Several of the land Mollusca have the power of exhaling a disagreeable smell, Hyalinia alliaria smelling strongly of garlic, and Stenogyra decollata of laudanum; but this need not be any argument for the sense of smell in the creatures themselves.
Position of Olfactory Organs in Pulmonata.—Most authorities are of opinion that the olfactory organs are situated in the tentacles. Moquin-Tandon considered that in the Helicidae and Limacidae the sense of smell is confined to the little knob or elevation at the end of the longer tentacles, close to the eye. He found that when he cut off these tentacles both in Limax and Arion, the creatures were quite unable to discover the whereabouts even of strongly-scented food. The same author believed that in the Basommatophora the sense of smell was present in the whole of the tentacle, which is covered with an exceedingly sensitive ciliated epithelium. Lacaze-Duthiers, however, places the olfactory sense in this group at the outer side of the base of the tentacles, near to the eyes. Some authorities[303] deny that the Helicidae have the olfactory organ at the tip of the tentacles, and locate it in a pedal gland near the mouth, which contains conspicuous sensitive cells. A Helix whose tentacles had been removed manifested its repulsion to the smell of spirits of turpentine, while another Helix, which was unmutilated, did not object to the turpentine being held between its tentacles. Altogether, then, the exact position of the smell-organ in the Helicidae must be considered as not yet thoroughly determined. Simroth holds that the sense of smell is distributed over the whole soft integument, and is especially concentrated in the feelers, and in the neighbourhood of the respiratory orifice.[304]
In nearly all marine Mollusca yet examined, the organ of smell or osphradium is in situation intimately connected with the breathing organs, being generally placed near their base, with the object, apparently, of testing the quality of the water before it passes over the branchiae. It consists of a patch of the epithelium, modified in a special manner, and connected by its own nerve with one of the visceral ganglia.
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.
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
Experiments made with a view to ascertain whether the Mollusca are sensitive to noises have usually led to the conclusion that they are deaf to very loud sounds. This is the more curious, because an undoubted auditory apparatus has been discovered in a large number of genera. In the case of an experiment, it is not easy to be sure that the animal is not affected, at least in part, by the shock or jar, rather than by the actual sound. In some experiments, however, conducted at the Plymouth Marine Biological Laboratory, Mr. Bateson found[306] that Anomia could be made to shut its shell by smearing the glass of the tank with the finger in such a way as to make a creaking sound. It was evident that the cause of alarm was not the jarring of the solid framework of the tank, for the same result occurred when the object on which the Anomia were fixed was suspended in the water by a thread. It was found that the sound had to be of a particular pitch to excite the attention of the mollusc.
As a rule the organ of hearing is nothing more than a small vesicle or sac (the otocyst), filled with a fluid secretion, in which are suspended one or usually more calcareous concretions known as otoliths. By means of cilia, which connect with sense-cells, these otoliths are given a peculiar movement or oscillation in the medium in which they are suspended. The number of the otoliths varies in different genera and species; there are several hundreds in Arion and Limax, about a hundred in Helix pomatia, nemoralis, hispida, arbustorum, rotundata, Succinea putris, and Limnaea stagnalis; about fifty in Planorbis contortus and Physa fontinalis, only one in Cyclostoma elegans. The number increases with age. In young specimens of Limn. stagnalis as few as ten, nine, and seven have been noticed.[307]