50 THE EVOLUTION OF THE METAZOA 



It is conspicuous that no new animal types have developed 

 in the plankton of the free water zone of the sea. All the major 

 types of animals have developed on the sea bottom. Neither 

 have any of the main types (phyla) developed by way of neoteny . 



For methodological reasons and because of its special con- 

 nection with Cnidaria we must deal here with one more 

 problem. We must differentiate between the two types of re- 

 lationship that exist between the stage that lives on the sea 

 bottom and the stage that lives in the plankton. On one hand 

 we have the transition which takes place within the frame of 

 one and the same generation. First wx have to imagine a case 

 in which the phase of the grown-up form which lives on the 

 sea bottom is primary, and the planktonic phase of a juvenile, 

 usually larval, stage secondary. In another case the form which 

 lives on the sea bottom is primary; naturally, it is able to pro- 

 pagate sexually and it can eventually develop its own juvenile 

 planktonic form; and there is a secondary generation which had 

 developed by way of gemmation or fission— in an asexual way — 

 before the sexual propagation has been adopted by the phase 

 which Uves on the sea bottom. The generation which had been 

 created by way of an asexual reproduction evolves into plank- 

 ton and adopts sexual reproduction while it becomes ad- 

 justed to life in the free water. This is the classical case 

 of the alternation of generations, or metagenesis^ where an ori- 

 ginally single phase which was for some time also the main 

 phase becomes, so to speak, degraded and it continues to live 

 a vegetative Life only. 



The subsequent development can pursue two or three diffe- 

 rent ways. Either we find the originally primary generation 

 withering away (this is the generation w^hich lives on the sea 

 bottom, in our case the generation of polyps), so that the 

 secondary, planktonic, form remains as the only one that 

 has survived (in our case the form of medusae) ; or, vice 

 versa we find that the secondary planktonic form and gene- 

 ration, i.e. that of medusae, is withering away, while the pri- 

 mary, benthonic generation, i.e. the generation of polyps. 



