768 DIGITAL AND OPHTHALMIC TENTACLES. 



inner sides which form the bases of the trihedra, so that a series of transverse ridges 

 is produced, by means of which the tentacles are able to adhere to surfaces in a manner 

 analogous to the action of the feet of a gecko. When attached to a rough surface by 

 means of the suctorial ridges the tentacles may be drawn out to a great length in all 

 directions, and hold on to their support with so much tenacity that sometimes when 

 I forcibly detached the animal some of the tentacles broke off and remained fixed to 

 the substratum. The suctorial ridges of the tentacles have a pale neutral tint in the 

 fresh state, while the crests and sides of the trihedra are distinguished by a deep 

 browTi pigment. The tentacles which thus act in a way resembling the arms of the 

 Dibranchiate Cephalopoda are the digital tentacles of the outer whorl. They have 

 an adhesive, prehensile and tactile function. 



In the middle of each tentacle runs a large axial nerve, which originates directly 

 from the ganglionic nerve-centre and presents, as was first noted by Griffin, a series of 

 slight ganglionic enlargements at its periphery, in correspondence with the annulation 

 of the tentacle. In sagittal section the axial nerve is seen ^vith its peripheral ganglion 

 cells which constitute an almost unbroken layer on the ventral side of the nerve, that 

 is on the side nearest to the suctorial ridges, but on the opposite side are segi-egated 

 into groups. The suctorial ridges appear as bold outstanding nearly flat-topped excrescences 

 slightly convex, with a tendency to imbricate at their edges. They are covered with a high 

 and very dense epidermis which passes abruptly into the indifferent layer which lines 

 the intervening grooves. Below the nerve the tentacle is traversed by an artery, and 

 below that again, a venous channel'. Owen (1843) gi'aphically described the dense central 

 tissue which protects the nerve and artery, fi-om whence radiating fasciculi proceed to 

 the periphery ^vith longitudinal muscular bundles in the meshes. He also pointed out 

 how similar requirements elsewhere are met by analogous structure. Thus " the mechanical 

 arrangement of the contractile fibres is very similar to that of the complex muscles 

 described by Cuvier in the proboscis of the elephant. The attenuation and elongation 

 of this brobdignagian [sic] tentacle must be effected without compressing the central 

 breathing-tubes and the transverse fibres accordingly radiate from the dense ligamentous 

 tissue which suiTounds the tubes." We may add that a closely analogous structure is 

 met with in the proboscis of the Enteropneusta. The arms of the other Cephalopoda 

 are also built upon the same plan. Owen duly noted the suctorial ridges which he 

 described as presenting " the appearance of a number of close-set transverse plates." He 

 says further that " this modification must increase the prehensile and sentient properties 

 of the inner surface of the tentacle, and it is on the corresponding part of the larger 

 and fewer tentacles of the Dibranchiate Cephalopods that the acetabula are developed." 

 Again on p. 310 (1843) he states that "the prehensile surface of the tentacula of the 

 Nautilus is made adhesive, after the type of the simple laminated sucker of the Remora." 

 Finally the primary import of the tentacles of Nautilus is " plainly indicated by the 

 direct derivation of their central nerve from the cephalic ganglion." 



These quotations serve to show that Owen had acquired a very just appreciation 

 of the general nature of the digital tentacles, although it should be noted that the fact 



' A good representation of a transverse section through a tentacle is given by Vayssi^re in Ann. Set. 

 Nat. (8) n. 1896, PI. 19, fig. 19. 



