10 THE EVOLUTION OF THE METAZOA 



hand one can observe a development towards a secondary oligo- 

 merization of individual pairs of antimeres or of individual 

 antimeres so that finally we see either a single pair preserved, 

 or a mr.rginal tentacle only, or even none whatever. In this 

 way a purely external similarity with Ctenophora can natunJly 

 develop. One such case was formerly described by Ernst 

 Haeckel who called an Anthomeduea with one pair only of mar- 

 ginal tentacles, C/^;/^n^ ctenophora (see Fig. 2); by this he washed 

 to express the palpable similarity that exists with a typical 

 Ctenophora and in this way point to the possibility that Cteno- 

 phora had perhaps developed from Hydromedusae. For some 

 time this comparison had a very great influence upon zoo- 

 logists; a more precise comparison, however, has finally shown 

 that these are inessential external similarities only. Ctenaria 

 remains a typical Anthomedusa and it does not stand in the 

 slightest degree closer to Ctenophora than any other Hydro- 

 medusa. The symmetry of the body is not a property which 

 can be considered unchangeable. On the contrary, it shows 

 comparatively rapid changes, e.g. when the sessile way of 

 life has been adopted, or, vice versa, during the transition from 

 a sessile way of life to that of a free movement. Many instances 

 could be mentioned in this connection yet it should suffice to 

 point to the case of echinoderms only. We shall later return to 

 this question to discuss it in greater details. 



It is in no way difficult to show how the bisymmetry has 

 been developed in Ctenophora. It is a result of the transition 

 from a benthonic to a pelagic way of life; during this transition 

 the difference between the ventral and dorsal sides has dis- 

 appeared. Thus we have here a combination of bilateral and 

 radial symmetries. The internal organs remained true to the 

 original bilateral symmetry while those situated on the peri- 

 phery leaned towards the radial symmetry. 



The second main difference between Ctenophora and Cni- 

 daria is in their tentacles. It is well known that both a majority 

 of Cnidaria as well as Ctenophora possess tentacles which are 

 strongly contractile and serve mainly for the capture of the 



