CTENOPHORA. A^ 



P'iiially Delage & Herouard (Traite de Zoologie concrete. II. 2. Les Coelenteres. 1901. 

 p. 760—765) regard the problem as "une question phylogenetique sans doute insoluble". "Les resem- 

 blances de Ctenoplana et de Coelop/aiia avec les Turbellaries semblent surtout adaptatives et sont 

 probablement secondaires". The}- are Ctenophorans adapted to a creeping mode of life, which have 

 acquired structural characters in accordance with this mode of life, characters which approach them 

 to the Planarians "uniquement parce que celles-ci sont aussi des animaux rampants . . . On .s'accorde a 

 dire que ces caracteres adaptatifs n'ont pas de valeur phylogenetique, en sorte qu'il ne reste gufere 

 en faveur des affinites planariennes des Ctenaires que les caracteres embryogeniques reconnus par 

 Selenka". (p. 761). 



The once so victorious theory of Selenka and Lang is thus far from being generally 

 accepted by the more recent authors. While everybody agrees that there are undoubted affinities 

 between Polyclads and Ctenophores, the direct phylogenetic relation between the two groups is not 

 accepted. As Abbott states (Op. cit. p. 62) "it is not probable that any morphologist accepts Lang's 

 hypothesis nowadays". . . . . :.. ■ 



We may now discuss the objections raised against the theor>' and see whether the researches 

 on Tjalficlla may not perhaps throw some light on the question. 



Let us regard firstly the assertion of Willey that Ctenoplana and Coeloplana are very primitive 

 forms, instead of very specialized as otherwise generally assumed, this being of fundamental impor- 

 tance for the whole question about the relations between Ctenophores and Polyclads. — The reason 

 adduced by Willey for this assertion is, indeed, not very convincing. It is taken as granted that 

 the "natural order of events" is, that the pelagic (and abyssal) fauna has originated from the littoral 

 fauna. "Ctenoplana approaches more nearly to a condition of bilateral symmetry than the Ctenophores 

 do, in that it possesses very clearl}- differentiated dorsal and ventral surfaces. And this is exactly 

 what we should expect to find in the littoral or sublittoral ancestors of such purely pelagic forms as 

 the Ctenophora, the pelagic habit, as is well known, often tending to produce a more or less radial 

 symmetry". This is, in fact, all that is said in favour of the primitive character of Ctenoplana. No 

 support of this assertion is sought for in its anatomical characters. The evident difficulty lying in 

 the existence of apparently reduced costte in Ctenoplana is met with the assertion that "the ctenophoral 

 plates must have put in their appearance for the first time in some form or other", and there is no reason 

 to regard them as reduced structures merely because they are smaller than in other Ctenophores. 



Against this rather too light reasoning Abbott's assertion that Coeloplana (and Ctenoplana) 

 is a very specialized form, resting on real, anatomical, facts (the presence of the otolith and of rosettes 

 comp. above p. 44) weighs very heavily. Also several other facts from the anatomy might be pointed 

 out against Willey; thus, e.g., it seems not very easily understood how the structure of the genital 

 organs in Ctenophores could have developed from those of Ctenoplana, as they are described by 

 Willey. The definitive answer to the question of the primitive or specialized character of Ctenoplana 

 and Coeloplana must be given by the development, which, unfortunately, still remains unknown. 

 However, we may safely draw some conclusions as to this point from the development of Tjalfiella. 

 It has been shown rather conclusively, I think, that Tjalfiella and Ctenoplana are nearly related. 

 The suggestion seems then quite justified that the young of Ctenoplana will likewise prove to be a 



