44 THE EVOLUTION OF THE META20A 



I will not enter here into the discussion of phylembryogene- 

 tic problems which have been already touched on. However, 

 since we are speaking about larvae, I wish to mention two 

 other extreme cases of such changes because they are important 

 from a methodological point of view. In one case which I have 

 already mentioned, the larval phase is simply omitted ; it has 

 been cancelled. It seems that this takes place comparatively 

 rapidly. 



The best known condition under which the larval phase 

 has become lost is that of the transition from sea to fresh water 

 (e.g. in Hydra, numerous Crustacea, MoUusca). To this should 

 also be added perhaps migration into great depths, and Hfe 

 in the far northern regions with their cold waters (a transition 

 to the nursing of the progeny taking place). Not infrequently, 

 however, traces of a former larval stage can in these cases be 

 observed in the development of the embryo. 



The opposite case is represented on one hand by the pheno- 

 menon of neoteny and on the other by the complete prevalence 

 of the larval phase. The essential difference between these two 

 cases can be seen in the fact that in the former the adult stage 

 is simply omitted, the development of gonads and maturation 

 are transferred to the larval stage. Dissogeny represents a tran- 

 sition to this stage; this is the double sexual maturation which 

 takes place once during the grown-up stage and once in larva 

 (it has been observed in Ctenophora). This change, too, is 

 connected with a change of the environment, usually in the 

 way that the animal remains in the plankton. Something similar 

 to neoteny can be found in animals with metagenesis (the al- 

 ternation of generations) when either one or the other genera- 

 tion is preserved as this can be frequantly observed in Cnidaria; 

 here the sexual function which had passed over from the poly- 

 poid to the newly formed medusoid generation is later trans- 

 ferred back to the polypoid generation; in case, however, of an 

 omission of the polypoid generation, the sexual function remains 

 with the medusoid generation which is then the only genera- 

 tion preserved (the so-called hypogenetic species). In these 



