722 CTENOPLANA. 



e. Ciliated ectoderm on ventral surface. 



f. Male gonads with tunica propria. 



g. Male genital ducts opening to the exterior on the dorsal surface. 



The fact of biradial symmetry means, of course, that there are two planes of 

 symmetry which divide the body respectively into two equal halves. One of these 

 planes passes through the muscular pinnate tentacles, and is called the tentacular plane 

 of Gtenoplana and of the Ctenophora generally. The other plane cuts the tentacular 

 plane at right angles, and corresponds to the so-called stomachal or gastral plane of 

 the Ctenophora. Corresponding with these two planes we have the two axes, namely, 

 the tentacular axis and the gastral axis. 



Armed with a few new facts, one is strangely tempted to enter the lists of 

 biological speculation and take one's chance in the meUe, or in other words, to make 

 some addition to the discussion concerning the relation of the above-named planes and 

 axes of biraclially symmetrical animals to the planes and axes of bilaterally symmetrical 

 animals such as Turbellarians and Annelids. The chief question which fixes the attention 

 of the observer is this: — Is it the tentacular plane or the gastral plane which is 

 homologous with the sagittal plane of a bilateral worm ? or, to put it more brusquely, 

 Is it the tentacular axis or the gastral axis which corresponds with the oro-anal axis 

 of a Nemertine or an Annelid ? 



It might be and I believe it has been contended that there is no ground for 

 comparison between the axes of a Ctenophore and those of a worm, in which case 

 the question propounded above has no point. Freely admitting that this may be true, 

 there is still room to suppose that it may not be true. Assuming therefore that one 

 or other of the biradial planes must represent the sagittal plane of higher forms, I 

 give it as my own conviction that the tentacular plane of Gtenoplana about which the 

 aboral ciliated sensory papillae are disposed in paired groups as shown in Fig. 13, 

 coincides with the sagittal plane of a bilateral Turbellarian, Nemertine, or Annelid. 



Much as the prominent pinnate tentacles of Gtenoplana 1 simulate transversely-paired 

 organs, they are not more paired than are the ciliated sensory tentacles of the same 

 animal. Both these organic systems, however, are paired about different planes and it 

 is to this circumstance that the discussion which I have outlined is due. I simply 

 desire to emphasize what I believe to be theoretically sound, namely, that if the 

 above-named planes are held to be respectively comparable, then the tentacular plane 

 of Gtenoplana and of the Ctenophores 2 represents the sagittal plane and the pinnate 

 tentacles occur in this plane, like the proboscis of the Nemertines. Either there is 

 no correspondence at all or the tentacular and sagittal planes are homologous, but to 

 my mind there can be no question of the gastral axis of Gtenoplana corresponding 

 with the antero-posterior or oro-anal axis of the bilateral forms. 



Whatever the truth may be, Gtenoplana remains a remarkably interesting type and 

 no zoologist could encounter specimens of it without experiencing a momentary thrill 



1 The same applies to tbe Ctenophora in a less obvious sense. For discussion of the axes of Ctenophora 

 the reader should consult Prof. Chun's Monograph, Die Ctenophoren des Golfes von Neapel, 1880. 



2 This is the view, as concerning the Ctenophora, which was first held by Professor Chun, but he seems 

 to have made a subsequent retractation. 



