586 



LECTURE XXIII. 



the oral sheath immediately in front of the mouth, appeared to me 

 to manifest the conditions of the olfactory organ more obviously than 

 do those short hollow internally lamellated processes which project 

 from the outside of the head beneath the eyes. If, however, the 

 cavity with tumid margins, opening near the eye of the higher Ce- 

 phalopods, be actually, as KoUiker conjectures*, the olfactory organ, 

 Valenciennes' idea of the sub-ocular hollow processes may be the 

 true one. 



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 the exercise of some degree 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 



216 



specially exercised by the nu- 

 merous cephalic tentacles, which 

 from their softness of texture, and 

 especially their laminated inner 

 surface {fig. 216, e, /), are to be 

 regarded as organs of exploration \ 

 not less than as instruments of 

 prehension. 



The calcareous extremity of the 

 upper mandible is sharp-pointed 

 and solid to the extent of five lines, 

 with a thinner layer of the hard white substance, which forms a den- 

 tated margin. The fossils termed " rhyncholites," are the homologues 

 of these calcareous extremities of the beak in cognate extinct cepha- 

 lopods. The muscular subspherical mass, which supports and moves 

 the mandibles, is provided wdth four retractors, and can be protruded 

 by a strong semicircular muscle, which is continued from the margin 

 of one of the inferior labial processes over the mandibles and their 

 retractor muscles to the labial process of the opposite side. 



The tongue is supported by a horny, slightly curved, and trans- 

 versely striated plate. The fleshy substance of the tongue forms 

 three distinct papillose caruncles anterioi'ly, into which the retractor 

 muscles are inserted. The dorsum of the tongue is incased by a thin 

 layer of horny matter, supporting four longitudinal rows of recurved 

 spines ; behind which the surface is again soft and papillose. Two 

 broad duplicatures of mucous membrane project forwards from the 



* CCCXCI. p. 107. 



Section of tentacle. Nautilus. 



The lower mandible is sheathed 



