CNIDARIA AS THE ONLY COELENTERATA 61 



their very numerous species that Uve in various depths of 

 the sea, yet they never have a typical bilaterally symmetric 

 form with a difference between ventral and dorsal sides. The 

 impression we get in the face of these immense riches of forms 

 of Radiolaria is that the life in free water is indifferent to the 

 symmetric conditions of basic forms and that it therefore 

 "allows" all kind of forms. 



In Cnidaria, as animals that primarily live on the sea bottom, 

 the radial symmetry has already been developed because of 

 their sessile way of life; after their transition, as medusae, to 

 a life in plankton, this radial symmetry becomes considerably 

 intensified. A more or less expressed bisymmetry can be occasion- 

 ally developed due to a reduction of their tentacles, and in some 

 cases even a state of asymmetry can be approached if only one 

 marginal tentacle has been preserved. The bilaterally symmetric 

 and bisymmetric medusae can be found as medusoids in 

 the cormi of Siphonophora only; it is only here that a poly- 

 morphism of medusoids can be observed. The individual swim- 

 ming medusae have never developed bilateral symmetry in spite 

 of the fact that they are excellent swimmers. 



The example of Echinodermata shows how long the old 

 symmetric conditions can be preserved during life in 

 plankton. Echinodermata are considered to have evolved out 

 of some bilaterally symmetrical ancestors that had crept over 

 the surface of the sea bottom. Because of a later adoption of 

 a sessile way of life, these Echinodermata have subsequently 

 developed more or less radially symmetric forms. Their larvae, 

 however, are purely bilaterally symmetric animals in spite of 

 the fact that they still continue to go into free water and 

 that they move slowly and only by means of cilia. This shows 

 that here we have a kind of conservatism; similar to the case 

 of starfishes which have adopted a secondary vagile way of 

 life while at the same time they still preserve an obvious radial 

 symmetry because of the slowness of their movements. On 

 the other hand, sea urchins which burrow in the soft sea bot- 

 tom have adopted a secondary bilateral symmetry. 



