and a pair 
of processes towards the hind end. It 
also has excretory tubules. A redia 
when developed bursts through the 
brood-sac or sporocyst and eats its 
way through the snail. Eventually it 
produces, by budding of its internal 
cells, a number of cevcari@ which are 
young or larval flukes. The cercarza 
escapes by the genital pore of the 
redia and out of the snail into the 
water. It has a rounded body and 
vibratile tail. Two suckers, a mouth, 
pharynx, and simple bilobed intestine 
can be distinguished, and there is also 
a flame-cell excretory system. The 
surface is dotted with cystogenous cells 
which produce the cyst. The cercaria 
works its way to the edge of the pond 
(the snail may be in grass already), 
up a blade of grass or other plant 
and there loses its tail, encysts and 
remains dormant. Should the cyst 
be introduced into the stomach of the 
sheep the cercaria escapes, passes up 
the bile-duct. and develops in a few 
weeks into a young fluke. 
We have to add that the sporo- 
cyst may produce fresh sporocysts by 
binary fission and that the redia may 
give rise to fresh generations of redize. 
Fig. 74.—SPOROCYST. 
Sporocyst containing Rediz. 
Fig. 75.—A Reva. 
Young Redia. Notice the 
mouth and alimentary canal, 
and two lateral processes. 
