ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA l6S 



Pteropoda,^ and yet the resemblance of the inter-alar appendages in 

 these two genera '^ to the foot of Gasteropods is so striking as at once 

 to point to their real nature. 



In truth, the foot, though very small in these genera, is exceedingly 

 well-marked, and shows a clear division into mesopodium and meta- 

 podium. It may be a matter of doubt whether the propodium is 

 developed or not, and this question can be settled by embryology 

 alone ; but for the present, I think, it may be fairly presumed that it 

 is represented by the tentaculigerous hood of Pneumodennon and by 

 the tentaculigerous lobes of Clio ; following in this case a very 

 common tendency (exemplified in all the Cephalopods and in many 

 Gasteropods) to become developed over and in front of the 

 mouth.^ 



As Cuvier demonstrated, there is no " mantle " in Pneumodennon 

 and Clio ; * the body of these mollusks answering precisely to that of a 

 Firoloides. The relation of the gill-laminae and of the small anomalous 

 shell in Pneumodennon sufficiently corroborates Cuvier's view. Gills 

 are never placed upon the outer surface of a mantle ; and if anything 

 answers to such an organ it must be the small space covered by the 

 rudimentary shell, so that the relations of the parts are, in fact, similar 

 in Pneumodennon to what we find in Firola and Carinaroides. 



Finally, new parts, the " als," make their appearance in these 

 genera and give its character to the order (figs, i, 2, 3, 4, 7, ep). 



Considering the position and relation of these organs as distinct 

 developments from the upper part of the sides of the foot, and the fact 

 that their nerves arise, like those of the foot, from the pedal ganglia, I 

 propose to consider them as parts of the foot and to call them the 

 " epipodia." It has been long since shown by Van Beneden and 

 others that they have nothing to do with the respiratory function. 



In this subtype the intestine opens on the right ventral side of the 

 neck, and dissection shows its first bend to be ventral, that is, towards 

 the side of the pedal ganglia. 



^ Mem. sur le Clio, p. 9 ; sur I'Hyale et le Pneumoderme, p. 10. 



2 This is fully recognized by Leuckart, " Ueber die Morphologie," &c. p. 149. 



3 The origin of the nerves of the acetabuliferous tentacula may probibly throw some light 

 iipon this matter ; I have not had the opportunity of dissecting sufficiently large specimens of 

 either Pneumodermon or Clio, carefully, with regard to this especial point. From the figures 

 of Eydoux and Souleyet, one would be led to believe that the nerves of the acetabuliferous 

 tentacles arise from the cephalic ganglia, which would be a very great objection to the view 

 advocated above, since all the other parts of the foot, the mesopodium, metapodium, and 

 epipodium are supplied by the pedal ganglia. (See Eydou.x and Souleyet, plate 15, fig. 30, 

 and plate 15 bis, fig. 8.) 



* Eydoux and Souleyet, however, call it " manteau " in the description of their figures. 

 Leuckart also opposes Cuvier's view, but I think without reason. {Op. cit. p. 146, note.) 



