neus; in some Aplysia they are blackish-red and granular; and in our native Aplysiæ they are yellow. Carus asserts, that those of the common fresh-water mussel are invariably bright yellow; in the Arcæ, Pinnæ, and a few others, they are tinted a rose-red, but in the majority of Bivalves they are yellowish, or almost white, and soft and transparent.

*

I will now tell you all I know about the senses of the mollusca, and will begin with that of touch, as it is common to the whole class, and consequently the most important. It is, indeed, doubtful whether the bivalved mollusca have any other sense; they have no undisputed eyes, no true tentacula, no tongue, slight vestige of an ear, and if they possess the perception of smells, we know no organ in which it is localized ;-"neither, indeed," says Bradley, "do I think the necessary organs for those senses can reasonably be sought for in such bodies as have a fixed state of life; the senses of feeling and tasting being sufficient for the maintenance and support of them."

I. TOUCH.

The skin of the mollusca is a soft, spongy, mucous membrane, wrinkled and thickish where exposed, smooth and very thin where covered with the shell. It is never in the slightest degree hairy, or villous, or horny, but always kept in a moist state by a glutinous secretion, exuded in some instances from "little, glandulous, unequal grains," profusely scattered over the surface; in others, from crypts or glands confined to particular parts. It is a homogeneous membrane, not divisible into epidermis and cutis vera, like the skin of the vertebrate animals; and it is so intimately fixed to, or rather interwoven with, the subjacent muscular layer, that it is contractile at every point, and in all directions. It invests every part, sometimes closely, but more commonly there is "ample room and verge enough" to form folds and expansions; from which circumstance it has received the name of mantle or cloak. The blood-vessels distributed in its texture are very numerous, and the nerves are presumed to be at least equally so.

From this structure we might have concluded that the skin would be peculiarly sensible to external impressions; and this we know is the fact. Let your experiment be made with the lightest hand and the softest instrument, yet it cannot come into contact with the mollusk which * Comp. Anat. trans. i. 53; also, Edinb. New Phil. Journ. vii. 229.

will not feel the impression, and evidence its alarm by intelligible signs. The intimations which they receive by this medium are probably of a very general character, and have respect only to the motion, the temperature, the hardness or softness, of the impressing body. To judge of the position, and perhaps in some degree of the form, of bodies, they are provided with special organs, which, when situated on the head, or about the mouth, are denominated tentacula, but when arranged along the sides or on the margins of the cloak, more commonly tentacular filaments. The former are two or four in number, very rarely six,* and in only one or two instances is there a pair, with an odd one behind them: they are of a cylindrical, tapered, or triangular figure, very flexible, and almost always capable of being withdrawn within a sheath or under the collar, at the will of the animal. The filaments are sometimes retractile, and sometimes not many species do not possess them; but, when they are present, they become the creature's chief ornament. The shell of the Haliotis, for example, if we except the splendid iridescence of its interior, is sufficiently plain and vulgar; but behold it borne along by the living tenant, its Fig. 32.

variegated garniture all displayed and vermicular in the smooth and crystal water, and it moves wonder and admiration (Fig. 32). The Cyprææ, Trochi, and the family of which the genus Turbo is the type or representative, afford equally fine illustrations, the filaments in some species

The Nautilus is remarkable for their unexampled number, surrounding the mouth in successive series, and amounting to little short of a hundred! They are also retractile within sheaths, and annulated; but it should be remembered that of this number four only seem designed for sensation, and these resemble the tentacula of Doris in their lamellated structure. Owen's Memoir; and Cyclop. Anat. and Physiol. i. 526.

See

being delicately ciliated. Among bivalves a fringe of these filaments is very general. In the genera which have the cloak completely open, as the oysters, and the sea and freshwater mussels, the filaments fringe it all round; and in those in which the cloak opens by a tube only, these appendices, either simple or variously scalloped, are attached to the circumference of its orifice. Such is the case in the genera Venus, Cardium, &c.

Now, these tentacula and filaments are exquisitely sensitive, and in all likelihood, convey impressions of a more distinct character than the general surface. When the mollusca walk abroad, these organs are all extended to the utmost, and in perpetual motion; sentinels alive to everything around, warning against foe or danger, and watchful of prey. By means of them, the Gasteropods likewise feel their way, and ascertain the nature of the ground they traverse, as it seems evident from the manner in which they use them; but to this purpose the proper tentacula are never applied, at least when they carry the eyes on their tips; and they appear to be organs of some other sense. If removed, the snail creeps on as if it were unmutilated; and there are tribes, among which we may instance the entire order of Nudibranches, in which their position is such, that they cannot possibly be applied to objects either in front or around them.

II. TASTE.

Swammerdam found, by experiment, that snails have "a nice appetite and taste;" and it seems necessary to suppose the existence of this sense in all mollusca, for they select particular articles of food in preference to others; and we know no other sense which is fitted to regulate the choice. It must reside, of course, in the mouth; but, whether diffused over the whole, or limited to a certain space, it were hard to determine. Blainville thinks that in the cephalous mollusca, the seat of taste may probably be in a knob or swelling at the lower end of the buccal cavity; and Cuvier conjectures that the tentacula, at the orifices at which the water, the vehicle of their aliment, enters, may exercise this sense in the acephalous ones.*

III. SMELL.

According to Swammerdam, snails have a very quick smell. "This I observed," says he, "when I moved a

* Comp. Anat. trans. ii. 694.

little fresh food towards them, for they immediately perceived it by the scent, and crept out of their little shells, and came to it."* Gaspard appears to have repeated this experiment without success +; but he is surely hasty in denying, on that account, the existence of the sense, seeing how positively the contrary is affirmed by one of the greatest and most honest of naturalists. Blainville says, in general terms, that the acephalous mollusca have no smell, but he admits that the Cephalopods and Gasteropods possess the sense, and the terrestrial species in a degree of considerable delicacy, since we observe that slugs and snails seek out particular plants, where sight could not have availed them. According to Carus, it appears to be fully proved by the observation of the aversion of these animals, the Sepiæ for instance, to strong-scented plants, that those mollusca which live partly in water and partly in air, have an olfactory organ, but he denies its existence in those which live exclusively in water. Admitting the existence of the sense in the cephalous families, there remains great uncertainty relative to its seat. Analogy is here at fault, for invertebrate animals have nothing similar to a nose. Cuvier thinks that a special organ may not be necessary, for the whole skin appears to resemble a pituitary membrane, and may, in consequence, be susceptible of receiving the peculiar impressions emanating from odorous bodies.§ If, however, a particular seat for the sense is to be fixed upon, he would place it at the entrance of their pulmonary cavity, because, in all vertebral animals, it is situated at the entrance of the organs of respiration; an argument of little value in the present instance. Blainville, whose opinion is always entitled to attention, states his belief that the proper tentacula are the olfactory organs, because the skin of them is more soft, smooth, and delicate than on any other part, and their nerves more considerable ; || argu

*Book of Nature, p. 49.

+ Zool. Journ. i. 179.

Comp. Anatomy, i. 74. According to Oppian the Octopus may be induced to leave the sea by placing branches of the olive-tree on the shore; and Cuvier thinks that this is a statement which deserves to be verified. Hist. des. Sc. Nat. i. 308. § Comp. Anat. trans. ii. 688.

Manuel, p. 107, or more particularly his excellent Principes d'Anatomie Comp. i. 341. Dr. Leidy places the sense, in the terrestrial Gasteropods, in a sort of cul-de-sac, "having its orifice beneath the mouth; between the inferior lip and the anterior extremity of the podal disk, and which in many species of different genera is elongated backwards into a blind duct, more or less deep, occupying a situation just above the podal disk within the visceral cavity." This opinion deserves re-examination.-Ann. & Mag. N. Hist. xx. 211.-The structure itself had been observed previous to Dr. Leidy's examinations. See p. 170, note §.

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 the tentacula do not support the eyes; nor are they, genera nor can they be, employed in tracking the way. This is the case in the family Dorida, 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 lamellæ, like the antennæ of some coprophagous beetles. The tentacula of the Tritoniada is equally complex, being cut into numerous fimbriated segments; and in Scyllæa these organs are in some degree cupped, a little conical appendage rising up out of the capsule. 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 unsuited 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 structure 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 papillæ. 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 papillæ; 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 laminæ compactly arranged in the longitudinal