148 THE EVOLUTION OF THE METAZOA 



secondarily into pelagic animals. Finally we must mention 

 here the MoUusca. In the primitive Mollusca an advanced 

 retrogression can be observed. In these, only the canals can 

 be observed but it is not clear if we are concerned with genuine 

 gonoducts or with coelomoducts, or with metanephridia ; 

 it is natural that under such conditions no pairing can take 

 place. Subsequently, however, a secondary complication of 

 the genital system appears in Mollusca, especially in the 

 dioecious species of gastropods. 



With regard to their genital system, there is in Cnidaria on 

 one hand a progressive simplification, and, on the other, an 

 increased complexity. The simplification does not only affect 

 the auxiliary organs but also the gonads themselves ; the 

 complications can be observed to take place in the bearers 

 of gonads and in their surroundings in the cormus, w^hich 

 is clearly a consequence of the sedentary way of life. 



With regard to the distribution of the two sexes there is 

 a clear trend towards the dioecious state. The hermaphroditic 

 state had been inherited at least partly by Anthozoa from 

 their turbellarian ancestors. It is not rare for this state to 

 become hidden and develop into a protandry ; the protandrous 

 individuals are therefore usually considered as monoecious. 

 It should be mentioned here that an inclination to protandry 

 appears as early in Turbellaria. On the other hand, we can 

 observe, according to Bresslau (1933:115), that in such Tur- 

 bellaria, with a lower status of general organization as in the 

 Acoela, l]ofstenia among the Alloeocoela and Polyclads also, 

 as the genital apparatus shows a lower stage of organization. 

 This trend in their evolution was inherited by the Cnidaria 

 from their turbellarian ancestors. 



The hermaphroditic Anthozoa occur above all among the 

 primarily solitary, and therefore, according to our interpreta- 

 tion, primitive species (Ceriantharia, Actiniaria) with polymeriz- 

 ed gonads. In the Anthozoa, just as in Ctenophora, we can 

 find the gonads of both sexes in one and the same sarcoseptum 

 (this name is better than that of mesentery) ; each gonad is 



