186 SENSES — SMELL. 



ments of too general a nature to have any influence when 

 adduced for a special purpose. He might have found, 

 perhaps, a better proof in their position, for in very many 

 genera the tentacula do not support the eyes ; nor are they, 

 nor can they be, employed in tracking the way. This is the 

 case in the family Doridae, in which the tentacula are 

 situated on the back, point directly upwards, are remarkably 

 large, and of curious and complex organisation, being formed 

 of a series of imbricated lamellae, like the antennae of some 

 coprophagous beetles. The tentacula of the Tritoniadae is 

 equally complex, being cut into numerous fimbriated seg- 

 ments ; and in Scyllaea these organs are in some degree 

 cupped, a little conical appendage rising up out of the cap- 

 sule. From a consideration of these and other instances 

 of peculiar organisation in the tentacula of the mollusca, 

 we cannot but suspect that they exercise some important 

 office in the animal's economy ; and since they are all un- 

 suited for vision or touch, no other sense but that of smell 

 remains to assign them, for we have found that the 

 mollusca in question, are not affected by noises of any kind. 

 This suspicion, long entertained on our part, has been 

 converted into an almost certain truth by the admirable 

 dissections of Mr. Albany Hancock and Dr. Embleton. 

 They have shown how closely similar the lamellated struc- 

 ture of the dorsal tentacles in the Nudibranchiates is to 

 the olfactory organs of some fish, and of the Nautilus ; and 

 the deduction which this similarity of structure would 

 warrant, they have further shown to be true, by the source 

 whence they derive their nerves, and the great size of them. 

 A confirmatory fact has been added by Mr. Alder, who 

 noticed " that the cilia on their surface vibrate in a direction 

 contrary to that of those on the surface of the branchial 

 papillae. On these the cilia move constantly from the body 

 towards the extremity of the papilla ; on those they act 

 from the point of the tentacle towards the body : thus, in 

 the former case, the water which has served for respiration 

 is drawn from the body and thrown off from the apices of 

 the papillae ; whilst in the latter the fluid which we may 

 suppose to contain odorous particles or qualities is attracted 

 to the end of the tentacle, and made to pass down over 

 the entire surface, and then thus to act upon the sentient 

 nerve within."* — Mr. Owen is of opinion that there exists 

 " a distinct organ of passive smell " in the Nautilus, formed 

 after the type of that organ in the inferior vertebrata, and 

 especially in fish. This part " consists of a series of soft 

 membranous laminae compactly arranged in the longitudinal 



