PREVIOUS INTERPRETATIONS OF CNIDARIA 75 



the third stratum, the mesoderm, had been first laid out as a 

 thin leaf which has later lost its leaf-like form, it has never- 

 theless remained a mesoderm, or something that has been 

 added later, as a final layer. The knowledge which had been 

 first arrived at in connection with Vertabrata was soon ex- 

 tended to invertebrates. And here a great mistake was made: 

 this was in the case of those lower Metazoa where no leaf-like 

 mesoderm could be found in their ontogenies and where for 

 this reason the presence of a third body layer has simply been 

 denied. In this w^ay the notion of Diploblastica was created, 

 i.e. Metazoa with two germ layers, in contrast to the higher 

 developed Triploblastica. Later I will try to show that there 

 is no real justification to believe in the existence of Diplob- 

 lastica. 



There has been a clarification of concepts during the second 

 period which developed under the influence of the theory of 

 evolution, due to the theory of recapitulation. It was very soon 

 (in 1859, when the notion of Coelenterata was created by 

 Leuckart) that the interpretation of these problems became 

 stabilized; and they have been preserved, with few changes 

 only, down to the present day. It is because of this generally 

 accepted situation that it is so difficult to weaken the erroneous 

 basis of this view and to propose something new in its stead. 

 This stabilized interpretation which sees in Cnidaria, and here- 

 with in all Coelenterata, primarily simple animals that consist 

 of two germ layers has been accepted by all the great authori- 

 ties in the field of zoology, beginning with Ernst Haeckel. 



It is certain that it was Ernst Haeckel who developed in 

 detail the basis of the interpretation of Cnidaria and of their 

 gradation. He accepted Leuckart's interpretation of Coelen- 

 terata because it seemed suitable to the principle of evolution: 

 whatever shows a simple structure is primary, and it is fol- 

 lowed by a progressive evolution. In this way it was not only 

 the sequence: Spongiaria-Cnidaria-Ctenophora which seemed 

 to agree with the "natural evolution," but there was even 

 within the group of Cnidaria the "natural" sequence Hydro- 



