TBS BYE OF PBOTEN. 4 4 3

The EYE of PECTEN. By SYDNEY J. HICKSON, B . S C ,Scholar of Downing College, Cambridge. (With PlatesXXXIV and XXXV.)

THE general absence of organs of vision amongst themembers of the class Lamellibranchiata meets with a curiousand interesting exception in the genera Pecten and Spon-dylus.

These genera have long been known to possess a greatnumber of eyes of considerable complexity, situated on theborder of the mantle. The number of these eyes variesconsiderably in different individuals, ranging in the genusPecten from eighty to one hundred and twenty. Theirposition also varies; for, although they are always situatedon the border of the mantle, yet sometimes they are placedat equal distances from one another, and sometimes they areclustered together in certain localities.

Notwithstanding this indefinite element, both in theirnumber and position, which might be expected to runparallel with a primitive and simple organisation, theiranatomy is exceedingly complicated, and exhibits all themost important structural elements of the eyes of the higherVertebrata.

The earliest investigations into the anatomy of Pecten'seye are those of Krohn,1 who gives a drawing of the courseof the optic nerve. This drawing is copied in many of thesubsequent papers on the subject by other investigators, and,as far as it goes, is correct. Duvernoy,2 in his description,of the nervous system of the Pectens, gives a short descrip-tion of the anatomy of the eye. This paper, however, ischiefly valuable for the excellent figures and descriptionsof the distribution of the nerves in the mantle, and thefilaments which are given off from the main trunks of theseto supply the tentacles and the eyes.

The researches of Blanchard8 and of Keferstein4 whichfollowed did not add very much to our knowledge on thissubject, and it was not until 1865 that any careful histo-logical inquiries were carried on. It was Hensen6 who first

1 Krohn, ' Miiller's Archiv,' 1810, p. 301, pi. xi.5 Duvernoy, 'Memoires de 1'Academic do Sciences,' t. xxiv, 1852.

' Memoire sur lc systeme nerveux des Acephales,' p. 73, pi. ii.3 Blanchard, ' Organisation du regne animal: Mollnsqucs Acephales.'4 Kcrfcrstein, 'Zeit. fur wiss. Zoologie,' 1863, p. 133.* licuseu, ' Zeit. fiir wiss. Zoologie,' 1865, p. 220. .

4 4 4 SYDNEY J. HICKSON.

gave figures of the characters of any of the histologicalelements. But as the eye of Pecten forms only a very smallpart of the paper, his figures and description are by nomeans complete, and in many respects they are incorrect.Finally, J. Chatin1 has contributed two short papers, with-out figures, on this subject.

Of the scanty literature Hensen's paper is by far themost important, and he alone gives any good figures ofsections of the eye, or of its elements ; the other observersgive remarkably few figures, and consequently I have had,owing to an imperfect knowledge of the German language,some difficulty in making myself acquainted with the sub-stance of their papers.

I have been encouraged in publishing the following re-searches chiefly by this scarcity of good figures, but alsobecause I believe, and will give my reasons for believing,that these eyes deserve more mention than is usually madeof them in our zoological text-books.

My investigations were chiefly carried on upon Pectenmazimus, but I have also had the opportunity of makingsections of and studying the eyes of two other species,Pecten jacobceus and Pecten opercularis. The eyes of thesethree species differ from one another in one or two notaltogether unimportant particulars, and, as I shall after-wards point out, they form an interesting gradation, thepoints of difference between P. mazimus and P. oper-cularis passing through intermediate stages in P. jacobceus.

The eyes of Pecten mazimus—are situated amidst a numberof tentacles, which run all round the border of the mantle.These tentacles are capable of considerable movement, andfrequently overhang the eyes and protect them from thelight. The eyes themselves are situated upon short stalks,which resemble very closely the basal part of an ordinarytentacle.

This similarity caused Duvemoy to name a tentacle atactile pedicel, and an eye an ocular pedicel, thus to a cer-tain extent implying that they are morphologically homo-logous organs respectively modified for a tactile and anocular function. This homology is justified by certainpoints in their anatomy, such as the course of the nerve andthe arrangement of the muscular fibres, and I believe thatwhen the development of these eyes is studied the homologywill be still further confirmed.

The border of the mantle which bears the tentacles and1 J. Chatin, 'Bulletin de la Societe Philomalique.' Paris, 1877.

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eyes is covered with an epithelium, consisting of columnar,non-ciliated, and slightly granular cells bearing nuclei,situated near the base of the cells. As this epitheliumpasses over the eye-bulbs, it undergoes two interesting modi-fications, tt becomes considerably thicker and filled with adark brown pigment (PI. XXXIV, fig. 1 c) as it passes roundthe sides of the eyes, but immediately in front of theeye (PI. XXXIV, fig. 1 a), it again diminishes in thickness,and becomes perfectly transparent. By thus surroundingthe eye on all sides with a dark-coloured pigment, leavingonly a round spot in front, clear and transparent, the epi-thelium, by limiting the entrance of the light to a smalldiaphragm in front, here performs the function of an iris.The epithelium which runs over this transparent part, andwhich forms the epithelial layer of the cornea, differs fromthe ordinary epithelium covering the rest of the mantle inthat their cells are rather larger, are perfectly transparent inthe living condition, and the nuclei are large and spherical,and situated in the centre of the cells.

The eye consists of the following parts, which I shall nowdescribe in order. The cornea, covered externally by itstransparent epithelium, protects a large elliptical lens.Close up to the lens is the retina, but separated from it bythe optic nerve, which spreads out over the anterior surfaceof the retina. The retina rests upon a tapetum, and behindthis, occupying all the posterior concavity of the eye-cup,there is a red pigment.

The cornea—consists of two parts, the outer epithelium,which has already been described, and a basement mem-brane, consisting of a thin layer of connective tissue. Asbefore stated, this epithelium is merely a modification of thegeneral epithelium of this part of the mantle; and the pig-mented epithelium surrounding the eye-bulbs (in like manner,a modification of the same tissue) is continuous with itall round its edge. The passage of the cells of the pig-mented epithelium into those of the corneal epithelium issignalised by two important changes in the characters of thecells. In the first place the pigment entirely disappears,and the nuclei, which in the former case were obscured bythe pigment, now becomes apparent, and in the second placethe cells are considerably diminished in their longitudinalaxis. The diminution in size of the cells causes the edge ofthe cornea to be sunk below the level of the pigmented epi-thelium ; and a shallow trough #runs round the line of itsjuncture with it (PI. XXXIV, fig". 3). The convexity of thecornea is not great, and the dome of it frequently only just

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446 SYDNEY J. HICKSON.

reaches the level of a line drawn from the highest points ofthe pigmented epithelium on either side of it. This appear-ance is not often seen in sections, as the pigmented epi-thelium rapidly shrinks, when the tissue dies, and undermost reagents ; but I am fully persuaded of the accuracy ofthis statement from an examination of the eyes of livingspecimens of Pecten maximus and sections of Pecten oper-cularis.

The delicate epithelial cells of the cornea, in consequenceof heing entirely unprotected by any membrane similar tothe conjunctiva of the higher animals, are quite naked, andvery liable to injury from the rough edges of the tentacleswhich surround them. The arrangement just described,however, probably prevents the tentacles from coming intoimmediate contact with them. The little trough which runsround the margin of the cornea always contains a littleliquid, even when the eye itself is removed from the water;and the pressure of the tentacles when folding over the eyecauses it to spread out as a thin layer over the cornea, andthus the cells are prevented from coming into immediatecontact with the tentacle.

Thus, the two remarkable modifications, namely, the pre-sence of a large quantity of pigment, and a greater longi-tudinal axis of the cells which the pigmented epitheliumexhibits, are of considerable value to the eye, firstly, to pre-vent very divergent rays from entering, and secondly, to pre-vent any damage to the cornea caused by the rubbing of theadjacent tentacles- over the sides of the eye.

The second layer of the cornea is about half as thick asthe epithelial layer, and, like it, is perfectly colourless andtransparent. It consists merely of a thin continuation ofthe connective tissue of the stalk. It may be called thebasement membrane of the corneal epithelium, as from theabsence of any definite cellular elements its only functionprobably is to support these cells.

Beyond the cornea this membrane becomes much thicker,and supports the pigmented epithelium, and at the sametime structural elements make their appearance in it. Fromthence it passes into the connective tissue of the eye-stalkwithout further modifications.

The lens—is one of the most interesting parts of the eye.It is comparatively large, and is composed of a number ofnucleated cells. In the fact that the lens is formed bymore than one cell the eye* of Pecten bears an interestingresemblance to that of the Vertebrata. The shape of the leush;is been n subject of much dispute amongst the authors

THE EYI! 01? PBCTBN. 447

who have written on this subject. Krohn and Kefersteinbelieved it to be spherical. Hensen has figured it filling upthe space between the cornea and retina, and consequentlyof an irregular bi-convex shape.

It is difficult to see how a controversy on such a simplesubject could have arisen, unless it is because differentauthors have examined different species, and described themfor the genus.

As regards Pecten maximus^n examination of the fresh eyehas convinced me that in this species the lens is elliptical, themajor axis being parallel to the plane of the mantle. A sectionof the eye made in a plane at right angles to the plane of themantle and the direction of its margin—that is, the plimewhich is most convenient for section-cutting, and the onewhich is apparently usually adopted—would consequentlycause the lens to appear circular in section. In the dia-grammatic representation of the eye (fig. 1) I have for con-venience sake represented the lens as being at right anglesto the plane of the mantle in order that the true shape ofthe lens may not be overlooked.

A fresh examination of the lens, when teased out from therest of the eye, exhibits one or two interesting points. Thelens is not, as in most eyes, perfectly colourless, but possessesa well-marked brown colouration, and a number of fine stritemay be seen running in the direction of the major axis.The lens does not appear so perfectly elliptical in the freshcondition as in certain sections I have made; it is drawnout somewhat longitudinally, so as to be more like a doublecone than an ellipse. This is probably due to the lens beingreleased from the ligaments and connective-tissue pressures,which cause it to retain its proper shape.

Hensen says that the lens is very soft, and the cells arelight, polygonal, and nucleated. A careful examinationof the lens of P. maximus has led me to a very different con-clusion. The lens seemed to be of exactly the same natureas in the higher forms, and when teasing it out I found somedifficulty in holding it with a needle, as it slipped away fromunder it when a slight pressure was exerted. As regardsthe shape of the cells composing the lens, they are not a}lpolygonal, as would be inferred from HensenJs remarks onthe subject. In the centre they are polygonal, but as theyapproach the periphery they become more and more flat-tened and elongated, until at the periphery they are strap-shaped. They are nucleated. As Hensen, I could find nomembrane covering the lens, and no muscular fibres con-nected with it; but in a few cases I have observed a liga-

4 4 3 SYDNEY J. HICKSON.

ment, such as I have represented diagrammatically infig. 1 / , which, I believe, forms a support for the lens. Thisligament is usually broken by the action of reagents, andthen hangs down by the side of the cavity, and thus becomesdifficult to observe; at the same time the lens sinks down,and rests upon the anterior surface of the retina.

The lens is suspended in the space which correspondswith the vitreous humour in the higher animals. This spaceis filled with an aqueous humour in Pecten. The lens islarger, and, consequently, the space occupied by aqueoushumour relatively smaller in P . maseimus than it is in eitherP.jacobtBus or P . opercularis, and in P.jacobceus it is largerthan in P . opercularis.

The retina—does not line the concavity of the eye-cup, asit does in most well-developed eyes, but is nearly flat, anda considerable space is left between it and the floor of thecup, which is filled up by the red pigment. In conse-quence of this the retina appears in section to be a thickband crossing the eye from side to side. Thus, just as thelens was remarkable for the way in which it approached theretina by hanging back into the cavity, so the retina is re-markable for the manner in which it leaves the posteriorconcavity of the eye-cup to approach" the centre. The eyeof Pecten, in fact, presents the interesting peculiarity of theapproach of the lens and the retina towards the centre, sothat in P . maximus they almost touch.

The anterior surface of the retina is convex at the sidesand concave in the middle, but these convexities and con-cavities vary in different species. The different layers of theretina will be described from behind forwards, as it will beeasier to trace the transitions in that way than if describedfrom before backwards. They are—1°. Posterior limbs ofthe rods. 8°. Anterior limbs of the rods. 3°. Spindle-shaped nucleated rods. 4°. Molecular and nuclear layer.5°. Nerves.

The posterior limbs of the rods stand upon a membrane,which runs along the posterior side of the retina; at theiranterior ends they pierce a very delicate membrane, and passinto the anterior limbs of the rods. The anterior limbs areabout twice as long as the posterior limbs, and are usuallysmaller in diameter, and situated farther apart than the pos-terior limbs. That they are circular in section may be seenfrom PI. XXXV, fig. 7«, which is a drawing of a sectionmade at right angles to the eye-stalk. The anterior limbsof the rods are sometimes swollen so as to appear oval;

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this condition occurs especially in the rods at the side con-vexities. Pig. 6 represents an isolated rod in this con-dition.

The anterior ends of the rods contract considerably, andagain expand into spindle-shaped bodies, each of which con-tains a nucleus; so that in P.jacobmus, where the retinalelements of this region are difficult to distinguish, theremay be seen a single row of nuclei running from end to endof the retina, and following its sinuosities (Plate XXXV,fig. 11 b).

In some of the rods at the side of the retina a secondspindle-shaped body follows the first one, as represented inthe isolated rods in figs. 5, 6, but usually the anterior endof the spindle is drawn out into a delicate thread, whichoccasionally possesses nuclear swellings. Finally, this threadbreaks up into a network, which bears a number of nuclear-like bodies at its nodes, and several round molecular bodiesappear to be caught in its meshes. These bodies are somuch like the ordinary nuclei of the network that I am in-clined to believe that they are, in reality, merely modifica-tions of them, and in some way connected with the network(fig. 6 a). Anteriorly the fibres of the network bend at rightangles and enter the nerve layer, which covers the anteriorsurface of the retina. This nervous layer will be describedwith the description of the optic nerves.

The above is a description of the retina as I found it inP. maximus, and I believe it holds good for the other mem-bers of the genus. The elements of the retina are so muchlarger in this species, and the spaces between the rods andnetwork, &c, so much more considerable, that it is a greatdeal easier to investigate; but I believe careful examinationof the other species would show that they do not differ fromthis in any important detail.

The tapetum—is placed immediately behind the retina, andmay help in its support. When fresh,1 the tapetum exhibitsa display of colours, and it is this membrane which givesthe eyes their beautiful metallic lustre. When examinedwith a -i-th-inch obj. it seems to be composed of a greatnumber of little black specks separated by a fine yellowmembrane, but careful examination with a higher powershows that it is composed of a great number of fine fibrilscrossing at right angles.

The space between the tapetum and the posterior part of1 I have one series of sections stained in osmic acid, and mounted in

Canada balsam, which has retained this display of colours.

4 6 0 SYDNEY J. HICKSON.

the eye-cavity is filled with a red fluid pigment. In the freshcondition the pigment readily flows on to the slide when theeye is pricked, but in sections of the eye which has beenhardened by alcohol or other reagents the pigment adheresto the tapetum or posterior wall of the eye-cup.

Hensen figures a layer of cells in this position, but I havenever been able to observe any thing of the kind ,• the pigmentcontains no cellular elements at all, nor is there a layer ofcells lining the cavity which contains the pigment. Thepigment consists of a number of bright red granules floatingfreely in a colourless fluid.

The nervous supply—of the eye of Pecten is perhaps themost interesting of the many peculiarities of this eye. Thenervous system of Pecten is well described by Duvernoy inthe paper referred to above. The mantle is supplied by anumber of branches given off from the principal ganglia.These branches all fall into a large nerve, which runs roundthe margin of the mantle, and which Duvernoy calls the" eircumpalial" nerve. This nerve is figured in section infig, 1, PI. XXXIV, one of the nerves joining this nerve beingfigured at fig, 1, q. This " circuinpalial" nerve gives offfilaments to supply the tentacles and eyes.

Krohn first gave a drawing of the optic nerve, and describedit as a single nerve passing off from this trunk, and dividinginto two branches as it aproaches the eye. Later observershave, however, drawn and described two nerves passing offfrom the "circuinpalial" nerve. My researches have ledme to believe that Krohn is right, and that such a figure asHensen gives in his paper, representing two main trunkspassing up to supply the eye is erroneous. Plate XXXV,fig. 9, of -P. rnaximus, shows the division of the single nerveinto its two branches. In fig. 1 the course of the optic nerve,before its division into two branches, was carefully drawnfrom one of a complete series of sections, and in none of theother sections could I find a trace of any other nerve pro-ceeding from the " circumpalial." The branching of thenerre takes place in a plane at right angles to the plane of themantle. When the optic nerve approaches the eye it dividesinto two branches, which may be called the " retinal nerve"and the " complementary nerve." The former passes up theside of the eye cftvity, and spreads over the anterior surfaceof the retina; the latter soon loses its sheath, and dividesup into a number of branches, which supply the tissues sur-rounding the eye. The course which the retinal branchtakes may be seen in PI. XXXIV, fig. 1, and in PI. XXXV,

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figs. 8 and 9. In figs. 8 and 9, the first section is cut throughthe optic nerve, and shows the manner in which the retinalhranch runs up the side of the eye-cavity; the second sectionshows the manner in which the branch bends over on to theretina and spreads out. The distribution of the comple-mentary branch is diagrammatically represented in fig. In;it seems to divide into a number of branches which enve-lope the eye-cup, and probably send filaments to the cornea,lens, tapetum, and epithelium.

Comparison of the eyes of the three species, P . maximus,P . jacobceus, and P . opercularis.—The eye of P. maximusis undoubtedly the most highly developed, the eye of P.opercularis is the simplest, whilst P . jacobceus, althoughmore like P . opercularis than P . maximus, shows manypoints in which it jg intermediate between the two.

The lens in P . opercularis is separated from the retina bya considerable space (PI. XXXV, fig. 10), and consequentlythe chamber containing the humour ia relatively large. InP. jacobceus the lens is larger than in P . opercularis, andthe chamber consequently smaller; and in P . maximus thelens is very large, and nearly touches the retina, the cham-ber of the eye being sometimes very small. A gradation isthus observed in the character of this part of the eye in thethree species. In P . maximus but a small space is filledwith humour, in P . jacobceus a much larger space is filledwith it, and in P . opercularis there is a larger space still.

Again, when the retinas of the three species are compared,a similar gradation is found. The retina of P . opercularis iscomparatively thin, and the concavity and convexities of itsanterior surface slight. In P . jacobc&us the retina is de-cidedly thicker, and the anterior surface is more convex atits side6 than in P'.opercularis ; moreover, it may be noticedthat the delicate membrane which separates the anteriorfrom the posterior limbs of the rods has become bent upjnthe regions corresponding with the anterior convexities ofthe retina. In P . maximus all these variations becomemuch exaggerated. The retina is much thicker than ineither of the other species ; and the side convexities of itsanterior surface are much bolder (PI. XXXV, fig. 11, a, b, c).The anterior concavity does not undergo much variation.

The shape of the membrane separating the anterior andposterior limbs of the rods is greatly altered. In P . opercularisthis membrane is observed, in section, to stretch from sideto side without any well-marked curves; in P . jacobceus twowell-marked curves, corresponding with the anterior con-

4 5 2 SYDNEY J, HICKSON.

vexities of the retina, are observed; but in P . maximus thesecurves are converted into two distinct folds, which run upinto the substance of the retina. The membrane betweenthe folds does not sink again as low as it is at the com-mencement of the folds, and consequently the centralpart of the retina is raised in the form of a table abovethe level of its sides. This elevation of the central partof the retina may be also seen in P . jacobaus, though itis not nearly so well marked. The folds which occur inP. maximus cause th; rods to appear to be given off in apinniform manner at the sides of the retina, and beforeI found the intermediate condition in P. jacobmus I hadsome difficulty in determining the true relationship betweenthe retinas of P . maximus and P. opercularis. (Comparea, b, c, fig. 11).

In addition to those just mentioned there are other minorpoints in which the eyes of these species differ from oneanother, such as in the shape of the cells composing thelens and in the distribution of the retinal nerve, &c, butthey are comparatively slight.

General considerations.—Having thus described, in somedetail the anatomy of the various parts which composethe eyes of Pecten, I shall, before leaving the subject, pointout some of their interesting morphological peculiarities.It is, in itself, a remarkable thing to find a large andvariable number of eyes situated on an area at some con-siderable distance from any central nerve-ganglion; and,when it is remembered that the class and even family (withone other exception, e. g. Spondylus) to which the genusbelongs, possess no organs of vision at all in the adult con-dition, it is altogether surprising that they should be of suchextraordinary complexity as they have proved to be. Thehigh structural development that this eye h.ts attained is,however, not so remarkable as the fact that in many waysit differs from the ordinary Invertebrate eye, and resemblesthat of the Vertebrata.

In the first place, the lens is built up of a large number ofdistinct nucleated cells, which undergo a flat'ening at itscircumference very similar to that found in the eye of theVertebrata. Whether the lens is developed from the cellscif the epiblast, as in the Vertebrata, or from the mesoblast,must at present be left unsettled, but it will probably befound, when the development of the eye is studied, that inthis respect also it resembles the eyes of the Vertebrata.The tapetum, a structure which is of considerable im-

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portance to animals which are nocturnal or aquatic in habit,has hitherto been described only in the Vertebrata. That?ecten possesses a tapetum as highly developed as anyfound amongst the Vertebrata is anatomically a point ofconsiderable interest; but it also indicates to a certainextent the physiological capability of the eye.

The chief interest, however, lies in the relative positions ofthe optic nerve, the retina, and the pigment. In the eyes ofthe Cephalopods the pigment layer is situated in front of therods, and the nerve-fibres enter the rods from behind. Inthe eyes of the Gasteropoda, the Crustacea, &c, down to thesimplest form of eye, such as that of the Rotifera, the samerelationship of these parts holds good. In the Vertebrata,however, their relative positions are reversed ; the optic nervepierces the retina, and distributes itself over the front of theretina, whilst the pigment^is situated behind it. In Pectenthe relationship of these parts is the same as that in theVertebrata; the nerve passing up the side of the eye-cupbends over, and spreads itself over the anterior surface of theretina. The pigment also is situated behind the retina.Pecten is not, however, the only Invertebrate whose eyesare built up on this type. Semper1 has recently pointed outthat on the backs of certain slugs (Onchidium) a number ofeyes are found, and that in these the nerves pass to the frontof the retina before being distributed. On account of thisdistribution of the optic nerve he says that they belong tothe Vertebrate type of eye (typus der Wirbelthieraugen), sothat two animals are now known, each belonging to a largeand important class of Invertebrata (Gasteropoda and Lamel-libranchiata respectively) that possess eyes which are builtup on this type. The eyes of Pecten are even more deservingof the name of Wirbelthieraugen than those of Onchidium,for they are much more highly developed, and possess, inaddition to this relationship of the nerve aud retina, otherVetebrate peculiarities. The lens is multicellular, a characterwhich, although not unknown amongst the Invertebrates, ismuch more characteristic of the Vertebrata. The tapetum,too, a structure which doubtfully exists in any other Inverte-brata is found in Pecten and some Vertebrates. But, althoughthe application of this word Wirbelthieraugen to these eyesis convenient for the adult condition, it must be carefullyremembered that the development of these eyes is essentially

1 Semper, ' Uber sehorgane vom Typus der Wirbelthieraugen auf demRiicken der Schnecken.' Wiesbaden, 1877.

4 5 4 SYDNEY J. HICRSON.

different from that of the Vertebrate eye. The Vertebrateeye is formed in the embryo from a hollow process given offfrom the brain, and the future eye-cup is formed by an in-vagination of this process. It is impossible for the eyes ofPecten or Onchidium to be formed by any process similarto this. Thus, in the young state these eyes are essentiallydifferent from those of the Vertebrata, and the resemblancein the adult is merely accidental, and by no means due tomorphological identity.

Little is known and little can be said concerning thefunction of the eyes of Pecten. The presence of such a•well-formed tapetum makes it probable that they arecapable of appreciating very diffused light, and the closeapproximation of the lens to retina makes it exceedinglyimprobable that any image is formed upon the latter.

A few experiments have been made on the extent of theirvisual power, which make it very doubtful whether they areof much value to the animal in avoiding its enemies. Themost reasonable theory of their function seems to be that,•when on the ebbing of the tide, a probability arises thatthey will be left high and dry on the shore, they can appre-ciate the fact by the growing intensity of the light, and,by that peculiar flapping motion of their valves thePectens are so remarkable for, move away into deeperwater.

These researches were entirely carried on in the morpho-logical laboratory of the University of Cambridge, and mybest thanks are due to Mr. Balfour for his valuable adviceand encouragement during the whole course of my researches.Owing to his kindness, also, I have been enabled to ex-amine some of Semper's preparations of the eye ofOnchidium, to which reference has been made in the text.

Methods.—For a general examination of the eye the bestmethod is to harden in alcohol and stain by immersion inhsematoxylin for twenty-four hours. Of the osmic-acid acidpreparations the best were obtained by immersion ina 1 per cent, solution for fifteen minutes, followed by abso-lute alcohol fov three or four days. This method is of greatvalue fov studying the retina and lens. I have also usedgold chloride for staining the nerves with some success.For examining the tapetum the best preparations I have weremade from some eyes given me by Mr. Haddon, which hadbeen treated with picric acid. This reagent seems to havedissolved away the red pigment, and consequently left thetapetum free from the numerous little red granules which

THE EYE OF PECTEN. 4 5 5

generally cling to it. For examining the isolated rods of theretina I have allowed the eyes to remain in a solution ofchloral hydrate for four or five days. I have then dissectedout the retina with needles as carefully as possible, andpoured a drop or two of hsematoxylin on to the slide. Whenthe retina had been standing in hsematoxylin in this mannerfor some hours it was washed with water, teased out "withfine needles, and mounted in glycerine.