The lengthened and delicate setæ with which they are furnished are, moreover, peculiarly adapted to receive and convey the most minute vibratory sensations from the medium in which they are suspended; and, on the whole, it seems to be satisfactorily settled by Mr. Spence Bate (to whose excellent memoir I am indebted for these explanatory details) that the inner antennæ are real _ears_. Having thus taken our crab by the ears, we will endeavor next to tweak his nose. But stay, we must find it first. We turn our horny gentleman up, and in his flat ancient face we certainly discern little sign of a nasal organ. Our friend Mr. Bate must assist us again. He will tell us to look at the outer or lower antennæ. We will look accordingly, magnifier in hand, while he makes it clear to us that these are a pair of noses. Each of these organs is formed of a stem consisting in general of five joints, and a filament of many minute joints. In the prawn and the lobster all the five joints of the stem are distinct; but in the crab the whole are, as it were, soldered together into a compact mass, with difficulty distinguishable into their constituent articulations; while in some species their position can be indicated only by the presence of the olfactory operculum. This important little organ varies in its construction in the different families of Crustacea. In the crab it is a small movable appendage, situated at the point of junction between the second and third joints; it is attached to a long, calcareous, lever-like tendon, at the extreme limit of which is placed a set of muscles, by which it is opened and closed; to assist in which operation, at the angle of the operculum most distant from the central line of the animal are fixed two small hinges. When the operculum is raised, the internal surface is found to be perforated by a circular opening protected by a thin membrane. In the prawn, shrimp, and lobster, there is no operculum, but only the orifice covered by a membrane, which is placed at the extremity of a small protuberance, and it is not capable of being withdrawn into the cavity of the antennæ, as in the crab. In the latter animal, the little door, when it is raised, exposes the orifice in a direction pointing to the mouth; and where there is no door, still the direction of the opening is the same, inward and forward, answering to the position of the nostrils in the higher animals. In each case it is so situated that it is impossible for any food to be conveyed into the mouth without passing under this organ; and there most conveniently the animal is enabled to judge of the suitability of any substance for food, by raising the little door, and applying to the matter to be tested the sensitive membrane of the internal orifice. Thus it is concluded that this lower or outer pair of antennæ are the proper organs of smell, as the upper and inner are of hearing.