52 



CTENOPHORA. 



sagittal canals, which also occur in Ctenoplana, are otherwise quite unknown in Ctenophores, and 

 the way in which Abbott (Op. cit. p. 62) seeks to explain them, does not appear to me very acceptable; 

 probably the study of their formation in the embryo will give the clue to their homology. But seeing 

 that we cannot thus homologize in a more detailed way the gastrovascular canals of Coeloplana with 

 that of other Ctenophores {— and we can also scarcely point out a direct homologue in typical Cteno- 

 phores to the branching canals of Tjalfiella — ), it can certainly not be expected that a more detailed 

 homologizing of the ramifying canals of Polyclads with those of Ctenophores should be possible. 

 A general homology is all that can be expected, and here Tjalfiella and Coeloplana undoubtedly show 

 the way along which the transformation of the peripheral gastrovascular system from the Ctenophoran 

 to the Polyclad type has taken place. It may perhaps be suggested that Ctenoplana in this respect 

 will be more primitive than Tjalfiella. — It is worth recalling here the peculiar respiratory tentacles 

 of Coeloplana, discovered by Abbott (Op. cit. p. 48. Taf. 8, fig. i), processes from the branches of the 

 sagittal gastrovascular canals. They are verj- much like the dorsal cirri of Thysanozoon, which like- 

 wise are in connection with the gastrovascular canals. 



While thus the peripheral canals do not and cannot be expected to afford more than a general 

 homology in the two groups, the case is different with the rest of the gastrovascular system. The 

 stomach ("Hauptdarm") of the Pohclads, of course, corresponds to the infundibulum of the Ctenophores, 

 both giving rise to the peripheral canals and both being of entodermal origin. The important physi- 

 ological accordance (comp. p. 39) may also be recalled here. Probably nobody would raise any object- 

 ion to this homology, so that it is unnecessary to say more about that. 



The "Pharyngeal tasche" of Polyclads is regarded by Lang as homologous to the stomodseum 

 of Ctenophores, both being of ectodermal origin. The "pharynx" of Polyclads, forming a ringwall 

 within the pharyngeal sac, is regarded as homologous to the stomodaeal folds of Ctenophores. In both 

 a narrow opening leads from the stomodseum to the stomach. This seems, indeed, highly plausible. 

 There are, however, some differences to be noticed. The epithelium of the pharyngeal sac of Polyclads 

 is not ciliated, while in the Ctenophores it is ciliated. The stomodteal folds of Ctenophores are paired 

 organs (situated in the transverse plane), in Polyclads they (the "pharynx") originate as a continuous 

 ringwall (a mesodermal thickening). The stomodaeum of Ctenophores is compressed in the sagittal 

 plane, in the Polyclads it is a wide sac, more or les complicated, but not compressed. — In view of 

 the higher specialization of the Polyclads the existence of these special structures is, however, not 

 more than what should be expected — and, furthermore, they are not unparalleled in the Ctenophores. 



In Mertensia ovum, which is generally thought to be one of the most primitive of Ctenophores, 

 the stomodseum is only in the lower part compressed in the sagittal plane; in the upper part it is 

 (on account of the strongly developed stomodseal folds) not thus compressed, the transverse diameter 

 being even larger than the sagittal. In Tjalfiella it has developed into the large "suboral" cavity, 

 which is no way compressed in the sagittal plane, the transverse axis being much the larger. Also 

 in Coeloplana it is stated by Abbott (Op. cit. p. 51) to be "not compres.sed in either plane but approx- 

 imately square in shape". In Ctenoplana it is evidently as in Tjalfiella, as may be concluded rather 

 safely from the figure copied from Korotneff (p. 26). Thus there is no difficulty for the homology 

 in the shape of the stomodseum. 



