TUBICINELLA. Lamarck. Annales du Museum, tom. 1 (1802), Tab. 30, fig. 1.
CORONULA. De Blainville. Dict. des Sciences Naturelles, (1824).
Compartments six, of equal sizes; shell sub-cylindrical, wider at the top than at the basis, belted by several large transverse ridges.
Hab. — Southern Pacific Ocean, Western South America, New South Wales, Cape of Good Hope; imbedded in whales, and often associated with Coronula balænaris.
This genus is closely allied to Coronula, and perhaps De Blainville was right in uniting them. But Coronula, as it now stands, is so natural a genus, that it seems a pity that a form so entirely different in general aspect, as Tubicinella, should be forced into it. The main difference between these genera consists in the walls being here not folded, in the simpler radii, and in the general shape of the shell and of the included animal’s body; but there are many other minor points of difference. The most novel character in Tubicinella consists in the shell being lined almost close down to the basis by the opercular membrane. In the opercular membrane thus forming a long tube, and in the general shape of the animal’s body, we shall presently see that Tubicinella is closely related to Xenobalanus. In numerous other respects Tubicinella is almost equally allied to the latter genus, to Platylepas, and to Coronula. Finally, several points of structure indicate that Tubicinella may be considered as a Coronula, with the shell much simplified in structure.
1. TUBICINELLA TRACHEALIS. Pl. 17, fig. 3 a-3 c.
LEPAS TRACHEALIS. Shaw. Nat. Miscell. (1789-1813), vol. 17 (1806?) tab. 726.
—— TRACHEÆFORMIS. Wood. General Conch. (1815), tab. 4, fig. 1-3.
TUBICINELLA TRACHEALIS. J. E. Gray. Annals of Philosophy, (new series), vol. 10, (1825.)
—— —— — MAJOR ET MINUS. Lamarck. Annales du Mus. Nat., tom. 1 (1802), Tab. 30, fig. 1-2.
—— —— — BALÆNARUM. Lamarck. Animaux sans Vertèbres, (1818).
—— —— — —— —— — Chenu. Illust. Conch. (Plate).
—— —— — —— —— — Sowerby. Genera of Recent and Fossil Shells (Plate).
—— —— — LAMARCKII. Leach. Encyclop. Brit. Suppl., vol. 3 (1824), Pl. 57.
CORONULA TUBICINELLA. De Blainville. Dict. des Sciences Nat., tom. 32, Pl. 117, fig. 5 (1824).
It may be observed that I have here broken through the great law of priority; for it appears to me too grossly incorrect to retain the specific name either of major or minus in a genus including a single species. Lamarck himself seems to have been of this opinion, by giving, in 1818, the new specific name of balænarum; but Shaw’s name of trachealis has the clear right to priority, if major or minus be rejected.
General Appearance. — Shell elongated, sub-cylindrical, with the upper end rather wider than the lower, and therefore widening in a direction the reverse of that usual with sessile cirripedes. The shell is often a little bent to one or the other side: it is surrounded by from two or three to about ten very prominent, strong, blunt ridges or belts, placed at rather irregular distances from each other. The surface is finely striated longitudinally. The six compartments are of nearly equal sizes and shapes. In full-grown specimens the parietes are not wider at the base, and often they are even a little narrower, than at the summit of the shell: in young specimens the parietes do widen a little downwards. The radii are narrow; but in young specimens they are proportionably much broader (Pl. 17, fig. 3 b) than in old specimens. The whole compartment, including the radius and wall, is always a little wider at the summit than at the base, in accordance with the shape of the whole shell. The operculum consists of four nearly equal-sized, similar valves, projecting above the upper end of the shell, which is always broken and jagged: the valves are united to the sheath by a very thick, much folded membrane. The aperture leading into the sack is bordered by very prominent lips, projecting above the opercular valves; the latter have their upper layers always scaled off. The shell is imbedded in the whale’s skin up to the level of its operculum. The largest specimen which I have seen was barely one inch (.95) in diameter at the summit, and 1.5 in length; the longest specimen which I have seen scarcely attained a length of one inch and three quarters.
Structure of the Shell. — The parietes are thin, and if the sheath (which extends to near the basis) be removed, they are rendered extremely thin. They are formed by an outer and inner lamina, united by fine longitudinal septa, projecting at the basis beyond the laminæ. The pores thus produced (which in a transverse section are oblong in outline), run up to the summit of the shell, and are not filled up by shelly matter; but I presume that the included tubular threads of corium are protected, at the broken upper end of the shell, by transverse membranous septa. The outer lamina of shell, as in Coronula, is formed, though obscurely, by the union of ledges projecting from the longitudinal septa. The circular prominent belts, surrounding the shell, are formed by the longitudinal septa, at certain, irregular and rather distant periods, growing outwards; the wall at each belt being increased to nearly twice its thickness in other parts. At each belt the threads of corium within the parietal pores lend off minute branches to supply the thickened wall. These belts, which continuously surround the shell, correspond (as is best seen in young specimens), with the little knobs or beads, which, in Coronula (Pl. 16, fig. 4), rise separately, and not quite regularly, on the longitudinal parietal septa, and which, I believe, are formed at every successive period of growth; here they are much larger, stand in straight transverse rows, become confluent, and are formed only at occasional intervals. The whole external surface of the shell is covered by membrane, stronger and more persistent than is usual with most cirripedes.
Internally the sheath extends almost to the basis of the much elongated shell, and terminates in a slight shoulder: it is divided as in common Balanidæ, and differently from in Coronula, into zones of growth, but these are very broad; at the upper end of the shell, which, as will hereafter be explained, is always breaking away, the sheath readily yields along the oblique planes, which separate the zones of growth and dip outwards: a similar but less strongly marked structure occurs in Platylepas, and in no other genus. We shall presently see that the sheath presents a much more anomalous character, in being lined down to its basal edge by the innermost and last-formed layers of the opercular membrane.
Radii. — The radii are narrow. The belts which surround the shell are prolonged, with slightly diminished prominence, across the radii, their formation being simply due to the radii being here thicker than in other parts. It can be seen more plainly in Tubicinella, than in other Balanidæ, that the membrane externally investing the shell, splits along the radii during the diametric growth of the shell, and is continually repaired and added to along these lines by new longitudinal slips of membrane. The radius consists (as usual) of an inner and outer lamina, which latter does not extend quite to the line of suture — a slightly gaping fissure being thus left. The two laminæ are connected by septa, which are not denticulated, but near the outer lamina bi- or tri-furcate, and the ends of the branches thus formed spread out, forming a sort of outer scalloped lamina, in advance of the true outer lamina. The fine threads of corium running between these septa, do not spring, as in all common cirripedes, from a fillet of corium occupying the actual suture, but from two nearly circular threads of corium occupying two tubes, which run along the line of junction between the radius and the compartment whence it springs. In Coronula alone we have a nearly similar structure; for the fine threads of corium occupying the proper radius, spring from a single very minute tube (Pl. 16, fig. 7, d′), occupying the same position with the two tubes in Tubicinella. I may further add, that the structure of the proper radius in Coronula is precisely the same as here just described, but being on so very minute a scale, I did not there describe it so carefully as I have here done.
Alæ: these are only remarkable from their extreme thinness; for they are not thicker than the inner lamina of the radius. Their sutural edges are quite smooth. Forming part of the sheath, they extend down close to the basis of the shell; where, instead of, as in general, ending abruptly in a rectangular shoulder, they slope off into their own compartment.
Basis. — The basal membrane is complicated, owing to the shell, when full-grown, barely, or not at all, increasing in diameter, and, in consequence, membrane after membrane, each with its own cement-ducts attached to it, are thrown down one nearly over the other. In the Introduction ( — Pl. 28, fig. 3), I have fully described the cementing apparatus, which is very curious from one of the ducts always having a loop with two spurs projecting from it. The basal membrane does not equal in diameter the base of the shell, for the membrane externally covering the walls is inflected inwards all round for a considerable width, and is then united to the basal membrane: in Coronula, the basal membrane extends only under the internal cavity of the shell, and not under the folded walls, and therefore presents a somewhat analogous structure.
Opercular Valves. — The scuta and terga are nearly of the same size and shape: they are mitre-formed, and higher than broad. They do not fill up the orifice of the shell. The scutum is a little larger than the tergum, and rather less symmetrical, the rostral corner of the valve being a little produced. There is no hollow or crest for the adductor muscle, which is small. In the tergum there is no trace of a spur. The two valves are not articulated together, but standing close to each other are united, as well as the scutum to the scutum, and tergum to the tergum, by thick, brown, tough, yet soft membrane, in layers continuous with, but differing in appearance from, the surrounding opercular membrane. The layers of shell, forming the valves, are thick, and only the three or four lower layers are usually preserved, the upper ones having symmetrically scaled off, leaving snow-white surfaces. Owing to the thickness of the successive shelly layers, and to the circumstance of each new layer being but very little larger than the last, the scaling off of the old upper layers is a quite necessary process; for otherwise the orifice into the sack would have been encumbered and almost closed by four long, slightly tapering points, prolonged upwards from the basal layers that form the four existing valves. The same scaling off process takes place in Platylepas, and amongst pedunculated cirripedes in Lithotrya. Microscopical examination does not exhibit any fine spines on the membrane investing the valves, or any tubuli in the shelly layers after their dissolution in acid: in this respect the valves resemble those of Coronula. The summits of the valves project freely for about a third of their own height, above the level of the membrane by which they are surrounded. The orifice leading into the sack is bordered by very protuberant lips, standing up even considerably above the upper freely projecting portions of the valves.
The Opercular Membrane, connecting the valves and the top of the shell, is thick and tough, and deeply folded in concentric wrinkles. As in Coronula, it consists of two or three separate membranes (each composed of many laminæ) one over the other, united to successive shelly layers of the opercular valves. As the upper shelly layer scales off, the membrane attached to it is likewise thrown off. The innermost laminæ of the last-formed opercular membrane extend down, closely attached to the sheath, to its basal edge, and therefore nearly to the basis of the shell; the outer and older laminæ, all closely attached one within the other and to the sheath, extend to a less and less distance downwards; consequently, the animal’s body is enclosed in a tube, thinning out downwards, formed by the laminæ of the successive opercular membranes, surrounded outside by the shell: only in the following genus, Xenobalanus, shall we meet with a nearly analogous structure. As the shell of Tubicinella increases in diameter, from the growth of the radii, the opercular membrane lining the sheath is necessarily split along the six lines of suture, in the same manner as is the membrane externally investing the shell; in a like manner, also, it is repaired and added to by new longitudinal slips of membrane. Of this structure, in the opercular membrane, I have seen no other instance; for in most genera the old opercular membrane is moulted, and a new and larger one formed at each period of growth; in Coronula, in which the opercular membrane is likewise for a time persistent, it does not run far down the inside of the shell, and each new membrane is formed large and extensible, so as to allow, without splitting, of some increase in the diameter of the shell. The opercular membrane at the summit of the shell, in Tubicinella, is folded in concentric lines, and so deeply, that the basal edges of the opercular valves are generally hidden: this folding arises partly from each last deposited and innermost membrane being originally formed slightly folded, but chiefly from the rapid downward growth of the shell, and the consequent downward movement of the whole animal’s body, together with the opercular valves to which the body is attached, and this necessarily tends to wrinkle and fold the opercular membrane. Owing to the opercular membrane extending far down inside the shell, and being firmly attached to the sheath, as the upper part of the shell breaks away and disintegrates (which we shall presently see is constantly taking place), small particles of shell are left adherent to the circumferential and folded parts of the opercular membrane; and this at first much perplexed me.
Muscles of the Sack: these extend down almost to the base of the shell, but in the lower part they spread out and become thin and very irregular, not even corresponding on the opposite sides of the body. The fasciæ in the upper part show very distinct transverse striæ, but lower down these become either obscure or entirely deficient. In all these characters the muscles of Tubicinella and Coronula resemble each other. The rostral depressor muscles of the scuta consist each of four small bundles of fasciæ; the lateral depressores run not quite straight down, but in a curved course towards the carinal end of the sack: the tergal depressores are proportionally smaller than in ordinary sessile cirripedes, but they project and form two crests (with some fasciæ between them), which support the Branchiæ. The membrane lining the sack, I may here mention, is unusually strong.
Branchiæ. — These are enormously developed; the two together covering two thirds of the area of the sack. Each consists of two folds, both deeply plicated. They are attached longitudinally to the two crests, including and formed by the muscles running from the terga to near the basis of the shell. The branchiæ are likewise attached transversely to the sack, under the basal margins of the terga.
These have been described by Professor Owen in the second volume of the ‘Descriptive Catalogue of the Museum of the Royal College of Surgeons.’
Mouth. — The labrum is very finely hirsute, without teeth; the palpi have a short row of moderately long spines along their exterior basal margins. The mandibles have four rather narrow, sharp teeth, which (excepting the first) have double points: between the second and third, and again between the third and fourth teeth, there is a single small intermediate tooth: the inferior angle is irregularly pectinated. The maxillæ are small; there is a small notch beneath the two upper great spines, and a second notch near the inferior angle. Between the outer maxillæ, there is a square-topped mentum. Hence we see that the mouth in all its few peculiarities, resembles that of Coronula.
Cirri. — These are short, with short and broad segments protuberant in front. The pedicel of the first cirrus is very broad, and exteriorly clothed with fine hairs: its rami are slightly unequal in length. The second and third cirri are very short. The three posterior pairs are remarkable from the pairs of main spines being placed so close one under the other, and in an oblique direction, that at first they appear to form a single crowded transverse row: the dorsal tufts are rather large.
Body. — The body is remarkable from its nearly vertical position, and from the much elongated pyramidal form of the prosoma, extending down nearly to the bottom of the sack. The membrane investing the prosoma, presents a few circular folds, falsely appearing like articulations. The œsophagus enters the stomach rather obliquely. With respect to the generative system, I have only to remark, that the vesiculæ seminales are of great length, and convoluted to a remarkable degree. The ovarian cæca, form a thick layer at the bottom of the sack; they do not appear to extend up the shell round the sack. The only other point, which I shall here mention, is that beneath the basal articulation of the first cirrus, there is a longitudinal swelling, ending in a freely projecting point, .06 of an inch in length; at first, I thought, that we here had a rudiment of a filamentary appendage like those found in several Lepadidæ; but closer examination showed an orifice at the apex, leading into the acoustic meatus, in which the singular, wrinkled, heart-shaped acoustic vesicle, mentioned in the Introduction, hangs suspended. Alongside the freely depending point, with an orifice at its end, there is a smaller upward projecting point, without any orifice, but hollow within and lined by corium; I believe it opens internally into the acoustic meatus.
Attachment and general Growth of Shell. — All the specimens which I have seen have been attached in groups. They are buried up to the level of the operculum in the whale’s skin; and their summits, I suspect, lie even beneath the general surface of the body of the whale. It is certain that the shells grow much at their basal ends. As in the case of Coronula, the flat membranous basis does not actually penetrate the skin; but the general pressure of the whole group of shells seems to push inwards the skin of the whale, and directly beneath each shell the formation of new epidermis is apparently checked. Between the shells, however, though close together, the epidermis continues to be formed, and is pushed upwards between them, in the same manner as it is forced into the flattened cavities on the under side of the shell of Coronula.
The manner in which a full-grown shell assumes its ordinary shape, at first appears very perplexing: it has to change from a cylinder, at first probably not much above the 1/50th of an inch in diameter, to a cylinder nearly one inch in diameter: and this is not effected by the growth of the radii, for the radii never reach the basis, and the basis of course has to increase in diameter like the rest of the cylindrical shell. The radii serve only to keep the summit of the shell wider than the basis, which is the natural shape of this species; and in large-sized specimens, this purpose is sometimes aided by the parietes during their downward growth decreasing slightly in width. In ordinary nearly full-sized specimens, the parietes are of the same width at the top and bottom, but in some large-sized specimens, as just stated, they even become narrower towards the bottom; as they grow only at the bottom, one does not at first see how they can ever increase in width, or how the older shells can have acquired their present diameter. But an examination of young specimens, from .1 to .3 of an inch in diameter, at once serves to show how the shell attains its full size and shape: for here the parietes are all found to increase downwards sensibly in width, though at a much slower ratio than in other sessile cirripedes; in larger, but not full-grown specimens, a similar increase can by care be detected: hence by long-continued growth at the base of the shell, with the removal of the upper part, a young Tubicinella of small diameter will be converted into an old one of large diameter, retaining during all the time its sub-cylindrical form, with its summit rather broader than its base. With respect to the removal of the upper part of the shell, this seems almost constantly going on, for the summit of every specimen invariably had a freshly broken aspect. The peculiar structure of the sheath, which is the strongest part of the shell, namely, its division into oblique layers, separable by a slight force, doubtless is subservient to the repeated breakage of the summit. In some species of Tetraclita and of Balanus, gradual disintegration of the upper part of the shell is a necessary element in the growth of the animal, in order that the orifice may increase in size, and here we have mechanical breakage equally necessary.
I am indebted to Dr. J. E. Gray for calling my attention to this subject, and explaining to me several points.
Some curious results follow from the peculiar growth of Tubicinella just described. At Plate 17, fig. 3 b, we have a careful drawing of a lateral compartment, together with its radius, (which latter does not here concern us), taken from a shell .2 of an inch in diameter. The two protracted dotted lines show the form which this compartment would have assumed, if it had continued growing downwards at the same rate of increase in width as hitherto. But the increase in width always seems to become less and less as the shell grows older; hence the dotted lines, representing the wall after long-continued growth, ought to have been drawn diverging or widening still more slowly than they do. The lateral compartment in fig. 3 a is the exact size of the compartment of a large specimen nearly one inch in diameter; in this specimen the parietes, far from increasing in width downwards, had commenced, as is represented, decreasing. Compartments of all intermediate sizes between those figured at 3 a and 3 b can easily be shown in different specimens. From these facts we may safely infer, that if the whole growth of the compartment 3 a had been preserved, instead of its upper end having been continually chipped away, it would have had even a more tapering form than that represented by the whole and dotted lines in the two figures, and would have exceeded six inches in length! this of course being also the length of the whole shell. The young Tubicinella, of which 3 b is a compartment, was imbedded in the whale’s skin nearly up to the level of its operculum; if it had lived, it would no doubt have grown to the length just specified, viz., above six inches, but as all the growth is at the lower end, the bottom of the shell, it might be thought, would necessarily have become buried in the whale’s skin to this same depth; and the summit of the shell, on this same view, would have been buried to a depth by as much less as the height or length of the old shell itself, namely, by about one inch and a half less than the six inches. As far as I can judge from an examination of several large groups of full-grown specimens, preserved in their imbedded condition, the summits of the shells seem always to lie a little beneath the surrounding level of the whale’s skin, but not nearly to the extent here just inferred. Nor can I believe that the epidermis of the whale had ceased being formed under these specimens, whilst it had gone on being formed all round them, to the thickness of between four and five inches, and that it had subsequently disintegrated to this same thickness, — which processes would account for the summit of the shell being still on nearly a level with the surface of the whale. The view which seems to me most probable, is, that the rapid downward growth of the shell, besides indenting the whale’s skin, at the same time slowly pushes the whole shell out of the skin, and thus continually exposes the summit to the wear and breakage which seems to be necessary for its existence. On this view, the very peculiar form of Tubicinella, which is retained during life, namely, the slightly greater width at top than at bottom, is beautifully explained, viz., for the sake of facilitating the protrusion of the shell; for the ordinary conical shape of sessile cirripedes, with the apex upwards, would have rendered the pushing out of an imbedded shell almost impossible; on the other hand, we can see that the likewise very peculiar, concentric, prominent belts may be necessary to prevent too easy protrusion.
Fossil Species. — I do not believe that this genus has hitherto been found fossil. The Tubicinella maxima of Ch. Morren, said to have been found (see Bronn, Index Palæontologicus) in the chalk of Belgium, I have good reason to believe does not really belong to this genus.