158 UNSEGMENTED "WORMS." 



main branches, themselves branched ; two ovaries, numerous yolk 

 glands and testes, a common genital aperture, e.g. Planaria and 

 Dendroccelum (in fresh water), the former sometimes divides trans- 

 versely ; Gttnda segmentata (marine), showing hints of internal 

 segmentation ; Geodesmns and Bipalium (in damp earth) ; Bipalium 

 kewense is an import often found in Britain. 



C. Polycladida. Large leaf -like marine " Planarians," with 

 numerous intestinal branches diverging from a central stomach ; with 

 numerous ovaries and testes, without yolk glands, mostly with two 

 genital apertures. 



e.g. Cycloporus (showing beginning of anus), Leptoplana (not 

 uncommon on the seashore), Thysanozoon. 



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 well-formed 

 and apparently cellular "cuticle" the presence of attaching 

 suckers (occasionally with hooks), and the rarity of sense 

 organs. 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 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 hepaticum or 

 Fasciola hepatica). 



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 



