PREVIOUS INTERPRETATIONS OF CNIDARIA 211 



The conditions that can be observed in the ontogenies of 

 Cnidaria had certainly not been developed from such trends, 

 but rather from others that were more primitive, more plastic, 

 more changeable, and less determined. It appears, in opposi- 

 tion to the previously widely accepted interpretation, that it 

 is the youngest stages in the ontogeny i.e. cleavage that are 

 changed first, and it is therefore necessary to be very careful 

 when attempts are made on this basis to derive conclusions 

 regarding phylogeny. 



We must mention here in passing the extremely interesting 

 example of a turbellarian (Prorhynchiis stagnalis M. Sch.) w^here 

 two rather different ontogenetic processes could be observed 

 in one and the same species (O. Steinbock and Sister Bernar- 

 dina Ausserhofer, 1950). Hyman (1957, 11:171) is right when 

 she emphasizes, "Practically nothing is known of the develop- 

 ment of these Rhabdocoela and Alloeocoela that have endole- 

 cithal eggs." To this it should be added that much depends 

 even in endolecithal eggs, on the quantity and on the type of 

 the distribution of the egg yolk. 



As a matter of fact, no larva clearly of a planula type can be 

 found among the Turbellaria. It can, however, be said that it 

 had been prepared in the Turbellaria as a transient stereogastrula 

 which was later developed in the benthonic and sessile Cnidaria 

 into a typical pelagic larva. Turbellaria as a w^hole show little 

 inclination to develop genuine larvae, and this is particularly 

 true for those species that have begun to live in the fresh 

 water or even on the land. It is only in the Polycladida that 

 planktonic larvae (Goette-Miiller's larva) have been developed 

 as a parallel (and nothing more !) to an actinula. 



If we disregard the special characteristic of Acoela (that 

 they have lost the second meridian cleavage which has led 

 in micromeres to the formation of a duet instead of a quartet) 

 we see that conditions that can be found in Cnidaria, and 

 especially in Anthozoa, show in reality close resemblance to 

 those that occur in Acoela. In these a stereogastrula which is 

 very similar to a planula is usually formed by way of epiboly. 



