180 THE EVOLUTION OF THE META/.OA 



groups of Anthozoa which have remained solitary animals. 

 The subsequent evolution took a retrogressive direction after 

 they had passed over to cormogony, a simultaneous decrease 

 of the size of individual zooids and, in connection with this, 

 of the internal surfaces. 



I think it useless to discuss whether the intestinal diverticula 

 of Turbellaria be homologous or homotypic to the intestinal 

 pockets of Anthozoa that can be found between their sarco- 

 septa. It is certain, however, that both these forms are due 

 to a tendency of these animals to increase the digestive or 

 resorptive surface of their intestinal tubes; these forms diverged 

 in their evolution w^hich in these animals also pursued different 

 directions: on one hand we have (in Turbellaria) a flattened 

 form of an animal which creeps over the ground, and on the 

 other hand (solitary anthopolyps) a cylindric form which 

 stands erect and which can move actively only slightly, 

 or not at all. 



On the other hand, we can find that even in Turbellaria an 

 inclination to develop longitudinal folds in their intestinal 

 tubes is not completely unknown to them. What other than 

 a longitudinal folding can finally be the forked form of that 

 part of the intestine of Tricladida w^hich can be observed in 

 the posterior half of their body? One could expect, if more 

 extensive investigations had been made in this respect, that 

 the formation of such longitudinal folds could be found even 

 more frequently in small species of Turbellaria whose trans- 

 verse sections show a rounded-off form. It would be permissible, 

 I think, to classify the longitudinal folds and herewith the septal 

 apparatus under the notion of the intestinal diverticule and 

 also the terminal diverticula (prochorda) as well as the chorda 

 dorsalis. The fact is not without a significance that during the 

 transition from anthopolyp to scyphopolyp the sarcosepta which 

 had been reduced into taeniolae had clearly obtained the form 

 of longitudinal folds. It should be added that the wrong in- 

 terpretation of the true nature of sarcosepta is partly due to 

 the fact that sclerosepta, w^hich are externally similar to sarco- 



