150 



THE EVOLUTION OF THE METAZOA 



of a former oviduct whose function had been changed. It is 

 therefore probable that we have here the remains of an earlier 

 stage in which evolutionary progress had reached a much 

 higher level. 



No regular sack gonad, i.e. a gonad with a central cavity 

 or with, a gonocoele and with a gonoduct, can be found 

 among the Cnidaria. It is in accordance with the progressive 



Fig. 26. A benthonic hydro medusa {Elentheria dichotoma), from 

 Hincks. 



retrogression of the middle layer (mesohyl) that the gonad 

 first loses its sharply circumscribed massive character. 

 It develops more and more into a simple agglomeration 

 of gametocytes. Those gonads which in the Turbellaria 

 had been situated in the depth of the parenchyma are trans- 

 placed in the Anthozoa into the sarcosepta which is more 

 usefull for the feeding of gametocytes. In this w^ay the 

 gonads have completely lost their contact with the skin layer 

 and they have entered into a closer union with the intestinal 

 epithelium. This, however, should not be understood as a 

 transition to an endocarpic state, as was formerly interpreted 



