TREMATODA. 173 



syncytium (i.e. without distinct demarcation into cells) which 

 secretes a thick cuticle, contains rhabdites, and rarely bears 

 cilia. The class seems to be intermediate between Rhab- 

 doccelid Turbellaria and Trematodes. 



Class TREMATODA. Flukes, etc. 



The Trematodes are leaf-like, or roundish external or 

 internal parasites. With their parasitic life may be associated 

 the absence of cilia on the surface of the adults, the thick 

 "cuticle" the presence of attaching suckers (occasionally with 

 hooks), and the rarity of sense organs. After embryonic life 

 the ectoderm degenerates, ceases to be distinctly cellular, and 

 sinks inwards. It is likely that they have arisen from free 

 Turbellarian-like ancestors, and they resemble the Turbel- 

 larians in being unsegmented, in having anterior nerve 

 centres, from which nerves pass backivard and forward, in 

 the rudimentary nature of the body cavity, in the ramifying 

 system of fine excretory canals, in the hermaphrodite and 

 usually complex reproductive system. The excretory and 

 nervous systems are, however, more complex than those of 

 Turbellaria. The alimentary canal is usually forked, 

 often much branched, and always ends blindly. In many 

 cases the animals are self- impregnating, but cross- 

 fertilisation also occurs. The development of the external 

 parasites is usually direct, of the internal parasites usually 

 indirect, involving alternation of generations. They occur 

 on or in all sorts of Vertebrates, but those which have 

 an indirect development, and require two hosts to complete 

 their life -cycle, often pass part of their life in some 

 Invertebrate. 



Type, The Liver Fluke (Distomum hepaticuni). 



The adult fluke lives as a parasite in the bile ducts of 

 the sheep. It sometimes occurs in cattle, horses, and 

 other domestic animals, rarely in man. In the sheep it 

 causes the serious disease called liver rot. The animal is 

 flat, oval, and leaf-life, measures almost an inch in length 

 by half an inch across the broadest part, varies from reddish 

 brown to greyish yellow in colour. As the word Distomiim 



