26 



stem by a common root, from which the group Cestoda is 

 given off. The existing forms are placed at the end of a 

 lin-e which is long, to indicate considerable divergence from 

 the ancestral form, and slopes downwards to show that the 

 group is a degenerate one. 



The Dendrocoelous and Rhabdocoelous Turbellaria are 

 probably the least modified members of the Platyelmian 

 branch, and are therefore placed most nearly in the direct 

 line of development. Lang* considers the Polycladidae as 

 the most ancestral forms known, and derives from them the 

 Tricladidse, the Rhabdocoela, the Acoela, and the Trematoda. 

 The three last named groups are probably all more or less 

 degenerate and have been placed in the table on side 

 branches sloping downwards from the ancestral Dendrocoela. 

 The Accela were given off first and have degenerated more 

 than the Ehabdoccela. 



The Trematoda are more divergent than any of the 

 Turbellarian groups. They show affinities in structure both 

 with the Dendrocoela and with the probable ancestral form of 

 the Cestoda. Probably they diverged from the Dendrocoelous 

 Turbellarians, and have been considerably specialised and 

 somewhat degraded. As a result of their generally parasitic 

 habits they have lost their original coating of cilia and have 

 acquired various organs of attachment. 



The Nemertida are the highest group of the Platyelmia. 

 They probably arose from the ancestral Turbellarians very far 

 back and seem to have undergone a considerable amount of 

 evolution. The body is elongated and more worm-like than 

 in the other Platyelmia, and the alimentary canal is more 

 highly differentiated. An anal opening is present, and a 

 protractile frontal proboscis, often of very large size and 

 armed with chitinous styles, is a characteristic feature. 



* Fauna und Flora des Golfes von Neapel, XI Monographie : Die 

 Polycladen. 



