326 THE EVOLUTION OF THE METAZOA 



been changed during evolution in the sense of a methodical 

 rationalization or of a "technical" improvement. Such a 

 development need not have been divergent, but rather 

 successive: it was first ecterocoelic (teloblastic) and later 

 enterocoelic; thus the formation of the "mesoderm" itself has 

 never had anything in common with the formation of the 

 intestinal diverticula, nor with the formation of lodges as they 

 can be found in the Anthozoa. 



The intestine too evolved according to the old interpreta- 

 tion as a new form along several lines; thus the Eumetazoa with 

 their intestines are supposed to have evolved several times 

 from a form that had no intestine (blastaea)! Can such an 

 interpretation be probable? The development of a uniform 

 intestinal cavity is one of the main characteristics of the 

 Eumetazoa (therefore the name Gastrobionta, according to 

 Rothmiiller). We consider that this cavity evolved first in 

 Protozoa as a transitional digestive vacuole; in the earliest 

 Eumetazoa this had led to an ad hoc (i.e., for each swallowed 

 piece) formation of a digestive cavity in their internal Plasmo- 

 dium; the final result of this development had been a perma- 

 nent digestive cavity which is surrounded by a specialized 

 epithelium that contains glands and which is able to resorb 

 food. It had originally one permanent orifice, and finally two 

 that are situated in the two opposite parts of the body. Nothing 

 is changed in this connection by the fact that the ontogenetic 

 type of formation had afterwards been repeatedly changed, 

 similarly as in the case of the development of the Anlage of the 

 middle body layer. The Metaphyta, however, have never been 

 able to develop anything that could resemble an intestinal 

 cavity. No new form has ever been evolved from the hollow 

 sphere of the Volvox. In those cases where some plant species 

 had begun to catch an organic prey (animals) and to digest the 

 same we find this digestion to be purely external, limited to the 

 surface of the plant. 



What is then the situation with Spongiae ? There can be no 

 doubt that the Spongiae are genuine animals, yet at the same 



