142 



PLATYHELMINTHES. 



its length into a large two-layered sac 

 called the sporocyst. 'I'he inner layer 

 buds cells into the internal cavity, which 

 develop into organisms called redice 

 through a 7norula and gastrula stage. 

 A redia 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 exc retory t ubules. 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 cercarice which are 

 young or larval flukes. The cercatia 

 escapes by the gen ital p ore 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 distii^a||j|ed, and there is also 

 a flame-cel^^Bltory system. The 

 surface is dotWo 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 ma)4. produce fresh sporocysts by 

 binary fission and that the redia may 

 give rise to fresh generations of redi?e. 



vSporocyst. 



Sporocyst containing Redia. 



l^^ig- 75-- A Redia. 



Young Redia. Notice the 

 mouth and alimentary canal, 

 and two lateral processes. 



