172 THE EVOLUTION OF THE METAZOA 



polyps with primitive forms that do not yet deserve the name 

 of sarcosepta but which resemble rather some longitudinal 

 folds, apparently took place in the period when no mineralized 

 skeletons had yet been developed; we can therefore not ex- 

 pect that we will ever be able to find such transitional fossils. 

 This makes it necessary for us to resort to a reconstruction. 



We can see considerable differences between the two forms 

 when wx compare sarcosepta such as they occur now in 

 Anthozoa with the intestinal diverticula of Turbellaria. These 

 differences seem to support the old interpretation of the evolu- 

 tion of Cnidaria, and they have actually been used by those who 

 adhere to the old interpretation in their critiques of my nevv^ 

 thesis. This new thesis, however, becomes clearer if we make 

 a detailed comparison, taking simultaneously into considera- 

 tion the radical change in the way of life, i.e. the transition from 

 creeping movements to the sessile way of life, and the erection 

 of the body. By means of this new^ interpretation many things 

 can now be explained which could not be explained on the 

 basis of the old theory. 



The tendency of the intestine to form folds that finally leads 

 to the formation of excrescences — the diverticula— which is so 

 generally prevalent among Turbellaria (with the exception of 

 the primitive groups of Acoela and the majority of Rhabdo- 

 coela) must be considered as the starting point for the deve- 

 lopment of sarcosepta w^hich are characteristic of Anthozoa. 

 Here we will first study that part of this tendency which is 

 connected with the formation of excrescences of the body skin 

 at the anterior end of a somewhat cephalized body. This ten- 

 dency is not so marked in Turbellaria as it is in Cnidaria. We 

 meet here the case of the Ctenophora which we derive from 

 the pelagic larvae of Polycladida, obviously from such where 

 no intestinal diverticula had yet grown into the pair of tentacles. 

 This has led to one of the most conspicuous differences that 

 exist between Cnidaria and Ctenophora. We must suppose that 

 the ancestors of Cnidaria had already developed the ten- 

 tacular diverticula, and this is the reason why this property 



