In Fishes, however, the plan is altogether changed, the organ of smell being no longer connected with the respiratory passages, but disposed in a cavity peculiar to itself, which opens externally by anterior nares, but has no internal communication by means of posterior orifices.
No distinct organ of smell has yet been dis covered in the Dibranchiate Cephalopoda; but in the Nautilus, a peculiar laminated organ, strongly resembling the olfactive organ of fish, has been considered by Prof. Owen as an olfactory apparatus. The inferior Mol lusca would seem to be altogether destitute of special organs of smell ; but as there is much reason to believe that some of them, espe cially the terrestrial Gasteropods, are guided to their food by its scent, it would not seem improbable that some part of the soft spongy glandular mantle, in which the entire body is enveloped, may be adapted to take cogni zance of odorous emanations ; or that in the air-breathing species, the entrance to the re spiratory sac should be endowed with a low degree of this power.
There is ample reason to infer, from ob servations of the actions of Insects, that these animals possess the olfactive power in no in considerable degree ; and yet no special organ for this sense has hitherto been satisfactorily made out. That many insects are guided to their food, to the proper nidus for their eggs, and to the opposite sex of their own species, and are even informed of the proximity of their natural enemies, by odorous emanations, can scarcely be doubted by any one who watches their habits, and who experiments upon their actions under a variety of circumstances. Thus, the flesh-fly will be attracted by the odour of decomposing meat, when this is completely hid from its sight ; and will depo sit its eggs on the envelope with which it may be covered. On the other hand, the same insect is deceived by the odour of the Stapelia, or carrion-flower, and is led to deposit its eggs in its petals. Again, many male insects will show that they are aware of the prox imity of their females, when the latter are shut up in boxes, so as to he hid from their sight, and utter no sound. And in like manner, when a predaceous insect or spider is shut up in a box that gives a sufficiently free pas sage to air, the small insects on which it preys will manifest their alarm at its proximity, and will endeavour to make their escape. Some entomologists have supposed the seat of the olfactory sense of insects to be in their an tennx, others in the palpi, and others in the entrances to the air-tubes. No evidence can
be adduced in favour of either of these sup positions that is satisfactory enough to prove it, and we have little other guide at present than a priori probability. In regard to the last of the three suppositions, however, it may be remarked that all analogy opposes the idea that the true olfactory apparatus should be thus scattered amongst the several segments of the body ; and the experiments which appear to favour it really lead to no other conclusion than this, that acrid or irritating vapours, taken in through the breathing-pores, may excite reflex movements which seem destined to expel them, or to withdraw the body from them. Such movements resemble those of coughing and sneezing in man, which are ex cited through the nerves of common sensa tion, and not through the first pair ; and they do not in the least indicate, therefore, that the sense of smell is in any way connected with the respiratory apparatus of insects, myriapods, &c. The use which many insects may be seen to make of their palpi, in taking cognizance of their food without actually touching it, suggests the idea that they are the true ollifctive organs ; and this idea is borne out by the fact, that these organs ter minate, in the living state of many insects, in soft bulbous expansions, which shrivel up and become horny in the dead specimen, thereby obscuring their real character. On the other hand, many insects are furnished with soft membranous appendages at the base of their antennx, which seem equally adapted to perform this flinction. And it is asserted by Duges*, that insects whose antennx had been cut off did not manifest the same cognizance of the neighbourhood of odorous substances, as did others of their kind whose antennm had been left entire. It would seem not a very improbable supposition that, as the antenn and palpi are organs of a shnilar class, the sense of smell may not be localised in one or other of them constantly ; but that it may be assigned to one or the other, according to the modifications they may respectively require for the performance of their other offices. The same doubt exists in regard to the olfac tive organ of the Crustacea. The manner in which crabs and lobsters are attracted by odorous bait placed in closed traps, makes it almost certain that they must possess some sense of smell; and the most probable locality of the organ would seem to be a cavity dis covered by Rosenthal at the base of the first pair of antennae.