PREVIOUS INTERPRETATIONS OF CNIDARIA 97 



amiss was seen in the fact that in Anthozoa there is one polyp 

 form only and no medusa form. Even among Hydrozoa as 

 well as among Scyphozoa there are species that possess no 

 medusa form, and it was possible that the Anthozoa, which 

 have no medusa form either, had completely lost this form 

 and generation. 



An important mistake was made in the evaluation of the 

 conditions that prevail in the Cnidaria. The generally valid rule 

 that progressive evolution takes place in connection with the 

 normal way of life, with free movement, and the type of 

 feeding, had been forgotten or at least neglected. There is 

 always a more or less distinct retrogression in the organization 

 — and thus a regressive development— when the free move- 

 ment is abandoned and the sessile or the sedentary way of 

 life is adopted. This, however, does not exclude the possibility 

 that in spite of a general retrogression of the organization 

 a progressive development could be observed in some details, 

 i.e. in the new developed strictly adaptive characteristics. 

 Thus this rule whose general validity has already been proved in 

 connection with the study of Protozoa (compare Euciliata with 

 Suctoria !) would not be valid in connection with Cnidaria ! 



The regressive, or, as it has also been less fortunately 

 called, degenerative development of Cnidaria refers to their 

 basic form, i.e. to the form of polyp. Yet the secondary form 

 of medusa supports, if it is interpreted correctly, the idea 

 of a retrogressive evolution in Cnidaria because the emergence 

 of the medusa form (which has actually twice taken place, 

 first in Scyphozoa and later in Hydrozoa) agrees with this 

 rule since the release of a polyp represents the beginning of 

 a new progressive development. The medusa as a freely 

 living form which swims actively in plankton has reached 

 an unquestionably higher level in its organization. The same 

 medusae, however— and this is true both for Scyphomedusae 

 as well as for Hydromedusae— can retrogress so that they 

 cannot even be discerned any longer. They even become 

 "degenerated" (in this case the usage of this word is approp- 



