longitudinal,--exterior and transverse--and oblique fasciæ; the former
are unusually strong; downwards they are attached to the basal
calcareous cup or disc, and upwards they extend all round to the lower
curved margins of the valves. They are, as usual, without transverse
striæ. Besides these, there are, (at least in _L. dorsalis_ and _L.
Nicobarica_,) two little fans of striæ-less muscles, which occur in no
other pedunculated cirripede; they are attached on each side of the
central line of the carina, near its base; they extend transversely and
a little upwards, and each fan converges to a point where the lower
margins of the carina and terga touch; of these muscles, the upper
fasciæ are the longest. Their action, I conceive, must be either to draw
slightly together the basal points of the terga, and so serve to open
their occludent margins, or to draw inwards the base of the carina:
these muscles apparently first shadow forth the posterior or carinal,
transversely-striated, opercular muscles of sessile cirripedes.
_Basal Calcareous Cup or Discs._--I have seen this part in all the
species, except _L. Valentiana_, and in this it probably occurs,
considering its very close alliance with _L. truncata_. The size, form,
and conditions of the cup or disc varies infinitely according to the
age, size, and position of the individual specimen. We will commence
with a full-sized animal, which has ceased to burrow downwards into the
rock, in which case the discs usually grow into a cup, and become
largely developed. In _L. dorsalis_ alone, I have seen many specimens,
so that the following description and remarks, though applicable I
believe to all the species, are drawn up from that alone. The cup (Pl.
VIII, fig. 1 _a´_, 1 _c´_) is hardly ever regular in outline, and is
either slightly or very deeply concave; I have seen one, half an inch in
diameter; it is formed of several thick layers of dirty white,
translucent, calcareous matter, with sinuous margins; externally the
surface is very irregular, and is coated by yellow membrane presently to
be described. The innermost and last-formed layer sometimes covers the
whole inside of the cup, and extends a little beyond its margin all
round; but more generally it projects beyond only one side, leaving the
other sides deserted. I have seen a _single_ new layer extending beyond
the underlying old layers, as much as one sixth of an inch; and again I
have seen a part of the cup, as much as a quarter of an inch in width,
deserted and covered with serpulæ. So irregular, however, is the growth,
that after a period an old deserted portion will occasionally be again
covered by a new layer, though of course without organic adhesion. Again
it sometimes happens that the last-formed layer, remaining central, is
very much less than the older layers; in one such instance the innermost
and last-formed layer (fig. 1 _a´_) had a diameter of only a quarter of
that of the whole cup, in the middle of which it was placed; the cup
thus tends to become filled up in the middle. The cup, in its fully
developed condition, is seated at the very bottom of the cavity in the
rock. From the aggregate thickness of the several component layers
forming the cup, the old and mature animal rises a little in its burrow;
for instance, the bottom of the cup in one specimen which I measured,
was 4/10ths of an inch in thickness.
In a younger condition, before the animal has bored down to the full
depth, and whilst the cavity is only of moderate diameter, the lower
part of the peduncle, instead of being attached to the inside of a cup,
adheres to small, irregular, nearly flat, calcareous discs, overlapping
each other like tiles (figs. 1, 2 _a´_). They are placed one below the
other, generally in a straight line, and are attached firmly to one side
of the burrow. The discs are oval, or rounded, or irregular, and are
commonly from 1/20th to 1/10th of an inch across: they usually form a
quite straight ribbon, widening a little downwards: each little disc
overlaps and extends beyond the one last formed, fully half its own
diameter. I have seen one row of discs an inch in length, but the upper
discs are always worn away by the friction of the calcified serrated
scales on the peduncle. It is very important to observe that the lowest
disc is not fixed, (as was the case with the cup,) at the very bottom of
the burrow, but on one side, just above the bottom, which latter part is
occupied by the blunt basal end of the peduncle.
In a valuable paper on _L. Nicobarica_, by Reinhardt, presently to be
referred to, the disc is said to be attached on the carinal side (see
fig. 2) of the peduncle; and this, I believe, is general. I have seen
one instance in which, during the excavation of a new burrow, an old
burrow was met with, and the row of discs turned down it, making, with
their previous course, nearly a right-angle. In another similar
instance, the discs, instead of turning down, became very large and
broad, and so fairly formed a bridge across the old burrow (fig.
1),--becoming narrow again as soon as the animal recommenced burrowing
into the solid rock. Sometimes, as it appears, the animal, whilst still
small, from some unknown cause, stops burrowing downwards, and then a
cup is formed at the bottom of the hole. As soon as the animal has got
to its full depth, the burrow increases only in diameter, and during
this process the linear row of discs is ground away and lost; a cup is
then formed. The little discs can be deposited or formed only at each
fresh exuviation; and as some of the burrows are above two inches in
depth, and as on an average each disc does not extend beyond the
underlying disc more than 1/15th of an inch, an animal which has bored
two inches in depth, must have moulted at least thirty times. I may here
remark that I have reason to believe, from some interesting
observations made by Mr. W. Thompson, of Belfast, that some sessile
cirripedes moult about every fortnight.
_Internal Structure of the Cup._--When the cup is dissolved in acid,
each shelly layer is represented by a rather tough, pale-brown membrane,
itself composed of numerous fine laminæ, which, under a one-eighth of an
inch object glass, exhibit generally only the appearance of a mezzotinto
drawing; but there often were layers of branching vessels, (like
moss-agate,) less than the 1/10,000th of an inch in diameter, and of a
darkish colour; these vessels are not articulated, but otherwise
resemble the same peculiar structure in the valves of the capitulum. The
exterior yellow membrane is marked, or rather composed of successive
narrow rims, which, in fact, are the lines of termination of the laminæ
of membrane, which in a calcified state form the cup itself. In most
parts, both on the borders and under the centre of the cup, but not
everywhere, there are imbedded in the yellow membrane, elongated,
irregular, top-shaped masses of bright yellow chitine, each furnished
with a tubulus, which penetrating the calcareous laminæ leads to the
corium; the little apertures thus formed, are clearly visible in the
layers of membrane, left after exposure to acid. In _L. Nicobarica_, the
innermost shelly layer of the cup was punctured, like the surface of the
shell in Chthamalus and many other sessile Cirripedes, by the internal
orifices of these tubuli. The top-shaped masses often have star-shaped
summits; and they differ in no essential respects from those on the
lower part of the peduncle, excepting that they are quite imbedded in
the membrane covering the under surface of the cup, whereas those on the
peduncle project freely. I found these top-shaped bodies in the outer
membrane of the cups in _L. dorsalis_, _L. cauta_, and _L. Rhodiopus_,
which alone I was enabled to dissolve in acid; and I mention this fact,
as indicating the probable presence of the more important star-headed
projections on the lower parts of the peduncle in these same species.
The basal calcareous cup resembles, in essential structure, the valves
of the capitulum; the chief difference being that in the former there is
a larger proportion of animal matter or membranous layers.
After the dissolution of the cups, in _L. dorsalis_ and _L. Rhodiopus_,
I most distinctly traced the two cement-ducts; they included the usual
darker chord of cellular matter; they were of rather small diameter,
namely, 2/3000th of an inch. The two (in _L. dorsalis_) ran in a very
irregular course, not parallel to each other, making the most abrupt
bends. They passed through the membranous layers, (as seen after
dissolution,) and running for short spaces parallel to the component
laminæ, were attached to them. In their irregular course, these
cement-ducts resemble those of _Pollicipes mitella_, but I could not
perceive that any cement had been poured out at the abrupt bends. In one
specimen of a basal cup, which I was enabled to examine whilst still
attached to the rock, I found under the very centre, (and of course
outside the yellow membrane,) a very small area of dark brown cement of
the usual appearance. In several specimens of full-sized cups, I was not
able to perceive any cement on the external surfaces of the upper and
later-formed layers; hence I believe that the cup is cemented to the
bottom of the hole only during the early stages of its formation; and
this, considering its protected situation, would no doubt be sufficient
to affix the animal. This probably accounts for the small size of the
cement-ducts, and for the facility with which, as it appears, the cups
can be removed in an unbroken condition from the rock. In the case,
however, of the small, flat, calcareous discs, which are formed whilst
the animal is burrowing into the rock, these are attached firmly to the
sides of the holes, in the usual manner, by cement. In this cirripede it
would be useless to look for the prehensile antennæ of the larva under
the cup, for the animal, during the formation of the successive discs,
must have travelled some distance from the spot on which the larva first
attached itself.
The membrane of the peduncle is continuous with the yellow membrane
coating the external surface of the cup; and this latter membrane is
continuous with those delicate laminæ which, in a calcified condition,
form the layers of the cup itself. In an exactly similar manner, in this
and other cirripedes, the membrane of the peduncle, at the top, is
continuous with that coating the valves, and is attached to the lower
exterior edge of the last-formed layer of shell. When a new shelly layer
is formed, both under the valves of the capitulum and inside the basal
calcareous cup, it projects beyond the old layer, and is included within
the old, as yet not moulted, membrane of the peduncle. Within the cup of
_L. Nicobarica_ I found a lately-formed layer of shell, projecting
1/10th of an inch on one side of the cup, and by its protuberance
distinguishable even through the old coat of the peduncle, which was
nearly ready to be moulted. In an analogous manner, in the capitulum of
_L. dorsalis_ and _L. truncata_, I have found a new peduncular membrane
bearing the usual, but then sharp, calcified scales, attached to the
lower projecting edge of the last-formed shelly layer, lying under the
old peduncular membrane, which was attached to the penultimate layer of
shell, and with its worn scales was just ready to be moulted.
The final cause of the moulting of the calcified scales, together with
the membrane of the peduncle to which they are attached,--a case
confined to Lithotrya,--I have scarcely any doubt is the reproduction of
a succession of scales, sharply serrated for the purpose of enlarging
the cavity in which the animal is lodged. The extreme thinness of the
membrane of the peduncle has been noticed; this may be partly related to
its protected condition, but partly, I think, to the necessity of its
being formed in a very extensible condition; for the new coat, owing to
the projection of the new shelly layers under the valves, and within the
basal cup, is by so much shorter than the old peduncle, yet after
exuviation it has to stretch to a greater length than the old membrane,
to allow of the growth of the Cirripede. Owing to the thinness and
fragility of this membrane, the basal attachment of the Cirripede is, no
doubt, chiefly effected by the unusually strong longitudinal muscles;
and the necessity of a surface of attachment for these muscles, stronger
than the external membrane of the peduncle, probably is one of the final
causes of the basal calcareous disc and cup, and likewise for the
unusual manner in which the valves of the capitulum are locked together
by folds and small roughened projections. The basal discs and cup,
however, apparently serve for several other purposes, namely, for
raising the animal a little in its burrow, (which is narrow and pointed
at the bottom,) at that period of growth when it has ceased to burrow
downwards, but still increases in diameter; also for carrying the
animal, as over a bridge, across any pre-existing cavity in the rock;
and lastly, perhaps, for removing lower down, in the intervals of
exuviation, the point of attachment for the longitudinal peduncular
muscles.
_Position of the animal in the rock, and its power of excavation._--A
specimen of rock, two or three inches square, in Mr. Cuming’s
possession, is full of Lithotryas; the cavities extend in every possible
direction, and several were parallel, but with the animals in reversed
positions; the same thing is apparent in some specimens of Mr.
Stutchbury’s, and it was evident that the positions occupied by the
animals were entirely due to chance. In Mr. Cuming’s specimen of rock, a
considerable portion of the external surface is preserved, and here it
can be seen that many of the specimens have their capitulums directed
from the external surface directly inwards. These individuals, which
were of full size, must have preyed on infusoria inhabiting the cavities
of the porous, calcareous rock. On the other hand, I have seen some
young specimens of _L. dorsalis_ with their valves not at all rubbed,
and others of full size with uninjured Balani and corallines on the tips
of the valves, and again a specimen of _L. truncata_ with minute
pale-green sea-weed on the summit of the capitulum,--all which
appearances induce me to believe that in these cases, the valves had
projected freely beyond the cavity in which their peduncles were lodged.
I may here also mention that in Mr. Cuming’s specimen, above alluded to,
the basal cups of five specimens touched and adhered to each other; I
was not able to make out whether there had originally existed separate
burrows, as I think is most probable, and that the walls had been wholly
worn away, or whether the five specimens had fixed themselves on one
side of a large pre-existing, common cavity. Young specimens seem to
burrow to the full depth, before nearly acquiring the diameter which
they ultimately attain. I measured one burrow, 1.2 of an inch in depth,
which, at its mouth or widest part, was only .17 in diameter.
The several species occur imbedded in soft calcareous rocks, in massive
corals, and in the shells of mollusca and of cirripedes. It has been
doubted by several naturalists, whether the basal calcareous cup at all
belongs to the Lithotrya, but after the foregoing microscopical
observations on its structure, it is useless to discuss this point. So
again it has been doubted whether the cavity is formed by the cirripede
itself; but there is so obvious a relation between the diameters of
specimens of various sizes, and the holes occupied by them, that I can
entertain no doubt on this head. The holes, moreover, are not quite
cylindrical, but broadly oval, like the section of the animal. The
simple fact, that in this genus alone each fresh shelly layer round the
bases of the valves, and therefore at the widest part of the capitulum,
are sharply toothed; and secondly, that in this genus alone a succession
of sharply serrated scales, on the upper and widest part of the
peduncle, are periodically formed at each exuviation; and that
consequently the teeth on the valves and scales are sharp, and fit for
wearing soft stone, at that very period when the animal has to increase
in size, would alone render the view probable that the Lithotrya makes
or at least enlarges the cavities in which it is imbedded.
Although it may be admitted that Lithotrya has the power of enlarging
its cavity, how does it first bore down into the rock? It is quite
certain that the basal cup is absolutely fixed, and that neither in form
nor state of surface it is at all fitted for boring.[66] I was quite
unable to answer the foregoing question, until seeing the admirable
figures by Reinhardt[67], (Pl. VIII, figs. 2 and 2 _a´_) of _L.
Nicobarica_, still attached in its cavity. Subsequently I obtained from
Mr. Stutchbury several pieces of rock completely drilled with holes,
many of small diameter, by _L. dorsalis_, and in these I found numerous
instances of the linear rows of little discs, like those of _L.
Nicobarica_, showing in the plainest manner, that each time a new disc
is formed, that is, at each exuviation, the animal moves a short step
downwards; and as the lowest of these little discs _in none of the
burrows_ was placed at the very bottom, we see that the lowest point of
the peduncle must be the wearing agent. In the peduncle of an
individual of _L. dorsalis_, nearly ready to moult, I found, it may be
remembered, beneath and round the basal disc, under the old membrane of
the peduncle, a new membrane studded with calcified beads, but with the
horny star-headed spines not yet developed, whilst on the old outer coat
these latter had been worn down quite smooth, and the calcified beads
worn entirely away. Here, then, we have an excellent rasping surface.
With respect to the power of movement necessary for the boring action,
the peduncle is amply furnished with transverse, oblique, and
longitudinal striæ-less muscles,--the latter attached to the basal disc.
In all the pedunculata, I have reason to believe that these muscles are
in constant slight involuntary action. This being the case, I conceive
that the small, blunt, spur-like portion of the peduncle, descending
beneath the basal rim of the lowest disc, would inevitably partake
slightly of the movements of the whole distended animal. As soon as the
Lithotrya has reached that depth, which its instinct points out as most
suitable to its habits, the discs are converted into an irregularly
growing cup, and the animal then only increases in diameter, enlarging
its cavity by the action of the serrated scales on the peduncle, and of
the serrated lower edges of the valves of the capitulum. With respect to
those reversed individuals attached with their capitulums downwards, I
suppose that the larvæ had crept into some deep cavity, perhaps made
originally by a Lithotrya, of which the rock in the specimen in question
was quite full, and had there attached themselves. Finally, it appears
that in Lithotrya the burrowing is simply a mechanical action; it is
effected by each layer of shell in the basal attached discs overlapping,
in a straight line, the last-formed layer,--by the membrane of the
peduncle and the valves of the capitulum having excellent and often
renewed rasping surfaces,--and, lastly, by the end of the peduncle (that
is homologically the front of the head) thus roughened, extending
beyond the surface of attachment, and possessing the power of slight
movement.
[66] Mr. Hancock, in his admirable account of his burrowing
Cirripede, _Alcippe lampas_, (‘Annals of Nat. Hist.,’ Nov. 1849,
p. 313,) came to this conclusion regarding the cup of Lithotrya,
and hence was led to think that this genus did not form its own
burrows, but inhabited pre-existing cavities. I am much indebted
to this gentleman, who has been so eminently successful in his
researches on the boring powers of marine animals, for giving me
his opinion on several points connected with the present
discussion.
[67] I owe to the great kindness of Prof. Steenstrup the sight of
this Plate, published in the ‘Scientific Communications from the
Union of Natural History,’ Copenhagen, January 30, 1850, No. I.
Since this sheet has been set up in type, I have received from
Prof. Steenstrup the memoir, in Danish, belonging to the figures
in question; and the greater part of this has been translated to
me by the kindness of a friend. My account of the means of
burrowing is essentially the same as that published by Reinhardt;
but the moulting of the scales on the peduncle, the presence of
scales and of points of a different nature, the method of
attachment by cement, the conversion of the discs into a cup,
&c., seem not to have been known to this naturalist. Reinhardt
states that the points on the peduncle will scratch Iceland spar,
and that, apparently, they are formed of phosphate of lime: in
the case of the closely-allied _L. dorsalis_, I must believe that
the scales or beads on the peduncle are formed of carbonate of
lime, for they were quickly dissolved with effervescence in
acetic acid; and the star-headed points, which are subsequently
developed under the calcareous scales, appeared to me, under the
compound microscope, to be formed of a horn or chitine substance.
Reinhardt states that the basal point of the peduncle is arched a
little under the lowest disc, and there forms for itself a slight
furrow (as represented in the lateral view, Pl. VIII, fig. 2);
but in the burrows examined by me, this furrow or depression did
not really exist, the appearance resulting from the basal margin
of the lowest disc, projecting beyond the wall of the cavity by
the amount of its own slight thickness.
* * * * *
We will now proceed with our generic description.--
_Animal’s Body._--This, as already stated, is partially lodged within
the peduncle. The prosoma is rather largely developed.
The _Mouth_ is placed at a moderate distance from the adductor muscle.
The _Labrum_ is moderately bullate, with a row of blunt bead-like teeth,
mingled with fine bristles, on the crest, which in the middle part is
generally somewhat flattened.
The _Palpi_ are blunt, and even squarely truncated at their ends; they
are of large size, so that, if they had been half as large again, or
even less, their tips would have met.
_Mandibles_ (Pl. X, fig. 2), with three nearly equal large teeth, and
the inferior angle produced, broad, and strongly pectinated: in the
interspaces between these teeth there are, in all the species, some very
fine teeth or pectinations, which are seated a little on one side of the
medial line. The mandibles are somewhat singular from the size of the
transparent flexible apodemes (_a_ _a_) to which the muscles are
attached; these are oval and constricted at their origins: in _L.
dorsalis_ they are roughened with little points; in _L. cauta_ and _L.
truncata_ they are large, of the same shape, but smooth.
_Maxillæ._--These are larger, compared to the mandibles, than is usual
with pedunculated Cirripedes; they differ in shape in the different
species, being either nearly straight on their edge, and notched or not
(fig. 10), or notched with the inferior part forming a double prominence
(fig. 12); the spines on the inferior angle, which is sometimes slightly
produced, are always crowded together into a brush, and are finer than
those on the upper parts. The apodemes are less straight than is usual,
and at their origin take, in all the species, a rather abrupt bend;
their extremity is enlarged into a little disc, which in _L. dorsalis_
is covered with strong points, but in the other species is, as usual,
smooth.
_Outer Maxillæ._--The inner margin is slightly concave, and in _L.
truncata_ alone, the bristles are hardly continuous, being interrupted
in the middle part. The olfactory orifices are only very slightly
prominent. The spines on all the trophi are more or less doubly
serrated.