(1577). To the above forty tentacula must be added four others of a different construction, which project immediately beneath the margin of the hood, like antennae, one before and one behind each eye (fig. 284, r.) These tentacles would seem at first sight to be constructed upon the same principles as the last; but on examining them attentively, they are found to be composed of a number of flattened circular disks appended to a lateral stem. Yet even all these organs of touch form but a small part of the tactile apparatus of the Nautilus Pompilius; for the mouth, lodged within the oral sheath, is surrounded with a series of tentacula even more numerous than those appended to the exterior of the head. Around the circular lip (fig. 289, m) which encloses the beak (n, o) are situated four labial processes (g g, i i): each of these processes is pierced by twelve canals, the orifices of which are disposed in a single but rather irregular series along their anterior margin; and every one of these canals contains a cirrus or tentacle rather smaller than those of the external digitations, although their structure is precisely similar (h h, k k.) These cirri, like the former, receive large nerves, - those supplying the external labial tentacles being derived immediately from the brain (fig. 291, 6 6), while those distributed to the internal labial tentacles proceed from a large ganglion (8) that is in communication with the oesophageal ring through the intervention of a considerable nervous trunk (7).

Nervous system of Nautilus Pompilius (after Owen): a a,bb, cut edges of the mantle.

Fig. 291. Nervous system of Nautilus Pompilius (after Owen): a a,bb, cut edges of the mantle; d, e, f, g, retractile tentacula; A, presumed olfactory organ; 1, the supracesophageal ganglion; 2 2, optic ganglia; 3 3, 4 4, suboesophageal ganglia; 5, 6, 7, 8, 9, nerves supplying the retractile tentacula; 10, nerves supplying olfactory organ; 11,12,13, nerves supplying the muscular integument; 14, 15, 16, nerves representing the sympathetic system.

(1578). In the Dibranchiate Cephalopods none of the above-described cirriferous processes are found to exist; but there is every evidence that the prehensile arms, and most probably the individual suckers appended to them, are highly sensitive to tactile impressions. Every one of the arms receives a large nerve, derived from the same portion of the oesophageal collar as that which gives origin to the tentacular nerves of Nautilus, which traverses its whole length, lodged in the same canal as the great artery of the limb. During this course the nerve becomes slightly dilated at short distances, and gives off from each enlargement numerous small nervous twigs which penetrate into the fleshy substance of the foot. Immediately after entering the arm and undergoing the dilatation above alluded to, every nerve furnishes two large branches, one from each side, which traverse the fleshy substance connecting the bases of the arms, to unite with the nerves of the two contiguous arms, so that all the nerves of the feet are connected near their origins by a nervous zone*, - an arrangement intended, no doubt, to associate the movements of the organs to which these nerves are appropriated.

(1579). There is little doubt, from the character of the soft and papillose membrane which forms a considerable portion of the surface of the tongue, that both in the Nautilus and in the Dibranchiate Cephalopods the sense of taste is sufficiently acute - far superior, indeed, to what is enjoyed by any of the Gasteropod Mollusca, and possibly even excelling that conferred upon fishes and others of the lowest Vertebrata that obtain their food under circumstances such as render mastication impossible, and the perception of savours a superfluous boon.

(1580). That the Cephalopoda are provided with a delicate sense of smell, and attracted by odorous substances, is a fact established by the concurrent testimony of many authors, although in the most highly organized genera nothing analogous to an olfactory apparatus has as yet been pointed out: nevertheless, in Nautilus, Professor Owen discovered a structure which he regards, with every show of probability, as being a distinct organ of passive smell, exhibiting the same type of structure that is met with in the nose of fishes, and, from the circumstance of its being the first appearance of an organ specially appropriated to the perception of odours, well deserving the attention of the physiologist. We may here premise that the exercise of this function in creatures continually immersed in water must depend upon conditions widely differing from those which confer the power of smelling upon air-breathing animals. In the latter, the odorant particles, wafted by the breeze to a distance and drawn in by the breath, are made to pass, by the act of inspiration, along the nasal passages, and, being thus examined with a minuteness of appreciation proportionate to the extent of the olfactory membrane, give intimations of the existence of distant bodies scarcely inferior to those obtained from sight and sound.

But, in an aquatic medium, information derived from this sense must be restricted within far narrower limits, inasmuch as the dissemination of odoriferous particles must necessarily be extremely slow, and the power of perceiving their presence comparatively of little importance, seeing that the extent to which it can be exercised is so materially circumscribed. Smell, in aquatic animals, is therefore apparently reduced to a mere perception of the casual qualities of the surrounding element, without any power of inhaling odours from a distance. Simple contact between a sufficiently extensive sentient surface and the water in which it is immediately immersed is all that is requisite in the case before us; and if an organ can be pointed out, constructed in such a manner as to adapt it to fulfil the above intention, there can be little hesitation in assigning to it the office of an olfactory apparatus.