PREVIOUS INTERPRETATIONS OF CNIDARIA 



91 



name (hypogenetic development according to Haeckel)— we 

 find that the circumstances (especially the closer relationship) 

 indicate that this direct development is a secondary phenome- 

 non only. Here it should be mentioned that the planktonic 

 way of life must be considered as secondary in comparison 

 to the benthonic way of life. On the basis of this view we can 

 explain that the polyp which became secondarily free, has 

 been developed into the medusoid form as a secondary form 



Fig. 16. Three examples of reduced free swimming hydromedusae. 



A, Pennaria sp. (without tentacles, after Hyman). B, Eugymnanthea 



/«^///7/«^Palombi (without tentacles and manubrium, after Palombi). 



C, Millepora sp. (after Delage & Herouard). 



whose organization is much higher because it is connected 

 with the free movement. It must be admitted, however, that 

 it is possible to imagine a retrogressive development by way 

 of which the form of medusa evolves into something that 

 resembles a polyp form. Such processes have actually taken 

 place both among Scyphozoa (Lucernariidae) as well as among 

 Hydrozoa (very numerous instances). Yet in all appearance no 

 genuine polyp has ever been developed in this way, but only 

 some polypoidal forms. In the Lucernariidae this has 

 led to a polypo-medusoid form which can be easily recognized 

 as such; in Hydrozoa we can find medusae which are in this 

 way strongly degenerated (resimplified) inasmuch as they have 

 remained freely swimming animals, (see Fig. 16) or they have 

 been degraded into the organs of cormi by becoming attached 



