1 8 THE CTENOPHORA 



A tendency towards doi'so-ventral compression is not unknown in 

 typical Ctenophora, for in Deiopca (Fig. YIII.) the main axis is 

 considerably shortened and the sagittal axis lengthened by the 

 development of the lobes. Ctenoplana is an undoubted Ctenophore 

 modified as a result of the assumjjtion of creeping habits. It 

 still retains the power of swimming, and has not lost the typical 

 Ctenophoran costae. Coeloplana is still more modified and has lost 

 the costae. The features in which Ctenoplana differs most from 

 Ctenophora are : the absence of meridional sub-costal canals, and 

 as a consequence the development of gonads in a more proximal 

 part of the gastrovascular system ; the presence of genital ducts 

 and the presence of a peripheral canal system, which, however, is 

 paralleled in the Beroidae and Lobatae. AVhilst there can be no 



I 



Fig. VII. 



Coeloplana Metgi^nikoirii (.slighUy .-iltered from Kowalev.sliv). o, mouth; i1, cavity of the 

 dige.stive canal ; i, islets of tissue ;" c, circular canal ; d', one of the four diverticula of the 

 digestive canal ; ss, c.ecal ofl'sets of the digestive canal, tcrminatint; in crescentic enlargements 

 about the otolith sac ; at, vesicle with a group of otoliths ; ts, tentacle sheaths ; to, nuiscular 

 fibre of tentacles. 



doubt that Ctenoplana is a Ctenophore, and not very distantly 

 related to the other members of the grouj), it is a question whether 

 it is a primitive or a much specialised form. Willey (22) is 

 decidedly of the opinion that it is primitive. He sees in it the 

 representative of the littoral ancestor from which both the pelagic 

 Ctenophora and the Platyhelminthes have been derived. In point 

 of fact we have no evidence as to whether Ctenoplana or Coeloplana 

 are primitive or derived forms ; such evidence can only be furnished 

 by their development and larval history, which are unknown. If 

 Ctenoplana should prove to have a cydippiform larva like the 

 Cestidae and Lobatae, (hen there can be no doubt that it is a 

 derived form ; if it should prove to have a direct develojmient 

 without a metamorphosis, then the probability will be that it is a 

 primitive form. In the present state of our knowledge it cannot 

 be said that the existence of Ctenoplana and Coeloplana gives any 



