1 64 UNSEGMENTED " WORMS." 



Relationships. Two remarkable forms Cceloplana (Kowalewsky) and 

 Ctenoplana (Korotneff) seem in some ways intermediate between 

 Turbellarians and Ctenophora. Thus they have an aboral sense organ, 

 and retractile branched tentacles ; the branching of the food canal is 

 slightly suggestive of that in Ctenophora ; and Ctenoplana has eight 

 dorsal bands of ciliated combs. The resemblance has been made much 

 of by Lang and others, but, apart from direct affinity, there are likely to 

 be resemblances of " convergence " (see p. 33) between forms not far 

 removed from a common stock that of the primitive Metazoa. 



The occasional presence of a retractile proboscis and of a ciliated 

 groove on each side of the brain is suggestive of two characteristics of 

 Nemerteans. 



The Turbellaria are also related to the next class the Trematodes. 



Class TREMATODA. Flukes, &c. 



The Trematodes are leaf-like or roundish external or in- 

 ternal parasites. With their mode of life we may associate 

 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 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 in or on 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 (Fasciola (Distomd) hepatica). 



The adult fluke lives in large numbers in the liver and 

 bile duct of the sheep. It sometimes occurs in cattle, 

 horses, and other domestic animals, and rarely in man. 



