CHAPTER VII

ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE FOOT--THE NERVOUS SYSTEM

_Organs of Sense_: I. _Touch_

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.

[Illustration: 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.

[Illustration: FIG. 82.--_Monodonta canalifera_ Lam., New Ireland, showing mantle lobes. (After Quoy and Gaimard.)]

[Illustration: 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 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.

[Illustration: FIG. 84.--_Idalia Leachii_ A. and H., British seas; _br_, branchiae. (After Alder and Hancock.)]

[Illustration: 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.

II. _Sight_

=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.

[Illustration: 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.

[Illustration: 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.

[Illustration: 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.

[Illustration: 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.)]

[Illustration: 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.

[Illustration: 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.

[Illustration: FIG. 92.--Dorsal eyes of _Chitonidae_, showing the various forms of arrangement in the first and fourth valves of 1, 1_a_, _Acanthopleura spinigera_ Sowb., E. Indies, × 2; 2, 2_a_, _Tonicia suezensis_ Reeve, Suez, × 3; 3, 3_a_, _Acanthopleura granulata_ Gmel., W. Indies, × 2; 4, 4_a_, _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 1/400 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.

[Illustration: FIG. 93.--_Pecten opercularis_ L., showing the _ocelli_ on the two edges of the mantle.]

[Illustration: 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.

III. _Smell_

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.

[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_

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.

[Illustration: FIG. 96.--Illustrating the otocyst in =A=, _Anodonta_, =B=, _Cyclas_; _ot_, otolith; _a_, _b_, _c_, _c´_, cellular layers surrounding the chamber; _ci_, cilia on interior walls of chamber: =C=, an otolith crushed. (After Simroth.)]

Delage extirpated the otocysts in certain Octopoda, and obtained some unexpected results. He found that remarkable effects were produced upon the animal’s powers of locomotion, so that it was unable to preserve its proper balance in the water when in rapid motion, but its body was forced to undergo a form of rotation more or less pronounced. He concluded that the otocysts must possess, besides their auditory functions, a power which stands in some relation to the proper orientation of the body in locomotion, a power which is not wholly supplied by sight and touch alone. The otocysts may thus regulate locomotion by stimulating muscular acts which tend to keep the body in the straight line during the process of movement.[310]

=The Foot=

One of the most characteristic organs of the Mollusca is the foot, which, under one form or another, occurs throughout the whole phylum. The foot is a thickening, on the ventral side, of a portion of the integument of the animal, modified to serve different forms of motion. It attains its maximum relative area in the Chitonidae, many Nudibranchs, and the slugs generally, in nearly all of which there is no portion of the body which is not subtended by the foot. Here too it presents the form of a regular disc or ellipse, which is more or less produced. In many cases, however, the foot becomes modified in such a way that we are enabled to recognise well-marked anterior and posterior portions, which have received the name of _propodium_ and _metapodium_ respectively, while the intervening central portion is termed the _mesopodium_.

[Illustration: FIG. 97.--_Sigaretus laevigatus_ Lam., showing excessive development of the propodium (_pr_) and metapodium (_met_) in a mollusc living in sand (the shell, which covers only the liver and adjacent parts, has been removed); _l_, liver; _s.ap_, aperture of proboscis, here deflected from the median line; _t_, _t_, tentacles. (After Quoy and Gaimard.)]

The propodium is most strongly developed in genera which crawl about in wet sand, _e.g._ _Natica_, _Sigaretus_, _Oliva_, _Harpa_, _Scaphander_ (Figs. 97 and 98, and compare Fig. 91). In such cases it seems to serve as a sort of fender or snow-plough, to push the sand away on both sides of the path the animal is traversing. In some species of _Sigaretus_ the propodium becomes as it were banked up against the head and proboscis, which are thus unnaturally elevated, or tend to disappear altogether. _Bullia_ (Fig. 62), which crawls about rapidly on wet sand, appears to attain its object by a wide extension of the foot on all sides, and so slides over the sand instead of ploughing through it; the little lappets at the end of the ‘tail’ probably serve as rudders.

In _Melampus_ and _Pedipes_ the propodium is marked off by a groove across the ventral surface. When the animal is in motion it first advances the propodium and then pulls the rest of the foot after it with the looping gait of certain caterpillars. In many Cyclostomatidae this groove, instead of being transverse, is longitudinal, and the animal advances first the right and then the left segment of the foot, which gives it a swaying motion from side to side.

Upon the metapodium lies the operculum, when it occurs. As a rule the metapodium is not sharply marked off from the rest of the foot. In _Strombus_ (Fig. 99) it becomes erected into a sort of hump or column, on the top of which the operculum is situated.

[Illustration: FIG. 98.--_Oliva textilina_ Lam., showing how the front part of the foot (_f_) is developed into a sort of fender, the propodium (_pr_); _e_, _e_, eyes; _m.ap_, front appendage of mantle; _m.ap´_, hinder appendage of mantle, folded into the suture when the animal is at rest; _si_, siphon; _t_, _t_, tentacles. (After Quoy and Gaimard.)]

The _epipodium_ is a prominent fold or border, which occurs upon the upper edge of the foot in most Diotocardia. In _Haliotis_ it is of considerable breadth, and is covered by a number of lobes which spring from a moss-like prolongation of the skin. From the epipodium are developed the lateral tentaculae of _Monodonta_ (Fig. 82, p. 178), and of other sub-genera of the Trochidae.[311]

In the Opisthobranchiata the lateral edges of the foot (the _parapodia_) are frequently produced into broad folds or wing-like extensions, which in many cases tend to fold over the shell, and, in conjunction with the mantle, eventually imbed it altogether. By the wavy motion of the parapodia the animal is enabled to progress through the water. The paired natatory lobes of the Pteropoda are simply the parapodia of the Tectibranchs modified for swimming purposes.

[Illustration: FIG. 99.--_Strombus lentiginosus_ Lam., showing the modified form of the foot (_f_): _e_, _e_, eyes on their pedicels; _mp_, metapodium; _op_, operculum; _p_, penis; _pr_, proboscis; _t_, _t_, tentacles. (After Quoy and Gaimard.)]

It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda, groups which have, for the most part, exchanged a crawling for a swimming life, that the modifications of the foot are most considerable. In _Oxygyrus_ and _Atlanta_, for instance, the propodium and metapodium are sharply distinguished from the mesopodium, and no doubt have acquired, as a means of propulsion, the power of separate movement, the animal swimming with these portions of the foot uppermost. In _Carinaria_ and _Pterotrachea_ the metapodium has probably become continuous with the long axis of the body, while the so-called ‘foot’ with its sucker represents only the original propodium. In the Cephalopoda the arms and funnel represent the modified foot, the sides of which are prolonged into a number of very long specialised tentaculae. In the adult Cephalopod some of the arms have assumed a position in advance of the mouth, the latter being in fact surrounded by a circle of arms. But in the Cephalopod embryo the mouth opens as in the Gasteropoda, _i.e._ in advance of the arms, and it is only gradually that it becomes encircled by them. Arms and funnel alike are found to be innerved from the pedal ganglion.[312]

The pointed axe-shaped foot, which is characteristic of the majority of Pelecypoda, is doubtless derived from a form more akin to the flattened ‘sole’ of the Gasteropoda. A foot with something of this disc-shaped base actually occurs in some of the Nuculidae, the parapodia being furnished with pleats which recall similar formations in other Orders (Fig. 100). The principal modifications of the foot are due to its employment as a burrowing organ. In genera which burrow but slightly it is small and feebly developed, while in genera which habitually excavate, it becomes the largest and strongest organ of the body. At the same time it has a tendency to shift its position from the ventral to the anterior margin, accompanied by a corresponding narrowing of the shell, until it arrives at the position seen in Mollusca of the shape of _Mya_, _Pholas_, and _Solen_. In sedentary or attached genera, _e.g._ _Pecten_, _Chama_, _Ostrea_, the foot tends to become aborted.

[Illustration: FIG. 100.--_Yoldia limatula_ Say, Greenland, showing the short plumed branchiae (_br_, _br_), the gasteropodous foot (_f_), and the large labial palps (_l.p_, _l.p_): =A=, as seen from the ventral margin; =B=, from the left side, with the mantle turned back; _a.m_, position of anterior adductor muscle; _i_, intestine; _l_, liver; _m_, _m_, mantle.]

The _byssus gland_, secreting a byssus of horny threads, is characteristic of many Pelecypoda, and may be observed by any one in the common mussel. It occurs in the larvae of many species which do not possess a byssus in the adult stage. The pedal gland of many Prosobranchiates, which secretes a tough and almost thread-like slime, is possibly homologous with the byssus gland of bivalves.

=Nervous System=

The Mollusca possess a nervous system, which usually consists of a number of nerve centres or _ganglia_, linked together by bands (the _commissures_) and sending out thread-like nerves which ramify into the various organs. The character of the nervous system varies greatly in different groups, ranging as it does from a condition of extreme complexity, in which the ganglia are numerous and the commissures equally so, to that of considerable simplicity, in which ganglia are almost entirely absent.

The most important ganglia are (1) the _cerebral_,[313] which are always placed above or on either side of the mouth, and from which proceed the nerves of the eyes and tentacles; (2) the _pedal_, which in Gasteropoda are situated below the oesophagus, in Pelecypoda at the base of the foot, and from which the nerves of the foot and sometimes the acoustic nerve arise; (3) the _pleural_,[314] whose position varies considerably, but is always below the oesophagus and slightly above the pedal ganglia; these innervate the mantle, branchiae, heart, and viscera generally.

=Gasteropoda.=--The simplest form of nerve system as thus understood occurs in the Amphineura, and more particularly in the Chitons. Here we find four longitudinal nerve-cords, parallel to one another for nearly the whole length of the mollusc. The two exterior cords probably represent the pleural, the two interior the pedal nervous system. There being no head or tentacles, but simply a mouth at the anterior end, the cerebral ganglia do not exist, but they are represented by the curved ring formed by the massing together of the two nerve-cords on each side. The only distinct ganglia are a pair of buccal ganglia (which are developed on a pair of commissures which pass forward from the cerebral mass and innervate the lips and buccal region), and a much smaller group, the sublingual. The two pedal cords are united by a number of transverse parallel connectives, which recall similar modes of connexion in the Chaetopod worms and in Arthropoda.

This quadruple set of nerve-cords is characteristic of all the Amphineura, but the absence of ganglia is most marked in the Chitons. In _Proneomenia_ and _Neomenia_ there is a distinct cerebral ganglion, formed by the massing of the two ganglia into one, while in _Proneomenia_ the lateral cords are joined to the pedal, as well as the pedal to one another, by connectives. In _Chaetoderma_ the cerebral ganglia, though adjacent, are distinct, and both the pedal and lateral cords connect directly with them, while there are no transverse connectives.

The remaining three great divisions of Gasteropoda, namely, the Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as comprising two distinct types of nervous condition, according as the loop formed by the two visceral nerve-cords is twisted over itself, forming a figure of 8, or continues straight and uncrossed. In the former case, we get the condition known as _streptoneurous_, in the latter that as _euthyneurous_.[315] The _Euthyneura_ include the whole of the Opisthobranchiata[316] and Pulmonata, the _Streptoneura_ all the Prosobranchiata.

[Illustration: FIG. 101.--Nervous system of the Amphineura: =A=, _Proneomenia_; =B=, _Neomenia_; =C=, _Chaetoderma_; =D=, _Chiton_; _c_, cerebral ganglia; _l_, _l_, lateral cords; _pc_, posterior commissure; _s_, sublingual commissure or ring, with ganglia; _v_, _v_, pedal cords. (Alter Hubrecht.)]

The simplest form of nervous system in the euthyneurous Gasteropoda occurs in the Opisthobranchiata. The cerebral, pleural, and pedal ganglia tend to become concentrated in a ring-like form, united by short commissures at the posterior end of the pharynx. The visceral loop is in some cases long, and the two or three visceral ganglia are then situated at its posterior extremity. The nervous system of the Pulmonata is of a similar type, the visceral loop being often much shorter, and tending to draw in towards the central group of ganglia. The tentacular and optic nerves are, as usual, derived from the cerebral ganglion, with which also the octocysts are probably connected by rather long nerves. A pair of buccal ganglia innervate the buccal mass, and are united by commissures with the right and left cerebral ganglia. The osphradial nerve springs from one of the ganglia on the visceral loop, the osphradium itself being situated (in _Limnaea_) immediately above the pulmonary orifice and adjacent to the anus (Fig. 102). This massing of the ganglia is still better illustrated by the accompanying figure of _Physa_ (Fig. 103), in which the animal is represented as if transparent, so that the ganglia and nerves are seen through the tissues.

[Illustration: FIG. 102.--I. Nervous system of _Limnaea stagnalis_ L. The oesophagus has been cut and pulled forwards through the nerve-collar, so as to expose the lower surface of the buccal mass(dissected by F. B. Stead).

=B.M=, buccal mass.

=B.G=, buccal, =C.G=, cerebral, =Os.G=, osphradial, =Pe.G=, pedal ganglia.

=Pl.G=, pleural ganglia.

=Op.N=, optic, =Os.N=, osphradial, =Te.N=, tentacular nerve.

=Ot=, otocyst; =V.L=, visceral loop.

=R=, rectum, dotted in to show its position relative to the osphiadium.

II. Right side of the head of _Limnaea stagnalis_. The overhanging flap of the mantle has been cut in the middle line, and the right half twisted back, so as to expose the pulmonary orifice, etc. The points =A= =A= on the mantle edge were continuous before the mantle was cut; the line =BA= is part of the free edge of the mantle.

=An=, anus; =F=, female generative orifice; =J=, portion of jaw; =M=, male generative orifice under right tentacle; =Os=, osphradium; =P.O=, pulmonary orifice.]

Of the streptoneurous Gasteropoda, the nervous system of _Fissurella_ and _Haliotis_ shows distinct points of similarity to that of the Amphineura. The pedal nerves are united by transverse commissures throughout their entire length, while a double commissure unites the cerebral ganglia to the mass from which the pedal nerves proceed. In the great majority of the Streptoneura the ganglia (except the visceral) are more concentrated and the commissures are consequently much shorter. The accompanying figure of _Cyclostoma_, in which the animal is represented as in that of _Physa_ just described, illustrates this grouping of the ganglia, the twist of the visceral loop, and the position of the visceral ganglia at its posterior end.

[Illustration: FIG. 103.--Nervous system of _Physa acuta_ Drap., showing the massing of the ganglia at the hinder end of the pharynx: _e_, _e_, eyes; _m_, mouth; _m.l_, _m.l_, mantle lappets; _o.f_, female generative orifice; _o.m_, male generative orifice; _os_, osphradium. (After Lacaze-Duthiers.)]

[Illustration: FIG. 104.--Example of a streptoneurous Gasteropod (_Cyclostoma elegans_ Drap.): _c.g_, _c.g_, cerebral ganglia; _e_, _e_, eyes; _os_, osphradium; _ot_, _ot_, otocysts; _p.g_, _p.g_, pedal ganglia; _pl.g_, _pl.g_, pleural ganglia; _sp.g_, supra-intestinal ganglion; _sb.g_, sub-intestinal ganglion; _t.n_, tentacle nerve; _v.g_, visceral ganglion. (After Lacaze-Duthiers.)]

=Scaphopoda.=--In the Scaphopoda the nervous system resembles that of the Pelecypoda. The cerebral and pleural ganglia lie close together, while the pedal ganglia are placed in the anterior part of the foot, connected with the cerebral ganglia by long commissures; the visceral loop is rather long, and the two visceral ganglia are adjacent to the anus.

=Pelecypoda.=--The nervous system in the Pelecypoda is the simplest type in which well-marked ganglionic centres occur. The ganglia are few, symmetrically placed, and are usually at a considerable distance apart. There are, as a rule, three distinct pairs of ganglia, the cerebral (cerebro-pleural), pedal, and visceral. The cerebral are formed by the fusion of the cerebral and pleural ganglia, which, however, in some cases (Protobranchiata) continue distinct.[317] They lie above or on each side of the mouth, united by a commissure of varying length. Another pair of commissures unites them with the pedal ganglia, which are placed at the base of the foot, and are usually very close together, sometimes (as in _Anodonta_) becoming partially fused. The length of these commissures depends upon the distance between mouth and foot; thus they are very long in _Mya_ and _Modiola_, and very short in _Pecten_. In cases where the foot is rudimentary or becomes aborted through disuse (_e.g._ _Ostrea_), the pedal ganglia may dwindle or disappear altogether. The visceral ganglia are on the ventral side of the posterior adductor muscle, beneath the rectum, and innervate the branchiae, osphradia, and the whole of the visceral sac. A pair of cerebro-visceral commissures traverses the base of the foot, surrounding it with a comparatively short loop (compare Fig. 106, _c.v.c´_), while a long commissure, which runs round the entire edge of the mantle, and supplies branching nerves to the mantle border and siphons (Fig. 106, _c.v.c_), may also connect the visceral and cerebral ganglia.

[Illustration: FIG. 105.--Nervous system of Pelecypoda: _A_, _Teredo_; _B_, _Anodonta_; _C_, _Pecten_; _a_, _a_, cerebral ganglia; _b_, pedal ganglia; _c_, visceral ganglia. (After Gegenbaur.)]

=Cephalopoda.=--In the Cephalopoda the concentration of ganglia attains its maximum, and may perhaps be regarded as approaching the point at which a definite brain may be said to exist. Another point of distinction is the formation of special small ganglia upon the nerve-cords in different parts of the body. In the Tetrabranchiata (_Nautilus_) the cerebral and pedal ganglia form a broad ring which surrounds the oesophagus, the former giving out the optic nerves, with their special optic ganglion, and a pair each of buccal and pharyngeal ganglia, the latter the nerves of the arms and funnel. The visceral loop is still present in the form of a separate band, which innervates the branchiae, osphradia, and viscera generally, forming a special genital ganglion in connexion with the reproductive organs. The principal ganglia of the Dibranchiata are still more concentrated, even the visceral loop being possibly united with the rest in forming an unbroken mass in which scarcely any trace of commissures can be detected. The pedal ganglion becomes separated into two portions, one of which innervates the arms, the other the funnel. Two peculiar ganglia (the stellate ganglia) supply a number of branching nerves to the mantle.

[Illustration: FIG. 106.--Nervous system of _Cardium edule_ L.: _a.m_, anterior adductor muscle; _br_, branchiae; _br.n_, branchial nerve; _c.g_, _c.g_, cerebral ganglia; _c.p.c_, cerebro-pedal commissure; _c.v.c’_, cerebro-visceral commissure; _c.v.c_, cerebro-visceral commissure of mantle; _l.p_, labial palps: _m_, mouth; _p.g_, pedal ganglion; _p.m_, posterior adductor muscle; _v.g_, visceral ganglion. (After Drost, × 3.)]

* * * * *

=E. L. Bouvier=, Système nerveux, morphologie générale et classification des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), iii. 1887, pp. 1–510.

=J. Brock=, Zur Neurologie der Prosobranchier: Zeit. wiss. Zool. xlviii. 1889, pp. 67–83.

=O. Bütschli=, Bemerkungen über die wahrscheinliche Herleitung der Asymmetrie der Gasteropoda, etc.: Morph. Jahrb. xii. 1886, pp. 202–222.

=B. Haller=, Zur Kenntniss der Muriciden. I. Anatomie des Nervensystems: Denksch. Math. Nat. Kl. Ak. Wien, xlv. 1882, pp. 87–106.

„ Untersuchungen über marine Rhipidoglossen. II. Textur des„ Centralnervensystems und seiner Hüllen: Morph. Jahrb. xi. 1885, pp. 319–436.

=H. Grenadier=, Abhandlungen zur vergleichenden Anatomie des Auges: Abh. Naturf. Gesell. Halle, xvi. 1884, pp. 207–256; xvii. 1886, pp. 1–64.

=A. P. Henchman=, The Origin and Development of the Central Nervous System in _Limax maximus_: Bull. Mus. C. Z. Harv. xx. 1890, pp. 169–208.

=V. Hensen=, Ueber das Auge einiger Cephalophoren: Zeit. wiss. Zool. xv. 1865, pp. 157–242.

=C. Hilger=, Beiträge zur Kenntniss des Gasteropodenauges: Morph. Jahrb. x. 1885, pp. 352–371.

=Lacaze-Duthiers=, Otocystes ou Capsules auditives des Mollusques (Gastéropodes): Arch. Zool. Exp. Gén. i. 1872, pp. 97–166.

„ „ Du système nerveux des Mollusques gastéropodes pulmonés aquatiques: _ibid._ pp. 437–500.

=P. Pelseneer=, Recherches sur le système nerveux des Ptéropodes: Arch. Biol. vii. 1887, pp. 93–130.

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