NEW INTERPPLETATION OF CNIDARIA 323 



transition from the monocellular species to the colonial 

 poly cellular forms. How can we be certain that the poly- 

 cellular heterotrophic Eumetazoa evolved in the same way 

 as was "invented" for an autotrophic regime ? Scholars should 

 refer in this connection only to developments that can be ob- 

 served in ontogenies. Yet are these really recapitulations of 

 some adult ancestors ? Contrary to the widely accepted opinion 

 which sees in Volvox pattern at least a plan of the primitive 

 ancestor of the Eumetazoa, wx consider such a suggestion as 

 completely improbable. 



Those who adhere to the old interpretation maintain that 

 during the second stage of the phylogeny of Metazoa, a two- 

 layered animal which was able to digest its food, the so-called 

 gastraea, had evolved from the primitive one-layered hollow 

 sphere whose members were able to perform, as epithelium 

 cells, all functions with the exception of the sexual function. 

 This tw^o -layered form was called a gastraea, after the name 

 of gastrula. Yet again no such animal exists. It was believed 

 that the typical evolution of this diploblastic form had been 

 similar to the development that can be occasionally observed 

 in the ontogenies w^here the posterior half of the animal (i.e. 

 the part of the animal situated opposite to the direction of 

 swimming) invaginates into its anterior half. Thus with one 

 action the animal succeeded in developing not only its two 

 layers, the ectoderm and the entoderm, but also a digestive 

 cavity and an oral opening which w^as unfortunately placed at 

 the posterior end of the animal. The gastraea as we find it thus 

 reconstructed is supposed to have lived as a planktonic animal 

 and it was radially symmetrical. This gastraea descended later 

 (partly simultaneously, according to Haeckel) to the sea bottom 

 where it evolved into various "Gastraeadae." The latter were 

 quite early given the unsuitable name of "Coelenterata." No 

 gastraea ever existed among plants; it is typical of the poly- 

 cellular animals. It possessed a skin and an intestinal layer and 

 showed a corresponding difference in its tissue and in its 

 cells. 



