DIGITAL AND OPHTHALMIC TENTACLES. 767 



wall of the siphon of Gonatus might be thought to militate against this view. Still 

 the anterior cornua of the cartilage of Nautilus penetrate into the ventral wall of the 

 funnel. 



9. Digital and Ophthalmic Tentacles. 



The head or cephalopodium of Nautilus comprises the better half of the animal, and 

 represents a complication of organs and appendages performing divers functions. The 

 body-wall in this region is composed in the main of a thick coriaceous integument, which 

 presents a dorsal convexity formed by the mottled brown-coloured hood, and three deep 

 concavities, anterior, posterior and ventral. The anterior deepest concavity is the fossa 

 buccalis, which is surrounded by the tentacular crown and lodges the great buccal cone ; 

 the posterior concavity is the fossa nuchalis which receives the anfractus of the shell, 

 and gives origin to a muscular flap, the semilunar nuchal membrane ; finally the ventral 

 concavity is the fossa infundibular is, which varies somewhat in depth according to the 

 state of muscular contraction of the parts concerned in its formation, and is exactly 

 suited to receive the funnel. 



The cephalopodium of Nautilus consists of a tentacular crown disposed about the 

 mouth in the form of an oral sheath, and composed of two whorls, outer and inner. 

 The outer circlet 1 of tentacles comprises the " digital processes " of Owen, while the inner 

 circlet consists of the so-called " labial processes," but the latter is an unfortunate term, 

 since the actual buccal orifice is provided with a prominent lip surrounding the mandibles 

 and richly furnished with labial papillae. I therefore propose to refer to the tentacles 

 generally, whether belonging to the outer or to the inner whorl, as digital tentacles. The 

 two tentacles, the one in front of, and the other behind the eye on each side, which 

 Owen named " ophthalmic tentacles," really belong to the outer whorl, but have become 

 so modified in function and position as to require separate treatment. 



The tentacles arise from the bottom of profound sheaths, within which those of the 

 outer whorl are completely retractile and those of the inner whorl partially so, but in 

 either case it is at once apparent that the tentacles are essentially marginal appendages. 



The digital tentacles have been more particularly described as to their structure 

 and distribution by Owen (1832), Valenciennes (1841), Owen 2 (1843), Lankester and 

 Bourne (1883), Vayssiere (1896), Willey 3 (1897), Griffin 4 (1898). These tentacles have 

 in general a trihedral form tapering slightly towards the tip, and are divided by shallow 

 annular grooves into a large number 5 of articulations, the grooves being deeper on the 



1 The outer whorl is the same as Lankester's "annular lobe" (Article "Mollusea," Encyc. Brit. 9th Edit.). 



2 Owen, B., " On the structure and homology of the cephalic tentacles in the Pearly Nautilus." Ann. 

 Nat. Hist. xn. 1843, pp. 305—311, 2 figg. 



3 Willey, A., " The pre-ocular and post-ocular tentacles and osphradia of Nautilus." Quart. J. Micr. Sc. 



Vol. 40, April 1897, p. 197; also in same volume (Aug. 1897), "The adhesive tentacles of Nautilus ," 



p. 207. 



4 Griffin, L. E., "Notes on the tentacles of Nautilus pompilius." Johns Hopkins Univ. Circ. xvm. 

 Nov. 1898, p. 11 ; reprinted in Ann. Nat. Hist. (7) in. Feb. 1899, p. 170. 



5 Sixty to one hundred, according to Mr Griffin. 



