PREVIOUS INTERPRETATIONS OF CNIDARIA 135 



ment with their radial symmetry and bell shape, at regular 

 intervals along the margin of their umbrellas. 



If we consider polyps as the primary form of Cnidaria, and 

 if we do not find in them any complex sense organs, we must 

 come to the conclusion that these sense organs had been lost 

 due to their sessile way of life. This indicates that in medusae, 

 which in Scyphozoa and Hydrozoa had been secondarily evolv- 

 ed out of polyps, these sense organs have been newly develop- 

 ed—probably only after they have long been adapted to the 

 free life in water. We must here point to the fact— without 

 any intention to enter into greater details — that there is 

 nothing unusual in the reappearance of complex sense organs 

 in the two subtypes of medusae. These sense organs represent, 

 according to Groselj, "extremely variable values," and they 

 reappear again and again whenever they are required by the 

 circumstances in which an animal lives; they frequently appear 

 in the most unusual parts of the body, e.g. in the tentacle- 

 like processes of the cloak margin of Pecten, or in the minute 

 feathers of the tentacle crown of some Polychaeta. The con- 

 sequences of considerable changes in the \\'ay of life are first, 

 a reduction of those sense organs that had become useless 

 and finally a complete loss of these; this can also be observed 

 in those species that live in underground caves, regardless of 

 the fact that they can belong to different systematic groups. 

 With the same facilitv such sense organs can again and again 

 be reintroduced. The explantation of this may be that the 

 mutations that are necessary for the development of these 

 sense organs return, even in those cases when such organs do 

 not appear to be very useful. If again they are ("something to 

 be desired"), useful because of the change of environment, the 

 corresponding mutations win the protection and guidance of 

 Natural Selection and they can again be further progressively 

 developed. 



It is therefore not a contradiction of facts when we main- 

 tain, on one hand, that Cnidaria have inherited their ability 

 to develop complex sense organs from their turbellarian 

 10^ 



