322 



LECTURE XXIII. 



The cavity in the cephalic cartilage, apparently that described in 

 my Memoir * as containing a sinus of the cephalic veins, but which 

 M. Valenciennes regards as the organ of hearing, is defective in the 

 structures which the uniform analogy of that organ in the molluscous 

 subkingdom leads us to conclude are essential to it : there were no 

 traces, for example, of otolithes.f This militates more strongly against 

 the idea of the French anatomist than even the place of origin and 

 anomalous number of the minute nervous filaments which he describes 

 as penetrating the cavity in question. 



With respect to the organ of smell, most physiologists will, I 

 think, admit that the structure and position of the soft close-set 

 membranous lamellae at the lower and inner part of the oral sheath 

 immediately in front of the mouth, manifest the conditions of the 

 olfactory organ more fully and naturally than do those short hollow 

 tentacles which project from the outside of the head beneath the eyes. 

 The complex and well developed tongue of the pearly Nautilus 

 exhibits in the papillae of its anterior lobes and in the soft ridges of 

 its root, the requisite structure for appreciating the quality of taste. 



The papillae upon the exterior surface of the two large confluent 

 digital processes forming the hood and of the two digitations next in 

 size immediately beneath them, form a remarkable character in the 

 Nautilus, on account of their obvious similarity to tactile papillae ; 



but the sense of touch must be 

 specially exercised by the nu- 

 merous cephalic tentacles, which, 

 from their softness of texture, and 

 especially their laminated inner 

 surface {fig- 131. /), are to be 

 regarded as organs of exploration 

 not less than as instruments of 

 prehension. 

 Section of tentacle, Nautilus. The calcareous extremity of the 



upper mandible is sharp-pointed and solid to the extent of five lines. 

 The lower mandible is sheathed with a thinner layer of the hard 

 white substance, which forms a dentated margin. The fossils termed 

 « rhyncholites,' are the analogues of these calcareous extremities of 

 the beak in cognate extinct cephalopods. The muscular subspherical 

 mass, which supports and moves the mandibles, is provided with four 

 retractors, and can be protruded by a strong semicircular muscle, 

 which is continued from the margin of one of the inferior labial 



i3i 



* On the Pearly Nautilus, 4to, 1832, p. 16. 



f '« Elle etait remplie d'une pulpe homogene " (coagulated blood?), " et ne con- 

 tenait aucune sorte de concretions." — Loc. cit., p. 291. 



