12 TROPICAL MEDICINE AND HYGIENE 



escape from the redia, and usually from their intermediate 

 host (or else migrate to another host), and after a brief 

 free life in water or encysted on blades of grass, enter 

 the body of their definitive host, and become sexually 

 mature, losing their caudal motile appendage or tail 

 early (fig. i, L). 



It will be seen from this that from the one fertilized 

 egg either several cercariae are formed, each capable of 

 becoming an adult trematode, or that the process of 

 asexual multiplication may be repeated, and from one 

 fertilized egg several rediae may be produced, and from 

 each redia many cercariae. Each cercaria may become 

 an adult sexually mature trematode (fig. i). 



For this process of development an intermediate host 

 is necessary. Such hosts are molluscs in the known 

 instances, with the exception that the development of 

 the Schistosomidae beyond the Miracidium is unknown. 



The life-cycle of the digenetic trematodes consists, 

 therefore, of alternations of generations. 



The adult is the sexual generation producing eggs and 

 fertilizing them. The eggs parthenogenetically produce 

 other generations. The process is known as Hetero- 

 genesis. 



The families into which the digenetic Trematodes are 

 divided are best considered after a brief review of the 

 anatomy of the adult forms of these flukes, as variations 

 in the structure of the different parts are the basis of the 

 scientific classification. 



Structures in Adult. The alimentary canal is composed 

 of a terminal mouth surrounded by a muscular ring 

 which forms the anterior or oral sucker. From the 

 mouth a short, 9 straight, muscular oesophagus passes 

 backwards, and in this oesophagus there may be lateral 

 pouches. 



The oesophagus terminates in a bifurcation to form 

 the intestine. This is thin-walled, and each half is con- 

 tinued backwards, terminating blindly. The intestine 

 varies. There may be numerous pouches, or diverticula, 



