102 THE EVOLUTION OF THE METAZOA 



Hyman finally proposes her own interpretation which she 

 naturally thinks to be the best and the one most acceptable. 

 Bilateral symmetry is a result of a secondary development 

 and it is due to the increased size of anthopolyps and, in con- 

 nection with this, to an increased need to send w^ater necessary 

 for respiration into the interior of the body by means of a quick 

 water current which is produced by siphonoglyphs. A normal 

 round oral opening was not suitable for this purpose; it was 

 for this reason that the mouth has become elongated. The bilate- 

 ral symmetry in the arrangement of sarcosepta is a consequence 

 of such a change. Even if we could accept as probable this 

 explanation of the origin of the elongated oral form and of the 

 siphonoglyphs, we must nevertheless reject it in view of the 

 fact that the bisymmetry (or the bilateral symmetry) does not 

 occur only in the arrangement of the sarcosepta but it can also 

 be found in the arrangement of the septal muscles which, are 

 not brought into connection with the water current — a fact 

 which has already been pointed out by Jagersten. 



Thus we can conclude that so far nobody has succeeded in 

 explaining the origin of the internal bilateral symmetry obser- 

 ved in the Anthozoa on the basis of the presumption that it is 

 a secondary phenomenon. 



Recently G. Jagersten (1955) has come to the conviction 

 that in the Anthozoa the bilateral symmetry must be a primary 

 property inherited from their ancestors. Jagersten, how^ever, 

 has come to this conviction along a different way from the 

 one I have taken and, as he maintains (Jagersten, 1955:333), 

 independently of me. In spite of this Jagersten uses the same 

 arguments I have used, and proposes to reverse the evolution- 

 ary sequence which has so far been considered as valid within 

 the system of the group of Cnidaria. 



Above all Jagersten was interested in the origin of Metazoa, 

 and especially of Coelomata. He believed that this problem 

 could be solved best if he modernized the old gastraea theory 

 invented by Haeckel. For the purposes of such a moderniza- 

 tion he constructed a hypothetical intermediate stage which he 



