CNIDARIA AS THE ONLY COELENTERATA 39 



form even if it does not have a feeding or a digestive system 

 and even if it lives for a while on the reserve food it had 

 obtained from its maternal organism. The structure, however, 

 of this planula is not quite so simple even if it does not yet 

 possess organs ; it is well differentiated both histologically and 

 cytologically. The tissue that can be observed in its interior 

 does not correspond to an anlage of an intestine, to an 

 entoderm; it is rather an "entomesoderm." 



The planula, however, is a pure larva and by no means 

 corresponds to a primitive form of the ancestors of Cnidaria ; 

 it is impossible to imagine the existence of a freely living 

 Eumetazoa without an active digestive apparatus. As is the 

 case with other animals that live on the floor of the sea, and 

 particularly those that lead a sessile way of life, so it was 

 with Cnidaria when they developed their specific larva which 

 lived in the sea plankton and which was characteristic of 

 them. We can speak here of a metamorphosis in Cnidaria 

 even if it is not a very radical one. In numerous Cnidaria, 

 with the exception of the Scyphozoa, the next higher ontoge- 

 netic stage, which is called the actinula, develops from the 

 planula during its free life in the plankton. This can be 

 explained as a case of paedogenesis and here we can see a 

 formerly older phase, which had developed on the floor of 

 the sea, transferred into the larval period. It is my belief 

 that it would be possible to find early stages in the onto- 

 genies of animal groups that stand higher in the animal 

 system, where the same stages would resemble that of the 

 planula in Cnidaria. It would only be necessary to make the 

 effort, though this seems hardly worth while. It is perfectly 

 clear that it would be wrong to make any conclusions about 

 relationships from this. Neither would it be sensible to 

 speak here about some modified planulae. 



The same is true for the trochophore, the next higher stage 

 in the ontogenetic development. It is by no means difficult 

 to show that all those stages that have been called trochop- 

 phores, are neither identical nor homologous. A modified 

 4* 



