TREMATODA. 183 



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 sometimes cylindrical 

 external or internal parasites. With their parasitic life may 

 be associated the absence of cilia on the surface of the adiilts, 

 the thick "cuticle" the presence of attaching suckers (occasion- 

 ally 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 Turbellarians in being unsegmented, in having anterior 

 ganglia, from -which nerves pass backward 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 hepattcum) 



The adult fluke lives as a parasite in the liver and bile 

 ducts of the sheep, causing "liver-rot." Unlike most 

 flukes, it has many occasional hosts, it sometimes occurs 

 in cattle, horses, deer, camel, antelopes, goat, pig, beaver, 

 squirrel, kangaroo, and rarely in man. The animal is flat, 

 oval, and leaf-like, almost an inch in length by half an 

 inch across the broadest part, reddish brown to greyish 

 yellow in colour. As the word Distomum suggests, there 



