134 THE EVOLUTION OF THE METAZOA 



condensation of nerve cells in the same spots of the nerve ring 

 which has led to the formation of (imperfect though they are) 

 ganglia (rhopalium ganglia). Something similar took place, it 

 seems to me, in the Mollusca in the case of an assymetrically 

 distorted bilateral symmetry. Data is available (Horridge, 1956) 

 which shows a nerve net to have been developed in the exum- 

 brellar subepithelium of the Scyphomedusae (nothing similar is 

 knoM n to me in the Hydromedusae). It has already been poin- 

 ted out that no sensory organs, and, in connection with this, 

 no nerve cell aggregates have been evolved in the pole of the 

 exumbrella, in spite of expectations that these should be deve- 

 loped in the anterior part of the body which comes into most 

 contact with the surrounding milieu when the animal swims. 

 It is therefore impossible that the so-called ''Hydroctena salenskii" 

 (Dawydoff) could be a Hydromedusa (Hadzi, 1959). In the Hyd- 

 romedusae, a combination of two different sensory organs 

 can sometimes be observed at the margin of their umbrellas 

 (e.g. Tiaropsis), a fact which has been used for an attempted 

 division of Hydromedusae into Ocellatae and Vesiculatae. It 

 should be also noted that in the subumbrella of Hydromedusae 

 an agglomeration of the nerve tissue can also take place along 

 their radial canals. It agrees with our interpretation of the 

 origin of the velum that no nerve net has been developed in 

 it; yet in spite of this the velum is able to perform rhythmic 

 contractions. 



The sense organs of medusae, if compared with those of 

 polyps, are, as this has already been stated, new formations. 

 Yet if we try to pursue the development further back we see 

 that those sense organs that occur in medusae had also been 

 essentially developed in their free living ancestors, in the 

 Turbellaria (tango-, stato- [or tono-], chemo-, and light-re- 

 ceptors). The main difference is that in the Turbellaria that 

 creep actively over firm surfaces, these sense organs show a 

 bilaterally symmetric distribution (distribution in pairs) in the 

 anterior end of their body which represents a primitive head, 

 while in medusae they are distributed in groups and, in agree- 



