PLATYHELMIA 49 



what has been called a parenchyma or mesenchyme. The 

 spaces in the mesenchyme allow of a circulation of metabolic 

 products. Some of the cells are converted into muscles, form- 

 ing especially a transverse and a longitudinal layer close to 

 the ectoderm, and the muscular coat of the pharynx. 



Eggs are produced in great numbers, each individual 

 providing, according to Thomas, some 4000. They undergo 

 development in the egg case, but complete it only in favourable 

 conditions namely, in wet places, as pools and ditches. In 

 such places the embryo escapes by the bursting of the lid or 

 operculum of the egg case, after a free period of about one month. 

 The larva, or miracidium, is ciliated. It is provided with a pair 

 of adposed eye-spots, and in front with a papilla and internally 

 with a pair of protonephridia. After being hatched the 

 larva rarely lives a day if it does not meet its intermediate 

 host, a common snail, Limnaea truncatula, found in ditches, 

 ponds, marshes, and streams, and on land near these. It 

 lodges in the pulmonary cavity of the snail, loses its cilia, 

 and, becoming ovoid in shape, grows quickly into a sporocyst, 

 the process lasting a fortnight in summer and a month in 

 autumn. The sporocyst may divide and thus form daughter 

 sporocysts. Cells are budded off from the inside of the wall 

 of the sporocyst, and these cells by further division form masses 

 which are converted into a number of rediae. The redia is 

 also sac-like, but is provided with a mouth leading into a 

 short alimentary canal. The rediae escape by rupture of the 

 sporocyst and pass from the pulmonary cavity into the body 

 of the snail and most frequently lodge in the liver. The redia 

 grows to a size of about 1J mm. and gives rise to daughter 

 rediae in summer, but in winter gives rise to cercaria. The 

 cercaria is provided with two suckers, a forked intestine and 

 a long tail. They make their way out of the snail, and, escap- 

 ing into the water, swim about by the action of the tail and 

 finally attach themselves to grass or water plants. When 

 attached the tail is lost and a cyst of mucus secreted around 

 the young fluke. The cyst hardens into a white envelope. 

 It is now in the state when further development can only 

 take place in the sheep. Eaten by the sheep the cysts are 



