46 THE EVOLUTION OF THE METAZOA 



highest categories which are characterized by phyla. The whole 

 phylum of Oligomeria with its numerous subtypes (situated 

 between Polymeria and Chordoma) is in my opinion a pro- 

 duct of a generally regressive development. In all probability 

 such a type of development has occured in the Cnidaria, as is 

 also the case of other animal types that have adopted a sessile 

 way of life. 



We speak here of circumstances which usually accompany 

 regressive development, or actually cause it. In a sessile way 

 of Ufe Natural Selection favours the minus mutanta which gradu- 

 ally reduce and eliminate all those organs and parts of the 

 structure that are causally or functionally connected with the 

 free mobiUty of the mobile or swimming animal types. 



Besides the sessile w^ay of Kfe in which the animal becomes 

 immobile, there is also the parasitic way of life which regularly 

 leads to a regressive development, particularly in the case of 

 internal parasites. It is well known that due to parasitism, cha- 

 racteristic form can be reduced so much that it is past recog- 

 nition. It should be mentioned that it is often the larval 

 stage, so far as this is preserved, which enables zoologists in 

 cases of a well advanced regression ("degeneration") to deter- 

 mine the history of a species and thus the membership in a 

 certain animal group. 



All this is well known and nothing new has really been said 

 here. Nevertheless, it has been necessary to discuss this point 

 because it was in connection with Coelenterata, and also with 

 Cnidaria, that many mistakes have been made. The rule— it is 

 better not to speak about laws in biology— according to which 

 a constantly sessile way of Ufe leads to a simplification of the 

 body structure has been believed to be true in connection 

 with other ammal groups; these are by no means few; yet it 

 has not been accepted in connection with Coelenterata, i.e. 

 Cnidaria. Later it will be seen that, due to historical reasons, 

 particularly to our deficient knowledge of the fine structure of 

 Cnidaria, the Cnidaria have been believed to be primarily 

 simple animals which are just on the border between animals 



