152 



THE EVOLUTION OF THE META20A 



In the Hydrozoa the location of gonads can again be changed 

 with a secondary retrogression of their medusoid generation. 

 In this connection we must mention the unique case of 

 Rleutheria medusae where the gonads have been transferred 

 into the exumbrella with a simultaneous development of a 

 brooding cavity during their transition to the benthonic way 

 of life (Fig. 27). We have, furthermore, to mention here the 

 Aeginidae, a group of Narcomedusae, which no longer possess 



Fig. 27. Half schematic longitudinal section of a benthonic 



hydromedusa (Eleutheria radiata) with brood pouch in its umbrella 



(after Lengerich), 



any distinct gonads and whose sex cells are scattered all over the 

 subumbrella. In Hydra it is a trace of former conditions— 

 when sex cells still had to move and when they were therefore 

 amoeboid — that the young egg-cells still move like amoebae, 

 simultaneously eating food cells. There are some authorities 

 who deny that an active migration of gametocytes really 

 takes place. All these singularities can be found in Hydrozoa; 

 they represent, in our interpretation, the final level that 

 retrogression of the sexual apparatus has reached in Cnidaria, 

 and not a primarily primitive condition. 



Parallel to this retrogressive development of the sexual 

 apparatus that can be observed in Cnidaria and in clear causal 



