18 THE EVOLUTION OF THE METAZOA 



proposed different opinions regarding Hydroctena. Some 

 have thought it to be an aberrant type of Narcomedusae, 

 others have considered that it is a ctenophore yet w ithout the 

 swimming apparatus in the form of eight meridional bands of 

 plates which is so characteristic for Ctenophora; the least ac- 

 cepted interpretation has been that it represents a transition 

 form. I have tried to submit all the known factual data (which 

 are very incomplete) to a renewed scrutiny (Hadzi, 1958 b). 

 In this way I have come to the conclusion that in all probability 

 we have here ctenophores which have developed crippled 

 forms because their embryos had been damaged by an external 

 event (storm). In this connection I have referred to obser- 

 vations, especially those made by C. Chun, according to which 

 the damaged embryos of Ctenophora continued to develop 

 into partial forms because of the strong determinism in 

 their ontogeny. Dawydoff maintained that Hjdroct^na had 

 no coUoblasts on its tentacles and that instead cnidae had 

 been found much as can be observed in Narcomedusae. 

 On the basis of this I have proposed that the supposed 

 Hydroctena could have developed from the embryos of Haeck- 

 elia rubra. I have therefore suggested that Hydroctena should be 

 set aside. 



In the meantime C. Dawydoff has published a short notice 

 where, on the basis of serial sections of Hjdroctena made by 

 another zoologist, he comes to the conclusion that we have 

 here a genuine Narcomedusan. The vertical sensory organ, 

 which was the main point in th:s revision, has been ex- 

 plained as a form siii generis that has nothing in common with 

 the aboral sensory organ in Ctenophora, but instead corre- 

 sponds to a pair of club-shaped marginal sensory organs of 

 Narcomedusae: this was first transplaced to the vertex of the 

 umbrella and later sunk into the interior of the same. This 

 interpretation seems to me completely improbable and at the 

 same time incompatible with the original description of the 

 structure of the supposed Hjdroctena as it had been given 

 by the same author (cf. the conditions of tentacles and of the 



