V. HEARING.

An auditory organ has been demonstrated in so many species, that it may be considered a very general possession of the mollusca. Its existence in the Cephalopods was known to John Hunter; MM. Eydoux and Souleyet have found it in several Heteropods, and, at least, in one Pteropod; and it has been seen by M. Laurent in several genera of this order. Van Beneden and Krohn have likewise described the hearing organ of several Heteropods; and its structure in the Nudibranches has been anatomized by De Quatrefages, and more especially by Messrs. Embleton and Hancock. However, we owe our knowledge of the organ principally to M. Siebold, who has pursued his researches more systematically than any other anatomist, and further has proved its existence in many Gasteropod and bivalve acephalous mollusca. Like the other organs of sense, the ear is always paired. It is formed by two hyaline, ovate, or orbicular capsules, situated on the head or neck at the bases of the tentacula, and is supplied with its specifically-endowed nerve from the cerebral ganglions.* In the capsule there are enclosed one or several (and sometimes they are numerous) oval or round crystalline bodies named otolites; and it is observable that the number varies not only in neighbouring genera, but even in nearly allied species. Siebold says that a concentric depression is evident in these otolites, and there may be seen in the centre of the greater number of them a shaded spot, or rather a minute aperture, which penetrates through the concretion from the one flattened surface to the other. Subjected to

* "En effet, cet organe se trouve chez tous les Gastéropodes à la partie postérieure des deux renflemens ganglionnaires les plus volumineux; on doit les chercher toujours auprès de la paire de ganglions antérieurs de cette portion de système nerveux, et il est plus facile de les trouver à la surface inférieure qu'à la surface supérieure, surtout chez les Gastéropodes (Limax, Helix), dont les divers ganglions sous-œsophagiens sont confondus de le manière la plus intime.”—SIEBOLD in Ann. des Sc. Nat. (1843), xix. 198.-In the Bivalves the organ is placed in the foot. Siebold thus describes it in Cyclas cornea: "On compressing the extremity of the foot of this species between two plates of glass, we bring into view a large or central nervous ganglion; and on each side of this there is a minute round reservoir, composed of an elastic, opake, and tenacious mass. In the centre of this there is again a perfectly transparent circular and flattened nucleus, which floats disconnected from the sides of the body that contains it, and has an oscillatory movement. This nucleus appears to consist of a crystalline salt." -Ann. des Sc. Nat. n. s. ix. 319.

a strong pressure, the otolites crack in radiating lines, separating often into four pyramidal pieces. This separation also ensues, after a longer time, when the otolites are immersed in a diluted nitric acid; and if we touch them with the concentrated acid, they suddenly dissolve with the disengagement of a gas, whence Siebold concludes them to be composed of carbonate of lime. The size of the otolites is not equal; and, in the same capsule there are always some which are smaller than others. Within the capsule they have, during life, a very remarkable and, in some respects, peculiar, lively, oscillatory movement, being driven about as particles of any light insoluble powder might be in boiling water. The otolites in the centre have the appearance of being pressed together so as to form a sort of solid nucleus; and towards this centre the otolites towards the circumference seem ever to be violently urged, their centripetal rush being invariably repulsed, and as often driven again into a centrifugal direction. Removed from the capsule, the motions of the otolites instantly cease. The cause of these

curious oscillations remains undiscovered. Siebold could detect no vibratile cilia on the surfaces of the capsule;* and the cessation of the motion when the otolites are removed proves them to be unciliated themselves, and, at the same time, distinguishes the motion from that of inorganic molecules as described by Mr. Brown.†

And in these discoveries you have a lively example of the nicety of anatomical researches in our times. In my student days, it was questioned whether any mollusk besides the cuttles had eyes; and it was agreed on all hands that they were earless and surd.§ Behold the change a few years has made in our knowledge of this branch of physiology! They have eyes and they see; and ears have they, yet, from the very rudimentary condition of the organ, it is adapted to communicate to the possessors only the most limited perception of sonorous undulations. Amusing myself with

"Kölliker has observed, that the motion of the otolites in the mollusca is dependent upon cilia, with which the internal surface of the cyst is covered."-Reports on Zoology for 1843-4, printed for the Ray Society, p. 216. + Observations sur l'Organe Auditif des Mollusques, par C. Th. V. Siebold, in Ann. des Sc. Nat. (1843), xix. p. 193-211.

"Vermium genus omne oculis caret; Plin. nisi forte Sepia ejusque Cymbium. Limaces videntur etiam oculis instructi, modo veri sint oculi, a nostris certe diversi."-LINN. Syst. Nat. 1069.

§ "None of the mollusca appear to possess, even in the smallest degree, the sense of hearing, if we except the highly organized Cephalopoda." ROGET's Bridgew. Treat. ii. 436.

some experiments, I found that Snails (Helix nemoralis) are not affected by loud and harsh noises about them; nor are the Periwinkle and Whelk (Purpura lapillus) more susceptible in this respect. It is affirmed, however, by the Rev. L. Guilding, that the Strombidæ possess the sense of hearing, or something allied to it. "I lately suspended," he says, "a number of large Strombi by the spire, that the animal, when dead, might fall from the shell. They had remained in this situation several days, till the body, weak and emaciated, hung down nearly a foot from the aperture, and the eyes had become dim. I found that even before my shadow could pass over them, they were aware of my presence, and endeavoured to withdraw into the shell. I then cut off the eyes, with the thick cartilaginous tentacula in which they were lodged, but the animals still continued to be sensible of my near approach, while hanging in this mutilated and painful condition."* The experiment is not conclusive, nor does it appear indeed that any noise was made. The impression which gave the alarm might perhaps be from the pulses of the atmosphere, thrown into motion by the approach of the experimenter, and acting on a skin morbidly sensible. On a summer evening I have observed the common Spout-fish (Solen siliqua), extended along the surface of the fine sand in which they burrow, enjoying apparently the calmness and mildness of the season, take alarm and instantaneously descend when I was yet distant several yards and I can explain this and similar facts only on the supposition of the existence of a sense of touch feelingly alive to impressions impalpable to our grosser "In the case of many animals," says Müller, "it may be doubted whether they really hear at all: for every reaction of nerves under the influence of vibrations cannot be called the sensation of sound, since the sense of touch is capable of perceiving the same vibrations as a tremor."†

sense.

Thus we explain the fear which was anciently ascribed to the pearl-mussels during the thunderstorm, when they were so shaken "with the feare of flashie lightenings," that they cast their pearls and became "emptie;" and thus, also, we explain a fact mentioned by Baster, who, on the authority of the seamen engaged in carrying mussels to Holland, tells us that these shell-fish are grievously affected by any violent motion and concussion of the air; for if the ship is overtaken by a thunderstorm, or sails too near any other vessel which at the moment discharges its guns, the mussels,

* Zool. Journ. iv. 172.

+ Physiology, trans. p. 1129.

frightened with the tremor, shut their valves too suddenly, so as to expel the contained fluid, and in consequence soon die.*

Molluscous animals are dumb. There are a very few exceptions. "The noise made by the cuttle-fish, when dragged out of the water, resembles the grunting of a hog." Two nudibranchial Gasteropods have been discovered to produce a sound, viz. the Tritonia arborescens and the beautiful Eolis punctata. The sounds (Fig. 35)

which the former species produces, when in a glass vessel, says Professor Grant, "resemble very much the clink of a steel wire on the side of the jar, one stroke only being given at a time, and repeated at intervals of a minute or two: when placed in a large basin of water the sound is much obscured, and is like that of a watch, one stroke being repeated as before at intervals. The sound is longest and oftenest repeated when the Tritoniæ are lively and moving about, and is not heard when they are cold and without any motion; in the dark I have not observed any light emitted at the time of the stroke; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke; the sound, when in a glass vessel, is mellow and distinct." The Professor has kept these Tritoniæ alive in his room for a month, and, during the whole period of their confinement, they have continued to produce the sounds, with very little diminution of their original intensity. In a still apartment they are audible at the distance of twelve feet. "The sounds obviously proceed from the mouth of the animal; and at the instant of the stroke we observe the lips suddenly separate, as if to allow the water to rush into a small vacuum formed within. As these animals are hermaphrodites, re

*Opusc. Subseciv. i. 109.

+ Barbut Gen. Verm. 73. Alder and Hancock Nudibr. Moll. fam. 3, pl. 12.

quiring mutual impregnation, the sounds may possibly be a means of communication between them; or, if they be of an electric nature, they may be a means of defending from foreign enemies one of the most delicate, defenceless, and beautiful Gasteropods that inhabit the deep." *

You must not confound these sounds of the Tritonia and Eolis with what has been absurdly called the "music of snails," a noise created by the crawling of them on a moist window-pane of glass. This is mechanical, and has no connection with the animal. As the source of the noise is often unsuspected, and as it is heard only in the evening after the dew has fallen, or at the witching hour of night, it has caused on some occasions a superstitious dread, and has afforded the ingenious Mrs. Bowdich a foundation for an interesting tale.

* Edinb. Phil. Journ. xiv. 186.

+ Mag. Nat. Hist. n. s. i. 107.