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Report on the Microscopic Structure of Shells. Part II.
By WILLIAM CARPENTER, M.D., F.R.S., F.G.S.

(Continued from Report for 1844, p. 24.)

Introductory Remarks.

In my former Report I gave an account of the principal varieties of elementary structure, which had presented themselves to me during a minute and extensive examination of the shells of Mollusks; and I described the peculiar combinations and arrangements of these elements, which are characteristic of the following groups of Bivalves, namely, the Brachiopoda, the Placunide, Ostraceæ, Pectinida, Margaritaceae and Nayadea. On the present occasion I shall enter into similar details in regard to the remaining families of the Lamellibranchiata; and shall state the results of my inquiries into the structure of the shells of the Gasteropoda and Cephalopoda. These last are, however, chiefly of a negative character.

Before proceeding, however, to this continuation of my former Report, I shall make a few additions to the facts contained in it, as to the structure of the shell in certain of the groups therein described, which have resulted from the continuation of my inquiries into their organic peculiarities.

I. Observations Supplemental to former Report.

57. Brachiopoda.-When drawing attention (§ 41) to the very remarkable system of perforations presented by the shells of certain species of Terebratula and allied genera, I was obliged to express my ignorance of the relation which these passages have to the structure and economy of the animal; not having had at that time the opportunity of examining a shell, the animal of which had been preserved in situ. This opportunity, however, by the kindness of Mr. Cuming and Mr. MacAndrew, I have since enjoyed; and I can now communicate the results of my inquiries, which, though not fully satisfactory, will be found, I think, to possess much interest.

58. The species on which my observations have been made, are the T. australis, and the T. caput serpentis (?) lately discovered to be a native of our own seas. When a thin portion of a shell of either of these (and probably therefore of any of the perforated species of Terebratula) which has been preserved with the animal in spirit, is ground down from the inner side, so as to leave the outer surface unchanged, it will be seen that each perforation in the shell is covered-in by an oval membranous disc, whose texture appears very firm (fig. 1). When a thin section thus made is exposed to the action of dilute acid, so as to remove from it the calcareous matter, it will be seen that these discs are connected together by a layer of very pellucid membrane, in which no distinct structure can be made out (fig. 2); this membrane, differing as it does from the membranous basis of the interior layers of the shell, is probably to be regarded in the light of an epidermis. When a portion of the shell, not reduced in thickness, is completely decalcified by immersion in dilute acid, and the membranous residuum is then examined, a very remarkable structure presents itself, such as is found in no shells of the Lamellibranchiate Bivalves. Attached to the membranous films are a series of tubular appendages, corresponding in diameter to the perforations in the shell, and arranged at the same distances (fig. 3). The free extremities of these appendages are much larger than those by which they are attached to the membrane, and have distinct cæcal terminations, which appear by the straightness of their border

to have been flattened against the discs that closed the orifices of the perforations in the shell. Indeed in some instances these discs have remained adherent to them, when the shell-membranes were torn asunder; and are seen edgeways, as in fig. 3, a. There can be no doubt, therefore, that these membranous cæca occupied, in the living animal, the perforations already described as penetrating the shell from one surface to the other. This will be still more evident on reference to fig. 39 of my former report; in which it will be seen how exactly the shape of the cæca corresponds with that of the perforations, when the latter are laid open lengthways by a section of the shell perpendicular to its surface. The lower margin of that figure corresponds with the outer surface of the shell, and the diameter of the perforations is seen to be there greatly increased. With regard to the office of these cæca, however, I am unable as yet to give any distinct explanation. Their contents are of a brown granular character, in which I have recognised distinct cells (fig. 4), such as are to be met with in the tubuli and follicles of ordinary glands; and their whole aspect satisfies me that they must be regarded as possessing a glandular character. I have not been able, however, to discover the nature or destination of their secretion *. The internal orifices of the perforations obviously constitute the outlets of the cæca; but there does not appear to be any system of tubes or canals for collecting the matters poured out from them, each cæcum having its distinct and independent termination on the internal surface of the shell. Although the unusual degree of adhesion between the mantle and the shells of Terebratulæ, first noticed by Professor Owen, formerly led me to suspect that the mantle might send prolongations into the perforations of the shell, I have not been able to discover any vestige of such. On the contrary, it has appeared to me that the mantle, which is a nearly homogeneous membrane where not traversed by vessels, is simply applied to the internal orifices of the cæca, and continued over them; no trace of any connection with them being visible when it is detached from the shell. I may mention, however, that I have found the surface of the mantle in contact with the shell to be scattered over with minute cells, corresponding in size and aspect with those contained in the cæcal tubes (fig. 4, a).

59. The physiological purpose of this curious structure, therefore, is at present a mystery; but there can be little doubt that it is a very important one in the œconomy of the animal, when we see the shell thus rendered subservient to the special protection of these cæcal appendages. And there is evidently strong reason for regarding the presence or absence of the perforations in the shell as a character of greater value in the subdivision of the genus Terebratula and its allies, than those more trivial indications furnished by the external conformation of the shell, which seem to have little to do with the structure or ceconomy of its inhabitant. I am very happy to find this opinion sanctioned by so high an authority on the classification of the Brachiopoda as Mr. Morris, who has laid great stress on the presence or absence of these perforations, as exactly corresponding with characters derived from the relation of the foramen to the deltoidal areat. And I am quite content to accept his correction of an error into which I had fallen (through an accidental disarrangement of my sections) in the classification

* Should I, however, be ever fortunate enough to have the opportunity of examining a fresh specimen, I shall be able to form a better idea of the former than is possible from specimens preserved in spirit.

See his paper on the subdivision of the genus Terebratula, in the Journal of the Geological Society, vol. ii. p. 382.

of species under the two heads of " perforated" and "not perforated," which I gave in my former report (§ 42); Ter. coarctata and Ter. subrotunda being perforated, whilst Ter. acuta is not perforated.

60. Placunida.-As the "prismatic cellular structure" appeared to be peculiarly characteristic of the group of shells with the lobes of the mantle completely divided-presenting itself most typically in the Margaritacea, and in a subordinate degree in the Ostraceae on the one hand and the Unionidæ on the other-I could not readily account for its apparently complete absence in the shells of this family. I have since ascertained, however, that a very thin but distinct layer of it may sometimes be traced on the exterior of Anomia ephippium (fig. 7); and I am disposed from analogy to believe that it is constantly formed in the first instance, but is subsequently more or less completely worn away. Since my former Report, I have also examined a specimen of Placunanomia; and have found that whilst its general texture closely resembles that of its congeners, there is one part of the shell-that which bears the muscular impression and surrounds the passage for the socalled bony attachment-which has a more solid and less laminated character, and is made up of a distinct cellular structure. The "bony attachment" of Anomia and Placunanomia differs entirely in structure from the shell to which it belongs; I have not been able, however, to satisfy myself fully in regard to the plan of its formation, and I therefore refrain from now attempting to describe it. I may state, however, that it has nothing in common with true bone in its texture, save the large proportion of animal matter which it contains.

61. Pectinida. For the reason just stated, I have always been on the outlook for indications of the prismatic cellular structure on the exterior of the shells of this group also; but I had not succeeded in distinctly tracing them at the time of my former Report. I was disposed to account for this by supposing that the first-formed or external portions of the shelly layers are usually abraded, perhaps in consequence of the comparatively active movements of these animals. My expectation has been in some degree justified by the discovery of a thin but beautifully distinct layer of this substance on the exterior of Pecten nobilis, as represented in fig. 6. I would suggest it to those who may have the opportunity of prosecuting the inquiry, to examine the shells of very young Pectens, which may be usually viewed by transmitted light without any preparation.

62. Margaritaceæ.-In describing the prismatic cellular structure, as it is presented in the shell of Pinna and its allies, I drew attention (§ 11) to the transverse markings which are seen upon the membranous walls of the pris matic cells, and also upon the calcareous prisms which they enclose, when these two elements of the structure are obtained in a separate form. And I stated my reasons (§§ 12, 13) for not assenting to the interpretation of this appearance offered by Mr. Bowerbank, who regards these striæ as indicative of the presence of tubes; but for considering them as produced by a simple thickening of the cell-membrane, at the points where successive layers of flat epithelium-cells have coalesced in piles, so as to make up the long prismatic cells or tubes which characterize the fully-formed shell substance. Since that time I have made repeated and careful examinations into this question; and I have now to present what I believe to be demonstrative evidence of the correctness of the view which I had taken. Amongst the various species of Pinna which I have examined, the Pinna rudis is most remarkable for the large quantity of animal matter interposed between the linings of contiguous cells, and also for the clearness and strength of the transverse markings on their walls. In making vertical sections of its decal

cified membrane, I have several times found them disposed to split, in the manner represented in fig. 5. The fissures have always taken place exactly in the line of the striations; showing just that tendency to separation at these parts of the prisms, which might be expected if the supposed coalescence had imperfectly taken place. In some instances it will be seen that the layers thus separated still contain two, three, or four of the single original layers; but at a, a will be seen the edges of single layers, which have become detached for a part of their length from those with which they were previously incorporated; and at b, b are portions of similar layers that are so twisted upon themselves, as to exhibit their surface as if it had been exposed by a section in the contrary direction. This surface presents the division into irregularly-hexagonal cells, which is characteristic of the structure; and we have thus an excessively thin layer, separating itself naturally, but showing us precisely the appearances of a similar layer artificially obtained. No trace of a vessel is here to be seen, the divisions of the cells being formed by the deposit of animal matter between them in unusual thickness.

63. Nayadea.-A vertical section of the shell of Unio occidens, in which the prismatic cellular structure is remarkably thick, has afforded me the opportunity of ascertaining that the prismatic cells, when brought into view in a longitudinal direction (fig. 9), have exactly the same characters as those of the Margaritacea; and also of showing the manner in which the shell is enlarged by the addition of new layers. The ordinary statement—that the shells of the Conchiferous Acephala are progressively enlarged by the deposition of new layers, each of which is in apposition with the internal surface of the preceding, and at the same time extends beyond it-does not express the whole truth; for it takes no account of the fact that most shells are composed of two layers of very different texture, and it does not specify whether both these layers are thus formed by the entire surface of the mantle whenever the shell is to be extended, or whether only one is produced. A reference to fig. 8 will, I think, clearly show the mode in which the operation is effected. This figure represents a section of the margin of one of the valves of Unio occidens, taken perpendicularly to its surface, and exhibits the two layers of which the shell is composed; the upper one being the outer or prismatic layer, the cells of which are traversed longitudinally by the section; and the lower one being the internal or nacreous layer. The latter is traversed by numerous lines, a a', bb', and cc', which evidently indicate successive formations of this layer; and these lines may be traced from the margin (the left-hand extremity of the figure) towards the umbo, in such a manner, that, at every enlargement of the shell, its whole interior is lined with a new nacreous lamina; the number of such laminæ, therefore, in the oldest part of the shell, indicating the number of enlargements it has undergone, as the age of an exogenous tree is indicated by the number of concentric rings in its stem. But it will be seen that each nacreous lamina is in immediate contact with the one which preceded it; so that the new lining thus produced is not formed of both kinds of shell-substance, but of that only which properly constitutes the inner layer of the shell. The outer or prismatic layer, on the other hand, is only formed where the new structure projects beyond th "oin of the old; and thus we do not usually find one layer of it overcapp..nother, except at the lines of junction between two distinct formations. Towards the margin, however, the arrangement is more complicated; and it appears to me to indicate that, even when the shell has attained its full dimensions, new laminæ continue to be formed one after the other, the entire shell being lined, as before, with a thin layer of nacre, whilst the lip

is thickened by successive formations of prismatic structure. The epidermis seems to be here present between the successive laminæ, as is better seen in fig. 10, which shows the appearances presented by a section of the lip on a larger scale. The position of the letter c shows the corresponding parts in figs. 8 and 10.

64. If we revert to the mode in which the well-known shell of Ostrea is enlarged, we shall perceive that there is such an essential conformity between the two cases, as may justify us in founding our general description of the process upon the indications which they present. A cursory inspection of the shell of the Oyster shows us that it is composed of two different substances; the principal part of its thickness being made up of the white inner layer (which I have termed sub-nacreous, § 29), the successive lamina of which have comparatively little adhesion to each other; whilst the margin of each of these laminæ is covered on its exterior with a border of a more elastic brownish-yellow substance, which the microscope shows to be composed of prismatic cellular structure. Now these external layers are quite disconnected, instead of being continued by the junction of their edges one with another as in Unio; and we thus see plainly that, as long as the shell continues to grow, it is only where the new inner layer projects beyond the margin of the old, that the outer layer is generated. But we may observe at the margin of a full-grown oyster exactly that which has just been described in the lip of Unio, namely, a succession of laminæ of prismatic cellular structure, superposed one upon another, although (like the other layers of this shell) with little or no mutual adhesion.

65. I believe, therefore, that the true history of the enlargement of the shells of the Conchiferous Acephala is this:-The margin only of the mantle has the power of giving origin to the outer layer of the shell, whilst its whole surface may generate the inner. Every new production of shell consists of an entire lamina of the latter substance, which lines the whole interior of the old valve, and of a border or margin of the former, which thickens its edge. So long as the animal continues to increase in dimensions, each new interior layer of shell projects so far beyond the preceding, that the new border composed of the outer layer is simply joined-on to the margin of the former one; so that the successive formations of the outer layer scarcely underlie each other. But when the animal has arived at its full growth, the new laminæ cease to project beyond the old; and as each is still composed of a marginal band of the external substance, attached to the edge of an entire lamina of the inner, these bands must now underlie each other, being either quite free as in Ostree, or closely united to each other as in Unio and most other Bivalves. We shall hereafter see that the additions to the shells of Gasteropoda are really made upon the same plan, although it has been commonly supposed that they are only attached to the edge of the old shell, instead of being continued over its internal surface.

II. Continuation of former Report.

66. In passing from the families of Bivalve Mollusca considered in the former Report, in which the lobes of the mantle are unadherent, to those in which they are partially or completely united, we encounter a gradual change in the texture and condition of the shell. We have seen that in the Saritaceae (which may probably be regarded as the types of the former 'uivision, § 56), the external portion of the shell is composed of a substance which always retains its cellular character; the partitions between the cells being firm, tenacious membranes, which altogether bear a large proportion to the calcareous substance they enclose. In the group we are now to consider, how1847.

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