CNIDARIA AS THE ONLY COELENTERATA 47 



and plants (Zoophyta). The conservatism and the authority of 

 prominent scholars, both basically useful to the science, can 

 become detrimental to it and an obstacle to the progress when 

 they exceed all bounds. This has been the case with Cnidaria. 

 The freshwater Hydra has for a long time been considered to 

 be the most primitive of all Eumetazoa, and consequently also 

 of Cnidaria. Even at the present time, Hjdra is often described 

 as a primitive simply built animal, a description w^hich is made 

 for didactic purposes if for no other reason. Finally, when the 

 idea has been accepted that the solitary character of Hydra is 

 a secondary phenomenon, and that Hydra is without its me- 

 dusoid generation, progress has stopped here and the scholars 

 did not try to draw the necessary consequences. This situation 

 has continued even after it had been proved by palaeozoologists 

 that the Anthozoa actually have an internal bilaterally symmet- 

 ric structure. 



The Significance of the Sessile Way of Life 



In our study of phylogenetic problems we must not be sa- 

 tisfied with an exclusive use of comparative morphology 

 even in those cases where a rich fossil material has been avai- 

 lable. We shall completely understand and comprehend a spe- 

 cies of animals— and this is true also for a systematic group 

 of animals— when w^e also know their way of life, and the en- 

 vironment in which they live. Even if this is an old and gene- 

 rally accepted truth one can never emphasize enough its great, 

 and decisive, importance. In this connection mistakes have 

 again been made, with regard to Cnidaria, which persist down 

 to the present day. It is therefore necessary to discuss this 

 problem at some length. All Cnidaria live in water, and all— 

 with a few exceptions— live in the sea; w^e shall therefore hmit 

 ourselves to the study of conditions that prevail in this lar- 

 gest and oldest of all environments in which the life of or- 

 ganisms, and thus also of animals, has been developed. 



