PREVIOUS INTERPRETATIONS OF CNIDARIA 101 



old interpretation— of a planula -like primitive form of Cnidaria 

 and of the primitiveness of the Hydrozoa. Yet at the same time 

 Hyman does not draw the necessary consequences. First she 

 brings for\^'ard some older attempts which had tried to explain 

 this bilateral symmetry. When she first mentions this phenome- 

 non (Hyman I. p. 539) she makes the correct statement that the 

 internal bilateral symmetry of Anthozoa must be a primary 

 property and that it could not have been later developed out 

 of a radial symmetry, a fact which appears especially clearly 

 in their embryology. However, Hyman explains at the end of 

 her discussion that this bilateral symmetry is a secondary phe- 

 nomenon. 



On page 640 Hyman discusses this problem in more 

 detail. She mentions as the first possible interpretation the 

 opinion that the bilateral symmetry could have been developed 

 secondarily as a consequence of the "colonial life." Hyman is 

 right when she rejects this interpretation because the same sym- 

 metry occurs also in Actiniaria and Ceriantharia which are pri- 

 marily solitary animals. To this we could add that in the 

 "colonial" Hydroidea and Siphonophora cases are not rare 

 where the externally developed bilateral symmetry is clearly a 

 secondary phenomenon. The second interpretation mentioned 

 by Hyman and which is, in my opinion, the right one, proposes 

 that the present Anthozoa must go back to ancestors that had 

 crept over the sea bottom, because bilateral symmetry appears 

 in such animals. It can be easily understood that Hyman cannot 

 consider this interpretation to be a good one because it stands 

 in opposition to her concept of the origin of Cnidaria. A third 

 interpretation which has been especially supported by some 

 palaeontologists is based on the idea that the old Anthozoa 

 which had otherwise "stood" erect, had lain down on one side, 

 as must be supposed for some Tetracoralla, and it was 

 in this way that the bilateral symmetry has been developed. 

 Jagersten (1955:331), however, believes that this does not 

 explain the origin of the bilateral symmetry because, "the ec- 

 centric attachment was simply a part of the bilateral symmetry." 

 8 



