374 THE EVOLUTION OF THE META20A 



ontomechanism as this has been suitably named by O. 

 Steinbock, or of the "morphogenetic technique" as I called 

 the same process even earlier. 



Viewed from our standpoint it appears as self-evident that 

 the principle of a simultaneous emergence of the third, the 

 middle body layer, or of the so-called mesohyl, with the second 

 body layer has the same validity. All the Eumetazoa are 

 without any exception three-layered animals if there is any 

 sense at all of speaking about three layers in the adult animals, 

 because there are frequently more than three layers present 

 even as early as in the ontogenies. The Eumetazoa inherited 

 all these three layers (zones, parts) from their prociliate ances- 

 tors, and these three layers had therefore not been newly 

 developed by the Metazoa. The pair Diploblastica : Triplo- 

 blastica is therefore completely superfluous. All the three body 

 layers evolved continuously and parallelly without an internal 

 struggle of the originally independent elements against the 

 secondary centralization combined with the subordination, as 

 had necessarily been the case with the evolution of the colonies 

 of the Protozoa. 



In connection with a general classification of the animal 

 world and with taxonomy it is important that we call attention 

 here to the subsequent evolution of these three layers. This is 

 necessary because one can frequently hear the objection that 

 too great an importance is usually attributed to the conditions 

 of the middle body layer, i.e. of the mesohyl and of the coelom. 

 As a matter of fact, however, we find that during the ontoge- 

 netic and phylogenetic developments the morphogeny is much 

 more intensive in the middle layer than in the two remaining 

 body layers. In order to be as brief as possible I wdsh only to 

 mention that it is above all the duty of the external layer to 

 "serve" as a means of protection (it is used as an organ of 

 movement bv means of cilia in the more primitive and small 

 forms only; this property had been inherited from the in- 

 fusorian ancestors); it also produces gland cells and the cuti- 

 cular formations. And what is produced by the middle layer? 



