132 THE EVOLUTION OF THE METAZOA 



If we disregard a clearly secondary retrogression of the 

 medusoid form that can be observed in Hydrozoa, and if we 

 disregard the case of Lucernariidae among Scyphozoa, we can 

 see all the remaining developments to have been clearly pro- 

 gressive, a fact which can be easily understood because it is 

 connected with a secondary adaptation of free life, in this case 

 in the free water zone and with a simultaneous preservation 

 of the essential characteristics of the older polyp form. On the 

 basis of this standpoint we are also enabled to understand the 

 conditions that can be found in the nervous system of medusae. 

 The progressive development of the whole nervous system 

 went parallel to the progressive development of the muscular 

 system and of the marginal sensory organs. In this connection 

 two regularities can be observed. This progressive development 

 did not lead back to the old conditions that had existed 

 in their freely moving ancestors (Turbellaria). The radially 

 symmetrical form has now been preserved by medusae which 

 developed the ability to swim freely and to creep over the sea 

 bottom. When they swim, they orientate their aboral end for- 

 wards. In this way the development did not lead to a cephali- 

 ^ation; the new anterior end remained empty, i.e. it did not 

 develop sensory organs. A new centre of sensory and nervous 

 functions has been developed around the margin of the swim- 

 ming bell, and in this way the nerve rings have been evolved. 

 The fact that there are two such nerve rings can be understood 

 if we take into consideration that there are two nerve nets 

 which are situated one above the other in the original polyp 

 form. The formation of nerve rings is simply due to an 

 intensive local thickening or condensation of the two nerve 

 nets. 



The second regularity (in the past people preferred to call 

 them laws), is the fact that the evolution of the polyp into me- 

 dusa has led to essentially identical results though it had taken 

 place twice. Thus both times a double ring of nervous tissue 

 has been developed along the margin of the umbrella, and 

 both times this evolution has led to similar marginal sensory 



