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._[127] 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).
[127] 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 (p. 143--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._[128]--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.
[128] 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:[129] 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.
[129] 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.