PREVIOUS INTERPRETATIONS OF CNIDARIA 205 



primitive form of all Metazoa. The moruloid and blastoid 

 forms have simply been presupposed and taken over from 

 the plant world when they could not be found among the 

 animals. It should be mentioned in passing that in my opinion 

 there have never existed any Gastraeadae. I even maintain 

 that the ontogenetic stage called gastrula does not represent 

 any recapitulation of a "gastraeadic" ancestor. 



It is quite surprising that various zoologists could not see 

 that the well-known facts in the field of the ontogeny of the 

 cnidaria do in no way agree with their concept of the origin of 

 Cnidaria from some gastraea-like ancestors, with Hydrozoa as 

 the earliest form. It was within this important sphere that it 

 was necessary to take into consideration the factor of "caenoge- 

 neses" from some unknown origins. One could expect, on 

 the basis of this preconceived scheme, that the ontogeny of 

 Hydrozoa as that of the supposedly most primitive Cnidaria 

 should have the most typical course, i.e. that the impregnated 

 and fertiUzed egg cell should pass, by way of a total and equal 

 segmentation, through the stages of a morula and of a blastula 

 (or coeloblastula) to grow finally by way of an invagination of 

 its vegetative half into a double -layered gastrula. The facts, 

 however, tell a different tale. Such a "typical" recapitulation 

 can be found nowhere in the ontogenies of all Hydrozoa 

 that have been investigated up to this point. Occasionally an 

 invagination-gastrula can be found in Scyphozoa and ia 

 Anthozoa; there is also everywhere an Anlage of the inter- 

 mediate layer, as yet not that of a third "germ layer," as a 

 mesoderm. Here the types of development show an extra- 

 ordinary variety and they depend strongly and obviously on 

 the quantity of the food substance which has here a purely 

 passive role. It can be therefore easily understood that a sharp 

 opposition to the gastraea theory soon appeared after this 

 theory had been formulated by Haeckel; Ilya Metschnik off,, 

 an expert in the ontogenies of Cnidaria, proposed his own 

 theory of a parenchymella, and in this he was followed by 

 several other scholars; Lankester (1877), on the other hand> 



