212 UNSEGMENTED WORMS 



to notice that the Turbellarians and TemnocephaUds are 

 free-Uving, except in the case of a few marine Turbellarians 

 which have taken to parasitism ; that the Trematodes are 

 all parasitic, either external hangers-on (ectoparasites) or 

 internal boarders (endoparasites) ; and that the Cestodes 

 are altogether endoparasitic. It is probable that the flukes 

 and tape-worms arose from Turbellarian-like ancestors 

 which adopted parasitic habits. Attention must be directed 

 to the flame-cells which are characteristic of Platyhel- 

 minthes. Each terminal twig of a branch of an excretory 

 canal leads into a large hollow cell, from the base of which 

 a bunch of cilia — with rapid movements suggesting a 

 flickering flame — projects into the cavity towards the 

 lumen of the twig. 



Class TuRBELLARiA. Planarians, etc. 



Turbellarians are tinsegmented " worms'' usually leaf- 

 like, living in fresh, brackish, or salt water, or in moist earth. 

 Almost all are carnivorous, a few are parasitic. They 

 represent the beginning of definite bilateral symmetry. 



The ectoderm is ciliated, often glandular, often with peculiar 

 rod-like bodies (rhabdites) which may be discharged on irrita- 

 tion. A pair of ganglia in the anterior region give off 

 lateral nerve-cords, and there are usually simple sense organs. 

 The food canal has a protrusible muscular pharynx, is often 

 branched, and is always blind ; digestion takes place partly 

 or wholly within the lining cells. There are no special 

 respiratory or circulatory organs ; the body cavity is not 

 represented, unless it be by intercellular lacunce in the 

 parenchyma ; the excretory system usually consists of two 

 longitudinal canals, whose branches end internally in flame- 

 cells. The Turbellarians are almost always hermaphrodite ; 

 and the reproductive organs usually show some division of 

 labour, e.g. in the development of a yolk gland, which may 

 have arisen as an over-nourished {hypertrophied) part of the 

 ovary. The eggs are usually enclosed in shells or cocoons, 

 and the development may include a metamorphosis. Some 

 forms multiply by fission. There seem to be affinities 

 between Turbellaria and Coelentera, especially the 

 CtenoPhora. 



