190 THE EVOLUTION OF THE METAZOA 



the intestine has been progressively developed into a rich 

 system of pockets and canals which corresponds to the change 

 of the whole form and to the increased size of their bodies. We 

 have here generally a further development of the intestinal 

 system such as had been evolved in their turbellarian an- 

 cestors. 



The Skin and Cnidae 



The skin of Cnidaria together with their characteristic cnidae, 

 which have given their name to Cnidaria, oflFer facts which 

 convincingly support our thesis of the origin of Cnidaria from 

 the Turbellaria and, as a consequence of this, of the evolution 

 in the direction Anthozoa -^ Scyphozoa -> Hydrozoa. It really 

 is suprising how^ it has been possible for these facts to have 

 been overlooked for so long. This attitude, which one is hard- 

 ly able to understand, can be partly attributed to an almost 

 hypnotic influence of Haeckel's gastraea theory and to the 

 authority of Haeckel. Quite similar is the situation about the 

 lore of the germ layers and with everything that is con- 

 nected with it. 



Cnidae have in their evolution reached their climax in Cni- 

 daria. No species of Cnidaria has been know'n which does not 

 possess cnidae distinctive of this same species. Their si^e, form, 

 organization, and the way they are distributed, as well 

 as the different places where they are developed and used, are 

 frequently characteristic of various taxons of Cnidaria. Attempts 

 have been made to solve taxonomic problems connected with 

 Cnidaria on the basis of conditions that can be found in their 

 cnidae. It is well know-n, however, that Cnidaria are not the 

 only animal group which produce cnidae. 



When w^e speak about cnidae, we must first know^ what a 

 cnida actually is. The most highly developed cnidae are w- onder- 

 ful products of very specialized individual cells of skin gland 

 cells. Their secretion becomes partly solid (the capsule, and 

 the long thin tube w^hich represents a continuation of the 



