An osphradium does not necessarily occur in all genera; for instance, it has not been detected in _Fissurella_. It is most highly specialised in the Conidae, and in the carnivorous Gasteropoda generally. In _Buccinum undatum_, for instance, it is very large indeed, and, from its plumed form, has sometimes been mistaken for an accessory branchia (Fig. 95). In _Haliotis_ it is paired, one lying in close proximity to each of the two branchiae, but in _Turbo_ it is single, corresponding to the single branchia. In _Chiton_ there is an osphradium at the base of each separate gill filament, making a total of twenty or more on each side. Its position in _Physa_ and in _Cyclostoma_ will be seen by reference to Figs. 103 and 104 (p. 205). In the Pelecypoda the osphradia are paired, and lie adjacent to the posterior adductor muscle, close to the hinder end of the axis of the branchiae. In the Tetrabranchiate Cephalopoda there are two osphradia, placed between the bases of the two pairs of gills. In the Dibranchiates on the other hand, a groove above the eyes has been regarded as the seat of the organ of smell. This groove contains sensory and ciliated cells, and appears to be connected with a special nerve centre of its own, which ultimately is derived from the cerebral ganglion. [Illustration: FIG. 95.--_Buccinum undatum_ L., deprived of its shell, showing the relative position of branchia (_br_) and osphradium (_os_); _m_, mucous glands; _s_, siphon. The portion of the mantle covering the osphradium has been removed.] Scarcely any instances of the exercise of the sense of smell on the part of bivalve Mollusca have been recorded. Something of the sort, however, seems to have been present in a case related by Mr. R. L. King.[305] A skull of a fox had been placed in a small ditch in order to soak, and after a few days, when taken out, was found to be covered with _Pisidium pusillum_ to the number of at least two hundred, which had been probably attracted from the water in the immediate neighbourhood by the smell of the decaying flesh. IV. _Hearing_