Tactile organs, although occurring in some of the Mollusca, do not appear to attain special or marked development, except in a few cases. The whole surface of the skin, and particularly of the foot, is very sensitive to the slightest impression. Nearly all Gasteropoda are furnished with at least two cephalic tentacles, projecting like horns from each side of the fore part of the head. At or near the base of these are generally situated the eyes. In the Helicidae the eyes are situated, not at the base, but at the apex of the tentacles, and in that case—except in Vertigo—a second pair of shorter tentacles appears beneath the longer pair. It frequently happens that several senses are centred in a single organ; thus the upper tentacles of snails not only carry the eyes and serve to a certain extent as tactile organs, but they also carry the organs of smell.
The edges of the mantle, which are sometimes specialised into lobes, appear to be keenly sensitive to touch in all Gasteropoda.
In Cypraea (Fig. 81) these lobes, or tentaculae, are a prominent feature of the animal, and also in certain genera of the Trochidae (Fig. 82). In most of the carnivorous land Pulmonata—e.g. Testacella, Rhytida, Ennea—there are developed, under the lower pair of tentacles, and close to the mouth, large labial palps or feelers. These are connected with the cerebral ganglion by a very large nerve, and may therefore be supposed to be of extreme sensitiveness. In some of the large carnivorous forms (Glandina, Aerope, compare Fig. 21, p. 54) these palpae are of great size, and curl upwards like an enormous pair of moustaches. When a Glandina seizes its prey, the palpae (see Fig. 83) appear to enfold it and draw it in towards the mouth.
Fig. 81.—Cypraea moneta L., showing tentaculae at edge of mantle, which partly envelops the shell: Si, siphon; M, M, mantle; F, foot; T´, tentaculae at edge of mantle. (After Quoy and Gaimard.) × 3/2.
Fig. 82.—Monodonta canalifera Lam., New Ireland, showing mantle lobes. (After Quoy and Gaimard.)
Fig. 83.—Glandina seizing its prey, with buccal papillae turned back. (Strebel.)
It is in the Opisthobranchiata that the organs of touch attain their maximum development. Many of this group are shell-less or possess a small internal shell, and accordingly, in the absence of this special form of defence, a multiplied sense of touch is probably of great service. Thus we find, besides the ordinary cephalic tentacles, clusters or crowns of the same above the head of many Nudibranchiata, with lobe-like prolongations of the integument, and tentacular processes in the neighbourhood of, or surrounding the branchiae (see Figs. 58 and Fig. 84, or even projecting from the whole upper surface of the body (Fig. 5, C).
In the Pelecypoda, the chief organs of touch are the foot, which is always remarkably sensitive, especially towards its point, the labial palps on each side of the mouth, and the siphons. In certain cases the mantle border is prolonged into a series of threads or filaments. These are particularly noticeable in Pecten, Lepton, and Lima (Fig. 85), the mantle lobes of the common L. hians of our own coasts being very numerous, and of a bright orange colour. In many genera—e.g. Unio, Mactra—this sensibility to touch appears to be shared by the whole mantle border, although it is not furnished with any special fringing. The ‘arms’ of the Cephalopoda appear to be keenly sensitive to touch, and this is particularly the case with the front or tentacular pair of arms, which seem to be employed in an especial degree for exploration and investigation of strange objects.
Fig. 84.—Idalia Leachii A. and H., British seas; br, branchiae. (After Alder and Hancock.)
Fig. 85.—Lima squamosa Lam., Naples, showing tentacular lobes of mantle (t, t); a, anus; ad.m, adductor muscle; br, br, branchiae; f, foot; sh, shell.
Taste.—The sense of taste is no doubt present, to a greater or less extent, in all the head-bearing Mollusca. In many of these a special nerve or nerves has been discovered in the pharynx, connecting with the cerebral ganglion; this no doubt indicates the seat of the faculty of taste. The Mollusca vary greatly in their likings for different kinds of food. Some seem to prefer decaying and highly odoriferous animal matter (Buccinum, Nassa), others apparently confine themselves to fresh meat (Purpura, Natica, Testacella), others again, although naturally vegetarian, will not refuse flesh on occasion (Limax, Helix).
Mr. W. A. Gain[284] has made some interesting experiments on the taste of British land Mollusca, as evidenced by the acceptance or rejection of various kinds of food. He kept twelve species of Arion and Limax, and eight species of Helix in captivity for many months, and tried them with no less than 197 different kinds of food, cannibalism included. Some curious points came out in his table of results. Amalia gagates appears to be surprisingly omnivorous, for out of 197 kinds of food it ate all but 25; Arion ater came next, eating all but 40. Limax arborum, on the other hand, was dainty to a fault, eating only seven kinds of food, and actually refusing Swedes, which every other species took with some avidity. Certain food was rejected by all alike, e.g. London Pride, Dog Rose, Beech and Chestnut leaves, Spruce Fir, Common Rush, Liverwort, and Lichens; while all, or nearly all, ate greedily of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries, Boletus edulis, and common grasses. Few of our common weeds or hedgerow flowers were altogether rejected. Arion and Limax were decidedly less particular in their food than Helix, nearly all of them eating earthworms and puff-balls, which no Helix would touch. Arion ater and Limax maximus ate the slime off one another, and portions of skin. Cyclostoma elegans and Hyalinia nitida preferred moist dead leaves to anything else.
Position of Eyes.—In the majority of the head-bearing Mollusca the eyes are two in number, and are placed on, or in the immediate neighbourhood of the head. Sometimes they are carried on projecting tentacles or ‘ommatophores,’ which are either simple (as in Prosobranchiata) or capable of retraction like the fingers of a glove (Helix, etc.). Sometimes, as in a large number of the marine Gasteropoda, the eyes are at the outer base of the cephalic tentacles, or are mounted on the tentacles themselves, but never at the tip (compare Fig. 60, p. 153 and Fig. 98, p. 199). In other cases they are placed somewhat farther back, at the sides of the neck. The Pulmonata are usually subdivided into two great groups, Stylommatophora and Basommatophora (Fig. 86), according as the eyes are carried on the tip of the large tentacles (Helix, and all non-operculate land shells), or placed at the inner side of their base (Limnaea, Physa, etc.). In land and fresh-water operculates, the eyes are situated at the outer base of the tentacles.
Fig. 86.—A, Limnaea peregra Müll.; e, e, eyes; t, t, tentacles; B, Helix nemoralis Müll.; e, e, eyes; t, t, tentacles; p.o, pulmonary orifice.
In the Helicidae, careful observation will show that the eyes are not placed exactly in the centre of the end of the tentacle, but on its upper side, inclining slightly outwards. The eye is probably pushed on one side, as it were, by the development of the neighbouring olfactory bulb. The sense of smell being far more important to these animals than the sense of sight, the former sense develops at the expense of the latter.
Organisation of the Molluscan Eye.—The eye in Mollusca exhibits almost every imaginable form, from the extremely simple to the elaborately complex. It may be, as in certain bivalves, no more than a pigmented spot on the mantle, or it may consist, as in some of the Cephalopoda, of a cornea, a sclerotic, a choroid, an iris, a lens, an aqueous and vitreous humour, a retina, and an optic nerve, or of some of these parts only.
In most land and fresh-water Mollusca the eye may be regarded, roughly speaking, as a ball connected by an exceedingly fine thread (the optic nerve) with a nerve centre (the cerebral ganglion). In Paludina this ball is elliptic, in Planorbis and Neritina it is drawn out at the back into a conical or pear shape. In Helix (Fig. 87) there is a structureless membrane, surrounding the whole eye, a lens, and a retina, the latter consisting of a nervous layer, a cellular layer, and a layer of rods containing pigment, this innermost layer (that nearest the lens) being of the thickness of half the whole retina.
Fig. 87.—Eye of Helix pomatia L., retracted within the tentacle; c, cornea; ep, epithelial layer; l, lens; op.n, optic nerve; r, retina. (After Simroth.)
Comparing the eyes of different Gasteropoda together, we find that they represent stages in a general course of development. Thus in Patella the eye is scarcely more than an invagination or depression in the integument, which is lined with pigmented and retinal cells. The next upward stage occurs in Trochus, where the depression becomes deeper and bladder-shaped, and is filled with a gelatinous or crystalline mass, but still is open at the top, and therefore permits the eye to be bathed in water. Then, as in Turbo, the bladder becomes closed by a thin epithelial layer, which finally, as in some Murex, becomes much thicker, while the ‘eyeball’ encloses a lens (Fig. 88), which probably corresponds with the ‘vitreous humour’ of other types.
Fig. 88.—Eyes of Gasteropoda, showing arrest of development at successive stages: A, Patella; B, Trochus; C, Turbo; D, Murex; ep, epidermis; l, lens; op.n, optic nerve; r, retina; v.h, vitreous humour. (After Hilger.)
In Nautilus the eye is of a very simple type. It consists of a cup-shaped depression, with a small opening which is not quite closed by the integument. The retina consists of cells which line the interior of the depression, and which communicate directly with the branches of the optic nerve, there being no iris or lens. This type of eye, it will be observed, corresponds exactly with that which occurs in Patella. It appears also to correspond to a stage in the development of eyes in the Dibranchiata (e.g. Octopus, Sepia, Loligo). Lankester has shown[285] that in Loligo the eye first appears as a ridge, enclosing an oval area in the integument. By degrees the walls of this area close in, and eventually join, enclosing the retinal cells within the chamber in which the lens is afterwards developed (Fig. 89). It thus appears that in some cases the development of the eye is arrested at a point which in other cases only forms a temporary stage towards a higher type of organisation.
Fig. 89.—Three stages in the development of the eye of Loligo; r, r, ridge, enclosing p.o.c, primitive optic chamber; or, orifice between the closing ridges; s.o.c, secondary optic chamber; ci, ci, ciliary body; l, rudimentary lens; R, retina. (After Lankester.)
Fig. 90.—Eye in A, Loligo; B, Helix or Limax; C, Nautilus: a.o.c, anterior optic chamber; c, cornea; int, integument; ir, iris; l, lens; l´, external portion of lens; op.n, optic nerve; op.g, optic ganglion; p.o.c, posterior optic chamber; r, retina. (After Grenacher.)
The developed eye in the dibranchiate Cephalopods consists of a transparent cornea, which may or may not be closed over the front of the lens. Behind the cornea is a narrow chamber (the anterior optic chamber) which is continued for three parts round the whole circle of the eye, and into which project the front portion of the lens and the folds of the iris. Throughout its whole extent, the anterior optic chamber is lined by the integument, the portion of which on the inner side is the choroid. The lens is divided into an outer and inner segment by a thin membrane, and is supported by the ciliary body which forms a continuation of the retina. The main portion of the lens lies within the posterior optic chamber, at the back and sides of which is found the retina (Grenacher).
There can be no doubt that the Cephalopoda use their eyes to observe, but there is nothing to show that any other Mollusca use their eyes for this purpose, the sense of smell in their case largely taking the place of visual observation. Madame Jeannette Power once saw[286] the Octopus in her aquarium holding a fragment of rock in one of its arms, and watching a Pinna which was opening its valves. As soon as they were perfectly open, the Poulpe, with incredible address and promptitude, placed the stone between the valves, preventing the Pinna from closing again, upon which it set about devouring its victim. The next day the Poulpe was seen, after crushing some Tellina, to stretch himself down close by a Triton nodiferus, and watch it attentively. After four hours the Triton emerged from its shell, when the Octopus sprang upon it, and surrounded it with its arms.
Powers of Vision in Land Mollusca.—The Helicidae are undoubtedly very short-sighted. Seldom emerging from their retreats except in twilight and darkness, they are naturally myopic, and see better in a subdued than in a bright light. Experiment has shown that a Helix can perceive an object better at 6 centimetres distance in a weak light than at 4 or 5 millimetres in a strong one. Cyclostoma elegans and Paludina vivipara are comparatively long-sighted, perceiving objects at a distance of 20 to 30 centimetres.[287] The increased power of vision is due, in these two cases, to increased elaboration in the construction of the eye, Paludina possessing a large and almost spherical lens, to which the vitreous humour closely adheres, while in Cyclostoma the lens is remarkably hard, and the aqueous humour very abundant. According to V. Willem,[288] the Pulmonata are very sensitive to the slightest movement of the air or jarring of the surface on which they crawl, but are so short-sighted as only to perceive a confused image of a large object at about 1 cm., and to distinguish the form of objects at not more than 1 or 2 mm. The senses of touch and smell are far more active than that of sight. A bean-pod enclosed in a narrow glass case and placed before a hungry snail was not noticed, but when taken out of the case and placed 8 cm. behind the snail, the latter at once turned towards it to devour it.
Some interesting experiments were conducted by the same author with the view of ascertaining whether snails avoid or court the light. He placed a number of species in different wooden boxes, which were divided into a light and a dark compartment, having previously well soaked the boxes in water to secure a humid atmosphere and surface, and so induce the snails to move about. The result showed that nearly all species have a marked predilection one way or the other, but not all in the same way. Helix aspersa, Arion empiricorum, six species of Limax, and three of Planorbis, are lovers of darkness, while H. nemoralis, Succinea putris, and two species of Limnaea are lovers of light. Physa fontinalis stands alone in being quite indifferent.
M. Willem endeavoured further to discover whether any of the Mollusca possessed ‘dermatoptic perception,’ or the faculty of perceiving variation of light by means of the skin alone. He accordingly repeated the above-mentioned experiments, having previously extirpated the eyes in all cases. The result was remarkable. In a few instances the experiment was not conclusive, but H. aspersa, A. empiricorum, several species of Limax, and one Limnaea shunned or sought the light just as they had done when their eyes were present. A few marine Mollusca (Littorina littorea, Trochus cinerarius, T. umbilicatus, Patella vulgata) were also shown to be exceedingly sensitive to the impact of a shadow, whether with or without their eyes.
Blind and Eyeless Mollusca.—In a large number of marine Mollusca which habitually creep about half buried in wet sand (Bullia, Sigaretus, Scaphander, Philine), eyes are altogether absent. In some species of Natica and Sigaretus, and in Doris, eyes are developed, but are enclosed in a thick layer of skin, through which they can probably do little more than faintly appreciate different degrees of light and darkness. Chiton has cephalic eyes in the embryo, but loses them in the adult stage. The two great Auricula, A. auris Judae and A. auris Midae, which habitually creep about in the liquid mud of mangrove swamps, have entirely lost their eyes. Certain pelagic Mollusca seem to have a tendency, which is not easily explained, to lose their eyes or the power of seeing with them. Thus Ianthina has no eyes at all. Pteropoda as a rule have no eyes, and the few that have (Creseis, Cavolinia) possess only certain pigmented spots placed near to the nervous centres. In the Heteropoda, however, and the Cephalopoda, many of which are pelagic, the eyes are unusually large.
Fig. 91.—Sigaretus laevigatus Lam., a species frequenting wet sand, and destitute of external eyes: F, anterior portion of foot. (After Souleyet.)
Eyes in Deep-sea and Underground Mollusca.—Deep-sea Mollusca, as a rule, possess no visual organs, or possess them only in a rudimentary state, but this rule has its exceptions. Dr. Pelseneer found[289] no trace of eyes in two species of Pleurotoma from 1850 and 1950 fath., none in a Fossarus from 1400 f., none in a Puncturella from 1340 f. A remarkable form of Voluta (Guivillea) from 1600 f. possessed eyes which could hardly be functional, as they were destitute of pigment, and exhibited other changes of structure. On the other hand, it is remarkable to notice that in three different species of Trochus from 450 f., 565 f., and 1375 f., the eyes were pigmented and well developed.
In land Mollusca which live beneath the surface of the ground or in absolute darkness, the eyes are generally more or less modified. Thus in Testacella, which usually burrows deeply in the soil, but occasionally emerges into the open air, the eyes are very small, but distinct and pigmented. Our little Caecilianella acicula, which is never seen above the surface, is altogether destitute of eyes. A species of Zospeum, a Helix, and a Bithynella from dark caves in Carniola have suffered a similar loss. On the other hand, a small Hyalinia from a dark cave in Utah (probably a recent addition to the cave fauna) has the eyes normally developed.
Eyes of Onchidium.—Many species of Onchidium, a naked land pulmonate which creeps on rocks near high-water mark, are provided with dorsal eyes of various degrees of organisation, and in numbers varying up to nearly one hundred. The tropical Onchidium are the prey of a fish (Periophthalmus) which skips along the beach by the aid of its large ventral fins, and feeds principally on insects and Onchidium. Karl Semper suggests[290] that the eyes are of service to Onchidium as enabling it to apprehend the shadow of the approaching Periophthalmus, and defend itself by suddenly contracting certain glands on the skin and expressing a liquid secretion which flies into the air like shot and frightens the Periophthalmus away. This theory for it is no more than theory—may or may not be true, but it is remarkable that Onchidium with dorsal eyes have precisely the same geographical distribution as Periophthalmus, and that where no Periophthalmus exists, e.g. on our own S.W. coasts, the Onchidium are entirely destitute of dorsal eyes. In those species of Onchidium which have no dorsal eyes, the latter are on the tips of the tentacles, as in Helix. The eyes are developed on the head, and afterwards ascend with the growth of the ommatophores, while in Helix the ommatophores are formed first, and the eyes developed upon them.[291]
Dorsal Eyes in the Chitonidae.—The remarkable discoveries of Moseley with regard to the dorsal eyes of Chiton were first published in 1884.[292] He happened to notice, while examining a specimen of Schizochiton incisus, a number of minute black dots on the outer surface of the shell, which appeared to refract light as if composed of glass or crystal. These ‘eyes,’ in all the species of Chiton yet examined, are restricted to the outer surface of the exposed area of the shell, never being on the laminae of insertion or on the girdle. In certain sub-genera of Chiton the eyes are scattered irregularly over the surface, in others they are arranged symmetrically in rows diverging from the apex of each plate, but in old specimens the eyes towards the apices are generally rubbed off by erosion or abrasion. Moseley regarded the occurrence of scattered eyes as indicating an original stage of development, when the eyes were at first disposed irregularly all over the surface of the shell; the gathering into regular rows showing a later stage.
Fig. 92.—Dorsal eyes of Chitonidae, showing the various forms of arrangement in the first and fourth valves of 1, 1a, Acanthopleura spinigera Sowb., E. Indies, × 2; 2, 2a, Tonicia suezensis Reeve, Suez, × 3; 3, 3a, Acanthopleura granulata Gmel., W. Indies, × 2; 4, 4a, Tonicia lineolata Fremb., Chili, × 2. From specimens in the Museum of Zoology, Cambridge.
The eyes appear to be invariably more numerous on the anterior plate. Thus in Corephium aculeatum there are about 12,000 in all, of which more than 3000 are on the anterior plate. In Schizochiton they are arranged in very symmetrical rows, six of which are situated on the anterior, and only two, sometimes only one, on the central plates. In Tonicia marmorata the eyes are sunk in little cup-shaped depressions of the shell, possibly to escape abrasion. As regards shape and size, in Ch. incisus they are circular, and about 1/35 inch in diameter, this being the largest size known; in Ch. spiniger and Ch. aculeatus they are oval, measuring about 1/100 x 1/600 inch. There are no eyes in Chiton proper, nor in Mopalia, Maugeria, Lorica, and Ischnochiton.[293] None of our English species appear to possess them.[294]
Eyes in Bivalve Mollusca.—Some, possibly most, of the Pelecypoda possess, in the larval state, true paired eyes at the oral end of the body. These become aborted as the animal develops, since that part of the body becomes entirely screened from the light by the growth of the shell. To compensate for their loss, numerous ocelli, or pigmented spots sensitive to the action of light, are in many cases developed on different parts of the mantle, functionally corresponding to the ‘eyes’ of Chiton described above. As in Chiton, too, we have here an interesting series of instances in which true eyes have suffered total obliteration, through disuse, and, as if to restore to the animal in some measure its lost sense, visual organs of a low power have subsequently been developed and are now observed in various stages of specialisation.
Concentration of Eyes in Special Parts of the Mantle.—Sharp has shown[295] that in several species of Ostrea, Cardium, Anomia, Lima, Avicula, Arca, and Tellina pigmented cells, with a highly refractive cuticle, are scattered over a considerable portion of the mantle. Experiment has proved the powers of ‘vision,’ i.e. of sensitiveness to different degrees of light, possessed by these organs. In Dreissena polymorpha, Tapes decussatus, and two species of Venus these cells are concentrated on that particular part of the mantle which is not always covered by the shell, i.e. the siphon, but since the siphon can be completely retracted within the shell, there is no special provision for their protection. A further step is shown in the case of Mya arenaria, where the siphon is scarcely capable of complete retraction. Here, while some of the pigment cells are scattered about over the surface of the siphon, the majority are placed in grooves at the base of the siphonal tentacles, forming an intensely black band round them. A higher stage still is shown in Solen vagina, S. ensis, and Mactra solidissima, where the cells are situated only in the siphonal grooves, which are more or less specialised in numbers and complexity.
Arca Noae, according to Patten, is very sensitive to any sudden change in the amount of light falling upon its mantle-edge. A faint shadow cast upon it by the hand is sufficient to cause it to close its valves quickly, but always one or two seconds afterwards, the promptitude in all cases depending upon the depth of the shadow. Sensitiveness in this direction was found to depend greatly upon the vitality of the animals themselves, since it always became less in those specimens which had been kept for long in confinement. A shadow was not always necessary to make them close. An ordinary black pencil, if approached within two or three inches with extreme caution, produced the same result, while a glass rod brought within the same distance, and even moved rapidly to and fro, appeared to cause no alarm. Sensitiveness to change in intensity of light was experimentally noticed by the same author in the case of Ostrea, Mactra, Avicula (to a special extent), and Cardium. It is very remarkable to find that increased elaboration in the structure of the eyes does not necessarily carry with it increased sensitiveness, i.e. higher visual powers. Avicula, which is only provided with a few scattered ommatidia, which would entirely escape the notice of any one who had not seen them better developed elsewhere, was considerably more sensitive to light and shade than Arca, with its eyes of conspicuous size and much more perfect organisation, instantly contracting the mantle upon the impact of a shadow so faint as to be invisible to the experimenter.[296]
Visual Faculties of Solen and Ostrea.—The visual power of Solen may be exemplified by any one who is walking along almost any of our sandy bays at extreme low-water mark. If the day be warm and sunny, numbers of Solen will be seen raising themselves an inch or two out of their holes; but if you wish to catch them you must approach very cautiously, and on no account allow your shadow to fall upon them, or they will pop down into their burrows in an instant, and it is vain to attempt to dig them out. ‘How sensitive,’ remarks Mr. W. Anderson Smith, with reference to oysters,[297] ‘the creatures are to the light above them; the shadow [of the boat] as it passes overhead is instantaneously noted, and, snap! the lips are firmly closed.’
Ocelli of Pecten.—In Pecten and Spondylus the ocelli are remarkably large and prominent, shining like precious stones, and are placed along the two edges of the mantle so as to receive the light when the shell gapes (Fig. 93). In Pecten opercularis, jacobaeus, and maximus their number varies from 80 to 120. In Spondylus gaederopus, a very inequivalve shell, 60 have been counted on the right or fixed valve, and 90 on the left or upper valve. Each ocellus is connected, by means of its optic nerve, with the large circumpalleal nerve, and so with the branchial ganglion. They possess a cornea, lens, choroidea, and optic nerve, and, according to Hickson,[298] bear a considerable resemblance to the vertebrate type of eye. In spite of this, the power of vision in these genera does not appear at all superior to that of other Pelecypoda.
Fig. 93.—Pecten opercularis L., showing the ocelli on the two edges of the mantle.
Fig. 94.—Compound eyes (c.e) of Arca barbata L.; m.l, mantle fold; omm, ommatidia. (After Patten.)
According to the elaborate investigations of Patten, the ‘eyes’ in Arca occur upon the middle or ‘ophthalmic’ fold of the mantle-edge, which is thickened at the end to admit of their reception. Along this is ranged a row of dark brown spots of various sizes, which are larger at the anterior and posterior ends of the mantle-edge, but smaller and more numerous towards the middle. These brown spots, or ‘eyes,’ are many of them compound, being made up of the fusion of a number of ommatidia (from 10 to 80) into one large round eye, which is generally elevated above the surface of the surrounding epithelium. Sometimes these eyes themselves tend to fuse together. In one specimen of Arca Noae, 133 of these faceted eyes were counted in one mantle border, and 102 in the other.
There can be little doubt that the development of these functional eyes, or sensitive spots, in bivalve Mollusca, is due to special needs. They appear to be entirely absent in fresh-water bivalves (with the exception of Dreissensia, which is obviously a marine genus recently become fresh-water), while they are most abundant in genera living between tide marks (Solen, Mya, Mactra), and most highly specialised in a genus that is, for a bivalve, of singularly active habits (Pecten). Now genera living in sand between tide marks, as the three above-mentioned genera are in the habit of doing, and also protruding their siphons, and occasionally a considerable portion of their shells, out of their burrow, are manifestly very much at the mercy of their watchful enemies the gulls, and anything which would enable them to apprehend the approach of their enemies would be greatly to their advantage. Here, perhaps, lies the explanation of the greater elaboration of these pigmented spots in littoral genera, as compared with those inhabiting deeper water. Pecten, again, a genus distinguished by great activity, which can ‘fly’ for considerable distances in the water by flapping its valves together and expelling the water from the apertures at either side of the hinge, may be greatly assisted by its ocelli in directing its flight so as to escape its enemies.
The sense of smell—touch at a distance, as Moquin-Tandon has called it—is probably the most important sense which the Mollusca possess, and is unquestionably far more valuable to them than that of sight. Any one who has ever enjoyed the fun of hauling up lobster pots will recollect that part of the contents was generally a plentiful sprinkling of Buccinum, Nassa, and Natica, attracted by the smell of the stinking piece of fish with which the trap was baited. According to Mr. J. S. Gibbons,[299] Bullia rhodostoma congregates in hundreds on gigantic medusae which are stranded on the sandy bays near the Cape of Good Hope. Dr. J. G. Jeffreys says[300] that quantities of the common Neptunea antiqua “are procured on the Cheshire coast by the fishermen placing a dead dog on the sands at low-water mark during spring tides. The bait is then completely covered with stones, which are piled up like a cairn. On the next turn of the tide the heap of stones is visited, and the whelks are found on the surface in great numbers, having been apparently attracted by the smell of the bait, but unable to get at it.” Mr. W. A. Lloyd kept specimens of Nassa reticulata in a tank in the sand, at the bottom of which they usually remained buried. If a piece of meat of any kind were drawn over the sand, the Nassa would appear above the surface in a few minutes. Half-picked beef or mutton bones, if placed in the tank, were covered in a few minutes. In fact, no animal matter, whether living or dead, could be introduced without the Nassa smelling it, and coming up to see what they could get.[301]
Any one can experiment for themselves on the olfactory powers of our common snails or slugs. Moquin-Tandon records[302] two interesting cases, one communicated to him by letter, the other occurring to himself. His correspondent, a M. Parenteau, was one day walking along a dusty high-road, when he noticed, near the middle of the road, an empty bean-pod and two Arions eating it. Attributing the meeting of feeders and food to mere chance, he was walking on, when he noticed a second bean-pod, and, about two yards away from it, a third Arion, hurrying straight towards it. When the Arion had yet more than a yard to traverse, M. Parenteau picked up the bean and put it in his pocket. The Arion stopped, raised its head, and turned in every direction, waving its tentacles, but without advancing. M. Parenteau then carried the bean to the other side of the road, and put it in a small hole behind a piece of stone. The Arion, after a moment’s indecision, started off straight for the bean. Again the position of the precious morsel was changed, and again the Arion made for it, this time without being further tantalised. M. Moquin-Tandon noticed, one rainy day in the botanical gardens at Toulouse, two Limax maximus approaching a rotten apple from different directions. He changed the position of the apple several times, placing it at a sufficient distance, to be sure they could not see it, but they always hit it off correctly, after raising their heads and moving their long tentacles in every direction. It then occurred to him to hold the apple in the air, some centimetres above the head of the Limax. They perceived where it was, raised their heads and lengthened their necks, endeavouring to find some solid body on which to climb to their food.
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.
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]
The otocysts are always paired, and, in Gasteropoda, are placed close to the pedal ganglia. The acoustic nerve, however, has been shown by Lacaze-Duthiers to connect with the cerebral ganglia in certain cases. The otocysts are never on the surface of the body and are rarely connected with it by any passage or tube; it is probable therefore that sound reaches them simply through the medium of the tissues.
In the Pelecypoda the otocyst is similarly situated near the pedal ganglion, and is probably (though this has not yet been proved) similarly connected with the cerebral. There is only a single otolith. Pelseneer finds[308] in Nuculidae alone a free communication between the otocyst and the exterior. Anodonta has been observed[309] to withdraw its foot into the shell at the noise of an opening door, a loud voice, or a shrill whistle, whether in a basin of water or lying on a study table.