Evolution 



body. There are frequently two pairs of sense organs, probably 

 rudimentary eyes and ears respectively. The main features of 

 the digestive, reproductive, excretory, and nervous system are 

 shown in the figures in Fig. 40. 



The Turbellarians probably arose from Ctenophora or from 



some nearly related form, a 

 view that receives support from 

 the occurrence of several ap- 

 parently intermediate types. 

 The differences may, in fact, 

 be partly accounted for as 

 adaptions to meet the change 

 of habitat from that of the 

 upper waters to that of the 

 sea floor. A spherical, or pear, 

 or bell shape is suitable enough 

 for a swimming animal, but 

 would be impossible for one 

 that was to crawl. The first 

 change, then, we may imagine, 

 was a flattening, which pro- 

 duced a disc-shaped animal, 

 with the mouth in the centre 

 of the lower aspect and the 

 sense organs in the middle of 

 the upper. Secondly, a definite 

 mode of progression, by which 

 one part of the body continually 

 went first, would be an advan- 

 tage, as permitting of a better 

 co-ordination of movements, 

 and an elongation of the body in the line of movement would have 

 the effect of diminishing resistance and of making progression 

 easier. Finally the sense organs, like the scouts of an army, 

 would be best in front, and would migrate thither, and the mouth, 



in order to get the full benefit of the food which the sense organs 



60 





FIG. 40. A simple Turbellarian 

 Rhabdocoelum (diagrammatic). 



m, Mouth ; </, digestive cavity ; nc, nephridia ; 

 an, eyes ; na, sense organs ; g, brain ; 

 ;/, nerves ; //, male, and ^, female, repro- 

 ductive glands. 



