On the Structure and Significance of some Aberrant Forms of Lamellibranchiate Gills

This Lamellibranch shows many departures from the structure which is generally regarded as characteristic of the class. Fig. 1 gives a fair idea of what is seen when the left valve of the shell has been taken away, and the mantle of the same side removed along the lower border of the visceral mass near the line xy. a. a. is the anterior abductor muscle made up of several fasciculi; p. a. is the posterior abductor. It will be noticed that Nucula possesses one of the few shells in which the umbo is turned toward the posterior end. In the specimen figured, the visceral mass (u. m.) shows convolutions on the surface, which, under the microscope, proved to be the male reproductive organ, probably enormously developed for the breeding season, and this character enables one to distinguish the sex of a specimen without difficulty. All the males have these convolutions, and, when preserved in alcohol, are of a greyish colour. The females show hardly any convolutions, and are much more darkly coloured. The foot (f) is folded longitudinally at its end, and can accordingly be spread out into a flat circular disc. The labial palpi (l) are unusually developed, and might at first sight be taken for gills. The inside of the outer and the outside of the inner palpus are raised into numerous parallel ridges, which, as shown in the figure, can be seen from the outside, and do not extend to the lower margin. At their posterior end there are two remarkable structures. One of them is a hood-like structure (l. b., figs. 1 and 2), which is the posterior prolongation of the united upper edges of the inner and outer palpi. The other (l. a., figs. 1 and 2), lying immediately below the first, is a long tentacular appendage. It is a hollow tube, open, however, along a line on its posterior aspect, and having its cavity continuous with the space between the two palpi. As it has been seen protruded, with the foot outside of the shell (Woodward’s ‘Manual of Mollusca,’ p. 426), and since, in alcoholic specimens, a great deal of dirt and sand is found along its length and between the palpi from its base to the mouth, it is no doubt a food-procuring organ, probably sending a constant stream of nutritive matters to the mouth by means of its cilia. It is interesting to notice in connection with this appendage that in Nucula, the gills, unlike those of ordinary Lamellibranchs, must be practically useless for obtaining food, as will be evident from the following descripion of them.

The gill (g., figs. 1 and 3) is comparatively small. It is situated quite posteriorly, and is suspended by a membrane (m., figs. 1 and 3), which is attached to the body along the broken line xyzw. It is united to the visceral mass (v.m.) from x to y, and to the upper part of the foot (f, fig. 3) from y to z (see figs. 1 and 3). At the last point, having come to the median line of the body, it joins with its fellow of the opposite side, and they continue in this way as far as Here they separate again, each proceeding to the posterior tip (p) of the gill of its own side. It should be remarked that, as the point x is further from the median line of the body than the point y (fig. 3), there is a considerable free space beneath the suspending membrane of the gill.

When we turn to the gill itself, we find an altogether unusual structure. Fig. 4 shows it dissected out and seen from below and slightly from one side. In general appearance it resembles a boat which is suspended by its keel. x c p, fig. 4 (seen in cross section at i j, fig. 5), is the line of attachment and corresponds to the keel; x d p, fig. 4 (seen in cross-section at d, fig. 5), represents the bottom line of the hollow of the boat. The latter is bounded by the two surfaces xapd and xbpd (fig. 4; seen in crosssection at b d and a d, fig. 5). The anterior end (x, fig. 4) is rather blunt, while the posterior end (/>, figs. 1, 3, and 4) is quite pointed. The resemblance of the gill to a boat is, however, only very superficial, as the gill is not one solid mass, but is made up of a series of paired plates of a peculiar shape, placed one after another from the anterior to the posterior end. A little dissection under a lens will show that the part above the line xdp (fig. 4) and below the line of suspension (x cp), is continuous along the entire length of the gill, and that, with this part for the stem, the plates are given off, one after another, in pairs to the two sides (see fig. 5). The plates constitute the proper respiratory parts of the organ. They are largest in the middle, and diminish in size toward the two extremities.

It is evident from this description that the gill in Nucula is of quite an exceptional nature. It does not, as in most Lamellibranchs, extend along the whole length of the side of the body, constituting the most conspicuous object of the mantle cavity, but is comparatively insignificant, being pushed back and freely suspended in the mantle cavity. It cannot, therefore, divide the latter into the suprabranchial and infrabranchial chambers, and is, of course, utterly devoid of any structure like the ciliated water-passages in the ordinary gill, for driving water from the lower to the upper. It cannot, also, as has been said, serve as an effective foodprocuring organ. The gill in Nucula must for these reasons be of vastly less functional importance to the animal than it is in common Lamellibranchs, and, so far as I am able to see, serves only as the organ of respiration. It seems to me, however, that the division of the mantle cavity into the upper and lower chambers is begun in the posterior part. It has been seen that ventral to the membrane suspending the gill (m, figs. 1 and 3) there is a large space continuous with the general branchial cavity, and there certainly is a space dorsal to this membrane. These spaces seem to be the rudiments of the supra- and infrabranchial chambers. Moreover, the arrangement of the different parts at the posterior end, as seen in fig. 3, recalls that of the corresponding parts in many of those genera in which the mantle cavity is divided into two parts. It is not difficult to conceive how the same division might be brought about in the case of Nucula, by proper development of the gill and the membrane.

Fig. 5 shows a pair of opposed plates considerably enlarged. The solid part (i d j) which I have called the stem, and which is continuous throughout the whole length of the gill, together with the suspending membrane (k i j l) is seen in cross section in the middle, and from this middle portion the paired plates (e. e.) are seen to proceed.*The coloured part at the bottom represents the complex chitinous framework. The membrane (k i j l) is made up of fibrous tissue, the bundles of which this is composed crossing each other in many directions. Its free surfaces are covered with columnar epithelium. The stem consists mostly of a solid mass of large irregular cells with rather large nuclei. There are, I am almost certain, two blood-channels excavated through it; a lower larger (n), and an upper smaller (o). The latter seems to be in connection with a free space (q.) found often in sections of the suspending membrane. The large channel (n) sends a branch (r) into each plate. The fibrous tissue found in the upper membrane dips down into this part at regular intervals, viz. between every branch (r) of the lower blood channel (n). How these fibres end below, when they reach the chitinous framework, I have not been able to make out. A few fibres (u) are sent down into the plate a little above the blood-channel (r), and gradually approach and finally touch the latter near its lower end. A few more fibres (t) are seen along the upper edge of the plate. Exactly what this fibrous tissue is I am unable to make out, but it seems to be some sort of tough connective tissue, with perhaps muscular fibres more or less intermixed. That it is very tough and serves as a support to the whole structure is seen by the fact that the fibres often stick out beyond the broken edge of the soft tissues. The trough of the chitinous framework is seen at s, in cross-section. It extends along the whole length of the gill and sends but two branches into each plate. I have obtained the appearances, in some sections, of a bundle of fibrous tissue running in it and filling it. The framework will he described more fully further on. The plates (e), the proper respiratory organs, are comparatively speaking very broad and quite thin, and hang down from the solid part of the gill. The epithelium of the plates which is represented in the figure as ending abruptly at the edges i d and j d, turns at a right angle at these lines to cover the stem, and is soon reflected outwards again to form the epithelium of the next plate in the series. This is evident from an inspection of fig. 8. Each plate may be said to be simply an enormously widened blood-channel (fig. 6), and as the blood is necessarily spread out in a thin layer over a large area, the purposes of aeration must be admirably served. The columnar epithelial cells seen at ad, fig. 5, are very characteristic of the plates under a microscope, and are the cells (d a, fig. 6) around the chitinous bars (h, figs. 5 and 6) seen in optical section. The surface of the irregularly rectangular cells placed just inside these columnar cells in fig. 5, ought therefore to be continuous with the outer edge of the columnar cells, but in order to avoid confusion is not so represented in the figure. This is also the case with the cubical cells along the upper edge. The chitinous support (h, figs. 5 and 6) of the plate runs near the lower edge (fig. 5) to its tip (a or b, fig. 5), and is made up of two entirely separate parts (seen in cross-section in fig. 6) applied closely together. Owing to the shape of these parts there is, however, a narrow oval space between them. This space, as will be shown further on, is continuous with the space in the trough (s, fig. 5) of the stem. The cells along the lower edge of the plate are columnar, and surround the chitinous support in a characteristic manner shown in fig. 6. Their surface outlines are irregularly rectangular, contrasting with the irregularly polygonal cells covering the rest of the plate. The branch (r, figs. 5 and 6) of the lower bloodchannel (n) in the stem, is seen to be circular in cross-section and to bulge out the surface of the plate. These points are not, however, constant, as the vessel is sometimes constricted into more or less separate channels, while the amount of bulging seems to depend on the quantity of blood present. The remaining part of the plate (e, figs. 5 and 6) is flat and quite thin, enclosing a broad blood-channel between its two epithelial surfaces. It is here no doubt that the aeration of blood is accomplished. The cells of this part are cubical, as seen in fig. 6. Some of them send processes inward to join others from the opposite side. This gives a labyrinthine appearance to this part of the plate. The course of the blood is evidently from one blood-channel in the stem to the other, through the space in the plate. For instance, the blood may start from the upper channel (o) in the stem, go to the broad flat part (e, fig. 5) of the plate where it gets aerated, then enter the branch (r), along its upper edge, and run up this to reach the lower blood-channel (v e) in the stem. This is, however, a purely hypothetical course. I have had no means of determining whether the blood goes from the upper to the lower channel or vice versa.

The framework which supports the gill can be separated out by heating it in dilute caustic potash, as it is insoluble in weak acids and alkalies. It is stained by carmine and other colouring reagents. Whether it is really formed of chitin I do not know, but as previous writers have described the substance as of that nature it will be convenient to use the term “chitinous support “for the present. The frame work consists of a trough (seen in cross-section at s, fig. 5; longitudinally from below in fig. 8; diagramatically represented in fig. 7), which runs along the whole length of the gill, and from which a pair of closely-applied parallel branches (A, figs. 5, 6, 7, and 8) is given off into each plate. The trough is divided into two unequal parts: an upper larger and a lower smaller, by a cross piece (c. p, figs. 5 and 7), which stretches from one side of it to the other, a little below the middle. This cross piece is not, however, continuous, but is pierced through by oral openings (o v, figs. 7 and 8) whenever branches are given off laterally to the plates. The space enclosed between each pair of closely- applied branches (see h, figs. 6, 7, and 8) is connected with the lower compartment of the trough by means of somewhat circular openings (op and o′p′, figs. 7 and 8) found near the bottom. In fig. 8 the letters a, a, a, are placed opposite each pair of the branches that go into a plate. It will be seen how one half of the chitinous support of one plate, after forming an arch at the trough, turns round to enter the next plate in succession, and to constitute there one half of the support of that plate. The framework treated with potash, and sometimes without any treatment, shows marked longitudinal striation (fig. 8), and some of its fibres sticking out at the broken edge beyond the others resemble in appearance the fibres found in the suspending membrane, at t and u, fig. 5, and give reasons for thinking that the whole chitinous framework is nothing but the fibrous tissue found in other parts cemented closely together and forming one cohering mass.

Although, owing to the state of the specimens, I have obtained only here and there evidences of cilia, it seems reasonable to suppose that the whole gill is covered with cilia. On two rows of cells (l. f., fig. 6 ; d. a., fig. 5) on the lower edge of the plate I believe there are larger cilia than on the rest, as 1 have now and then seen their remains, and as, without any question, cells in the corresponding positions in Yoldia have long and conspicuous cilia.

Yoldia resembles Nucula in several structural peculiarities—in its well-developed labial palpi, with their peculiar food-procuring appendage, in its feather-like gills, in the posterior position and comparatively small size of the gills, and the consequent absence of the division of the mantle cavity into the supra and infrabranchial chambers. It differs from Nucula in having a siphon, and further shows its departure from the ordinary lamellibranchiate structure in having a highly specialised tactile organ in the siphon.¹

The gill, although different in details from that of Nucula, is essentially of the same structure as the latter. It is suspended by a membrane, as in Nucula. Fig. 9 shows it dissected out by itself. The line of suspension is xcp; xdp is the ventral median line, and corresponds to xdp in fig. 4. As in Nucula, the gill is made up of a series of paired plates, placed one after another, and attached to the central solid stem continuous throughout the whole length of the gill. The plates do not, however, project downward, as we have seen in the case of Nucula, but here turn upward (see fig. 11). The plates are largest in the middle, and gradually become smaller toward the extremities. At the front end (x, fig. 9) there is a rather interesting arrangement. Fig. 10 shows diagrammatically the relations of the various parts at the anterior termination of the gill. It will be seen that the plates of the gill gradually become smaller and finally die out toward the front, and the gill is continued simply as a flat membranous structure (x, fig. 10), which goes into the visceral mass (v.m., figs. 9 and 10). A cross-section of this part shows that at its lower portion, at least, there is a blood-channel, probably continuous with one of the channels in the stem of the gill. In some specimens this membranelike portion of the branchia is longer than in others, and goes some distance around the visceral mass.

Owing to the rather poor state of preservation of the alcoholic specimens, I have not been able to make out the histology of the Yoldia gill as fully as I should like, but the following description I believe to be correct in essential points :—Fig. 11 represent an opposed pair of plates, and corresponds to fig. 5 of the Nucula branchia. The suspending membrane (k i j l) consists of fibres crossing each other in several directions, and is covered on its two surfaces by columnar epithelium. The solid stem (i d j) of the gill has two blood-channels, an upper (n) and a lower (o). The latter seems to be in communication with a comparatively free space (q) in the middle of the suspending membrane. Directly below the upper blood-channel (o) there is a bundle of tissue, which appears to be fibrous, running the length of the gill (seen in cross-section at f, fig. 11). It serves no doubt for support. The floor of the lower blood-channel (r) is covered by a V-shaped bundle of longitudinal fibres (s). This would seem to be homologous with the trough-shaped chitinous structure in Nucula, but seems to be formed of the same fibres already referred to several times, which are found in the suspending membrane and other parts of the Nucula and Yoldia gills, and I cannot establish any connection between this bundle and the chitinous bars (h, fig. 11) in each plate. The latter, when they reach the longitudinal bundle (s), make a bend and turn out again to enter the next plate in the series. In some sections I have obtained indications of a very thin layer of chitin beneath the fibrous bundle (s), which may, therefore, correspond to the fibres found in the trough of the framework in the Nucula gill (see above). If, however, this V’Shaped structure is really homologous with the trough of the Nucula gill, it goes far in support of the view advanced above, that the chitinous framework is really made up of the fibrous tissue which is found in other parts, here cemented into one compact mass. In such a case fusion has gone further in Nucula than in Yoldia, and we see in the first genus the trough well united with the branches (h) in each plate. The plates (e, fig. 11) in Yoldia spread themselves upward instead of downward, as in Nucula. The chitinous bars (h), of which there are two in each plate, follow the curve of the plate and end rather bluntly about half way up, at the point a. That the part from d to a corresponds to the lower inner edge of the Nucula plate (da, fig. 5) is shown by the characteristic rows of columnar cells having longer cilia than those found in other parts of the gill. There is another system of chitinous structures (ch, fig. 11). Many fine chitinous filaments come down together in a bundle on each side from the suspending membrane, and as soon as each bundle reaches the plate of its own side filaments spread themselves out like the frame of a fan over the whole plate. Several fibres sometimes proceed together, and then separating give the appearance of branching. They are found directly beneath the epithelial cells that cover the plate. The effect of this framework must be to keep the pliite well spread out for the purpose of aération. I have not succeeded ip obtaining any single section which shows the structure of the plate well, but from the comparison of a good many sections which I have made, I feel tolerably sure that the whole space between the epithelial surfaces is pervaded by what Peck¹ calls “lacunar tissue “(fig. 12). It is a loose trabecular tissue with many nuclei and within whose network blood can flow. The space between the chitinous bars (h, fig. 11), which is quite large in Yoldia, seems to be tolerably free from this lacunar tissue. Fig. 11 a gives the outline of the plate seen from one side.

The gills, here described, of Nucula and Yoldia are, 1 think, the most rudimentary of any that have been studied so far. In fact, at first sight, the resemblance to the ordinary Lamellibranch gill is not apparent, and they suggest more the Cephalopod gill. But I believe, the homology of their various parts with those of more complex gills in Unio, Mytilus, Area, &c., is not difficult to make out. After consulting the articles by Peck (loc. cit.), Posner,¹ Lacaze-Duthiers,² Bonnet,³ and others, and also after examining the sections I myself have obtained of Unio, Modiola, Scapharca, &c., I have no doubt whatever that the plates in Nucula and Yoldia represent the descending or attached limb of the filaments in the outer and inner gillplates in forms like Mytilus, Modiola, and Area, and accordingly are homologous with the folds on the inner lamella of the outer gill-plate, and on the outer lamella of the inner gill-plate in Unio, Anodon, and Dreissena. If a comparison is made of my fig. 6 with any of the cross-sections of gillfilaments given by Peck, it will be seen at once how similarly the paired chitinous bars are placed, how almost identically the epithelial cells are arranged around them, how two rows of those cells (l.f., fig. 6)—called by Peck latero-frontal epithelial—have longer cilia than the rest. In fact, Peck’s fig. 12 (a transverse section of a filament of the Anodon gill) agrees with my fig. 6 in all essential points. The left hand-figure in his fig. 5 (the superficial view of the edge of a gill-filament of Mytilus showing the latero-frontal and other epithelial cells) and the upper part of his fig. 20 (the same view of a gill-filament of Anodon) would pass very well for the corresponding part in Nucula. So far as I can make out from rather poor specimens, the latero-frontal cells in Nucula are strikingly like those represented in Peck’s fig. 20. If, then, the plates in the gills of Nucula and Yoldia represent the gill-filaments in other genera, it follows from the embryological observations of Lacaze- Duthiers’ (loc. cit.), and from the position of the chitinous bars in the plates, that they are homologous with the descending limb of the gill-filaments in ordinary Lamelli- branchs. Professor Huxley seems to have no doubt whatever of the homology stated here, as will appear from the quotation given further on. Admitting, then, that this supposition is correct, and that the gills in Nucula and Yoldia are in an unusually rudimentary condition, what light, if any, do they throw on the organogeny of the Lamellibran- chiate gill ? But, before proceeding to the discussion of this point, let us review briefly what theories have been advanced as to what is the most primitive type of the branchiae of this group. Setting aside older authors like Williams and Hancock, I consider the articles, already alluded to, by Peck, Posner, and Lacaze-Duthiers as having the most important bearing on the subject. Posner, after a careful histological examination of the gills of Anodon, Unio, Cardium, Mya, Mytilus, Ostrea, Pecten, Pholas, Pinna, Scrobicularia, Solen, Solecurtus, and Venus, puts forward, although with hesitation, the theory that the pouchlike gills of the Unionidse are the most primitive type of the Lamellibranchiate gill. Stepanoff,¹ so far as I can gather, inclines to this view. Peck, on the other hand, after an investigation of Area, Mytilus, Anodon, and Dreissena, comes to the conclusion that “the gill-plates of the Unio- nidse are a highly modified form derived from a simple condition in which the gills consist not of plates but of a series of juxtaposed independent filaments, such as we see in a less modified state in Area and Mytilus.’’ This view is the more generally accepted of the two. The only complete history of the development of the Lamellibranchiate gill by Lacaze-Duthiers (loc. cit.) and all the fragmentary embryological observations on the organ show that the gills are at first of a tentacular or filamentary character. Those who read carefully Mr. Peck’s paper, will, I think, feel convinced by the arguments he brings forward. So high an authority as Professor Huxley is entirely of this view. He says:—” In its simplest form, the branchia of a Lamelli- branch consists of a stem fringed by a double series of filaments (e.g. Nucula). The next degree of complication arises from these filaments becoming, as it were, doubled upon themselves at the free ends, the reflected portions lying on the outer side of the outer, and on the inner side of the inner, series of filaments … (Mytilus Pecten). In most Lamellibranchs, the gills are four elongated plates, each of which is in fact a long narrow pouch, with its open end turned toward the haemal face of the body” (‘Invertebrates,’ pp. 408-9, Am. Ed.). My own observations lead me to the same conclusion. In fact, it is difficult to see how the pouch-like gills of Unio can give rise to such forms of branchise as are found in Nucula and Yoldia. By a very circuitous route they may have degenerated into their present rudimentary state, it is true, but all recent observations tend to show that while other organs in the Lamelli- branchiata have been steadily degenerating, the gills, on the contrary, have become highly developed and perform functions which the probable change of the animal from the motile to the sedentary habits of life has forced on these gills. If, then, there has been no considerable degeneration, and if the homologies of different parts of these branchise are, as I have stated above, the filamentary character of the primitive Lamellibranchiate gill is placed beyond doubt.

I believe further light is thrown on the subject by the gills of Nucula and Yoldia. Peck shows that the gills primarily consisted of a series of filaments, but does not attempt to account for the fact that these filaments have come out in long rows on the side of the body. I venture to suggest an explanation. If we reflect for a moment, I think we shall see that the gills of Nucula and Yoldia may be considered as a stem which, being folded on either side to increase the surface of contact with the water, gives rise to the flat plates which I have homologized with the descending limb of the gill-filament of Mytilus and other like forms. The plates are, strictly speaking, nothing but the epithelial covering of the stem raised into folds and enclosing between the two sides of the folds a blood-channel. In the case of Yoldia mesoblastic lacunar tissue is carried out into the folds. According to this theory, the gill of the Lamellibranchiata was originally a longitudinal ridge on the side of the body. Probably in this a blood-vessel ran, and must have served as the organ of respiration. In course of time, however, this ridge became folded for the increase of the surface of contact with the water and thus produced papilla on its two sides—rudiments of the future gill filaments. The gills of Nucula and Yoldia have gone but little beyond this stage. I think there is much to support this view. Stepanoff (loc. cit.) observed in Cyclas that the gills arise first as a ridge on each side of the body. Leydig¹ makes the same statement. M. Lovén’s² observations have a still more important bearing on the point. He says :— “Nous avons, si je ne me trompe, vu la première formation des branchies; nous en savons assez pour être sûr qu’elles se montrent sous la forme d’un cordon fin, renflé à certains intervalles ; que ces renflements se contournent plus tard en anses, qui s’allongent de plus en plus, et sur lesquelles se développent les cils vibrátiles régulièrement disposés et d’un forme particulière.”³ “Un cordon fin renflé à certains intervalles” is, it seems to me, nothing but a ridge with slight swellings or papillae. Lovén’s figures are not exactly clear to me, but what he designates as the gills are certainly in favour of my view. In all the fragmentary embryological observations, the gills are generally seen as papillæ, or nothing but the folds of a blood-channel. I have already called attention to the anterior part of the Yoldia gill where the plates die out and the gill is continued simply as a ridge containing a blood-channel. Whether this is a remnant of the primitive ridge or not it is difficult to determine, but the fact that there can be on the side of the body a thinwalled ridge which, containing a blood-channel, must serve more or less for respiration, goes far in support of the view here advanced.

To review the whole matter, the Lamellibranch gill was perhaps originally a simple ridge on the side of the body, but to increase the surface of contact with the water folds may have arisen on two sides of this ridge. If such was the case, Nucula and Yoldia are still in a stage only very little advanced from this primitive condition. In course of time, however, as some of the Lamellibranchiata, either owing to degeneration or some other cause, become incapable of extensive locomotion, these buds or folds were perhaps prolonged to form tentacular filaments, which, going on in their development, finally produced such complex gill structure as we see in Mytilus, Unio, Ostrea, and other forms, taking on at the same time functions totally foreign to their original one. Between the simple gills of Nucula and most complex ones known, there are a great many intermediate stages, some going more in one direction, others in another. For instance, Lucina and Corbis are said to have only one gill-plate on each side (‘Owen’s Inverteb.’). According to Sars, Pecchiola is in the same condition (‘Remarkable Forms of Animal Life,’ G. 0. Sars). Chamostrea and Myochama are described by Hancock (‘Ann. and Mag. of Nat. Hist.,’ 1852-3) as having the inner gill-plate complete, but the outer plate lacking the outer lamella. In these tentacular filaments seem to be fused with each other. On the other hand, although Area, Mytilus, Modiola, have all the lamellae present, the filaments composing them have not fused with one another. It is interesting to notice that Nucula and Yoldia, in which the gills have remained rudimentary, have, as Dr. Brooks first pointed out to me, an unusual power of locomotion, while forms wholly or almost wholly unable to move, as Ostrea, Pholas, &c., possess highly- developed gills.

For some reason the inner gill-plate seems to develop further than the outer. For instance, in many genera, the inner is much larger than the outer. In Chamostrea and Myochama, already referred to, it is the inner gill-plate that is complete, and the outer gill-plate that lacks a lamella. It will also be seen a little further on that in Anodon the inner gill-plate has gone further than the outer in its development. In the embryological study of the branchiæ of Mytilus Lacaze-Duthiers observed that the filaments of the inner gill budded out first.

It is very instructive to see the process of secondary folding going on in higher varieties of the gill. The two lamellae of a gill-plate are, in such a case, no longer parallel, but wavy, and the surface of a lamella is thus considerably increased. In Anodon this process is perhaps going on, for Peck shows that in that genus the cross-section of the outer gill-plate has parallel and straight edges, but that the outer lamella of the inner has a wavy margin. Posner shows successive stages of secondary folding in the gills of Pholas dactylus, Venus (sp), Mya arenaria, Ostrea edulis, Solen vagina, Cardium edule, Pinna nobilis.

Diametrically opposite, as the views advocated by Posner and Peck may seem, it is not difficult to reconcile the two.

If we look over the list of the genera examined by Posner, we shall find all of them, except Mytilus and perhaps Pecten, to possess more complex gills than Unio, and starting, as he did, from the last genus, it is no wonder that he considered it to possess the primitive gill. On the other hand, Peck investigated forms simpler than Unio, and arrived at the- probably true conclusion. Posner simply began where Peck ended. The two investigators, therefore, supplement each other, and now, with the addition of the extremely simple gills of Nucula and Yoldia, the series is fairly complete, and it seems to me that the filamentary character of the primitive Lamellibranch gill is made tolerably certain.