CTENOPHORA. 



55 



the eyes of Polyclads is indicated here. (If Heteroplana were really a Ctenophore, related to Ctenoplana, 

 as maintained by Willey, it would afford the direct connection between Polyclads and Ctenophores 

 in this point, having undoubted eyes along the margin. But, as stated above, it can scarcely be 

 doubted that it is a true Polyclad). 



The presence of a general ciliation of the epidermis in Polyclads and its absence in Cteno- 

 phores would in itself not mean a difference of so much importance as to imply a difficulty to the 

 theory. Lang rightly points out that there are, in any case, traces of ciliation in Ctenophores, both 

 in the grown and the embryonic stages. However, it is by no means without importance to notice 

 that Tjalfiella, Ctenoplana and Coeloplana also in this regard appear to represent a transitional con- 

 dition. In Tjalfiella the basal surface, the suboral cavity and the "chimneys" are ciliated. In Cteno- 

 plana and Coeloplana the whole underside is ciliated; according to the first descriptions (by Kowa- 

 levsky and Korotneff) they are ciliated over the whole body, but this is denied by the later 

 researches of Abbott and Willey, who maintain that only the ventral surface is ciliated. 



This leads to a couple of questions of very great morphological importance : to what corresponds 

 the flat underside of Ctenoplana and Coeloplana, and how has the conspicuous flattening of the body 

 in these two forms originated? The most obvious explanation of the transformation of the high body 

 of typical pelagic Ctenophores into the flat body of the creeping Ctenophores would be, that it has 

 been brought about through a simple shortening of the main axis of the body. This is, however, 

 scarcely the right explanation; the facts revealed by the study of the development of Tjalfiella decid- 

 edly point in another direction. In the young Cydippid of Tjalfiella the lower part of the body is 

 divided through the deep transverse furrow into two large lobes, which may be folded out, so that 

 the animal becomes quite flat (PI. Ill, fig. 6); it is doubtless with the inside of these lobes that the 

 animal attaches itself to the Umbellula, the furrow thus becoming converted partly into the basal 

 surface, partly into the "suboral" cavity and the chimneys allowing that there is no distinct limit 

 between the widened pharyngeal cavity and the furrow 1 ). The epithelium of the furrow is ciliated and 

 remains so in the grown animal in the parts derived from the furrow. Ctenoplana completely corre- 

 sponds with the Cydippid of Tjalfiella; the two lobes of the body can be opened or folded up; the 

 inside of the lobes (lower side of the body) being ciliated. The only difference from Tjalfiella is, besides 

 the absence of the "chimneys", that the pharyngeal cavity appears to be more distinctly limited from 

 the furrow (comp. fig. 3, p. 26). The conclusion seems then inevitable that also in Coeloplana 

 the whole ciliated underside corresponds to the transverse furrow of Tjalfiella and 

 Ctenoplana. The definite proof of this can, of course, only be given by the study of the development 

 of Coeloplana; but the facts available do not seem to leave any doubt that this is the explanation 

 of the ciliated underside of Coeloplana, the ciliation being thus in itself not a special adaptation to 

 the creeping habit - The flattening of the body in the creeping Ctenophores thus 

 appears to be due, not to a direct shortening of the whole main axis of the body, but 

 mainly to a splitting up of the lower part of the body. That also some shortening of the 



') In the grown specimens it would seem that the whole of the "suboral" cavity and the chimneys represent the 

 pharyngeal cavity, the flattened basal surface alone corresponding to the transverse furrow of the young (comp. PI. Ill, 

 figs- 5. n)- This interpretation, however, is not directly supported by the facts of the development. But further material may 

 perhaps give the proof of it. 



