organs in the Lepadidæ. The Capitulum is usually much flattened, but
sometimes broadly oval in section. It is generally formed of five or
more valves, connected together by very narrow or broad strips of
membrane; sometimes the valves are rudimental or absent, when the whole
consists of membrane. When the valves are numerous, and they
occasionally exceed a hundred in number, they are arranged in whorls,
with each valve generally so placed as to cover the interval between the
two valves above. Of all the valves, the scuta are the most persistent;
then come the terga, and then the carina; the rostrum and latera occur
only in Scalpellum and Pollicipes, and in a rudimentary condition in
Lithotrya, and, perhaps, in the fossil genus Loricula. The valves are
formed sometimes of chitine (as in Ibla and Alepas), but usually of
shell, which varies from transparency to entire opacity. The shell is
generally white, occasionally reddish or purple; exteriorly, the valves
are covered by more or less persistent, generally yellow, strong
membrane. The scuta and terga are always considerably larger than the
other valves: in the different genera the valves differ so much in shape
that little can be predicated of them in common; even the direction of
their lines of growth differs,--thus, in Lepas and some allied genera,
the chief growth of the scuta and of the carina is upwards, whereas in
Pollicipes and Lithotrya, it is entirely downwards; in Oxynaspis, and
some species of Scalpellum, it is both upwards and downwards. Even in
the same species, there is often very considerable variation in the
exact shape of the valves, more especially of the terga. The adductor
muscle is always attached to a point not far from the middle of the
scuta, and it generally has a pit for its attachment. In several genera,
namely, Pæcilasma, Dichelaspis, Conchoderma, and Alepas, the scuta show
a tendency to be bilobed or trilobed. The valves are placed either at
some distance from each other, or close together; but their growing
margins very rarely overlap each other, though this is sometimes the
case with their upper, free, tile-like apices; in a few species the
scuta and terga are articulated together, or united by a fold. The
membrane connecting the valves, where they do not touch each other, is
like that forming the peduncle, and is sometimes brilliantly coloured
crimson-red; generally, it appears blueish-gray, from the corium being
seen through. Small pointed spines, connected with the underlying
corium by tubuli, are not unfrequently articulated on this membrane: the
tubuli, however, are often present where there are no spines. To allow
of the growth of the capitulum, the membrane between the valves splits
at each period of exuviation, when a new strip of membrane is formed
beneath, connected on each side with a fresh layer of shell,--the old
and outer slips of membrane disintegrating and disappearing: when there
are many valves, the line of splitting is singularly complicated. This
membrane consists of chitine,[13] and is composed of numerous fine
laminæ. After the valves have been placed in acid, a residue, very
different in bulk in different genera, is left, also composed of
successive laminæ of chitine. It appears to me that each single lamina
of calcified chitine, composing the shell, must once have been
continuous with a non-calcified lamina in the membrane connecting the
several valves: at the line where this change in calcification
supervenes, the chitine generally assumes some colour, and becomes much
harder and more persistent; and as the whole valve is formed of
component laminæ thus edged (the once continuous laminæ of non-calcified
chitine connecting the valves, having disintegrated and disappeared) the
surfaces of the valves are generally left covered by a persistent
membrane, constituted of these edgings: this membrane has been called
the epidermis. In some genera, as in Lepas, this so-called epidermis is
seldom preserved, excepting on the last zone of growth: in Scalpellum
and Pollicipes it usually covers the whole valves. It appears to me that
the laminæ of chitine, and of calcified chitine composing the valves,
are both formed not by secretion, but by the metamorphosis of an outer
layer of corium into these substances.
[13] Chitine is confined to the Articulata. It was Dr. C. Schmidt
(Contributions, &c., being a Physiologico-Chemical investigation:
in Taylor’s ‘Scientific Memoirs,’ vol. v), who discovered that
the membrane connecting the valves and forming the peduncle, and
the tissues of the internal animal, were composed of this
substance. But Dr. Schmidt says that the valves in Lepas are
composed of 3.09 of albuminates, and 96.81 of incombustible
residue; I cannot but think that the existence of the albuminates
is an error caused by Dr. Schmidt’s belief that the Cirripedia
were intermediate between Crustacea and Mollusca, in the shells
of which latter, the animal basis consists of albuminates. For
after placing the valves of Lepas and Pollicipes in cold acid, I
found that the membrane left could _not be dissolved_ in boiling
caustic potash, but could, though slowly, (and without change of
colour,) in boiling muriatic acid; and these are the main
diagnostic characters of Chitine, compared with albuminous
substances. I may add, that Schmidt was also induced to consider
the shells of Cirripedia as having the same nature with those of
Mollusca, from finding that in the above 96.81 of incombustible
matter, 99.3 consisted of carbonate and only 0.7 of phosphate of
lime; but Dr. Schmidt’s own analyses prove how extremely variable
the proportions of these salts are in the Crustacea, as the
following instance shows:--
_Lobster._ _Squilla._
Phosphate of Lime 12.06 47.52
Carbonate of Lime 87.94 52.48
And, therefore, it is not very surprising that Cirripedia should
have still less phosphate of lime in their shells, than has a
lobster compared with a squilla.
Within the capitulum is the sack, which, together with the upper
internal part of the peduncle, encloses the animal’s body. The sack is
lined by a most delicate membrane of chitine, under which there is a
double layer of corium; this double layer is united together by short,
strong, transverse bundles of fibres, branched at both ends:[14] in some
genera, the ovarian tubes extend between these two layers. We have seen,
under the head of the Metamorphoses, that the delicate tunic lining the
sack is simply a duplicature of the thick membrane and valves forming
the capitulum, the whole being the posterior portion of the carapace of
the larva slightly modified.
[14] I am much indebted to Mr. Inman of Liverpool for having
kindly sent me excellent specimens illustrating this structure.
_Peduncle._--Its length varies greatly in different species, and even in
the same species, according to the situation occupied by the individual;
its lower end is sometimes pointed, but generally only a little narrower
than the upper end. In outline, the peduncle is usually flattened, but
sometimes quite cylindrical. It is composed of very strong, generally
thick, transparent membrane, rarely coloured reddish, and often
penetrated by numerous tubuli. The underlying corium is sometimes
coloured in longitudinal bands. At each period of growth a new and
larger integument is formed under the old one, which gradually
disintegrates and disappears; the extreme lower point is often deserted
by the corium, and ceases to grow, whilst the whole upper part still
continues increasing in diameter: in length the chief addition is made
(as is clearly seen in those genera having calcified scales), round the
upper margin, at the base of the capitulum. The surface of the membrane
is either naked or superficially clothed with minute, pointed,
articulated spines, or it is penetrated by calcified scales or styles,
(in Ibla alone formed of chitine,) which pass through it to the corium,
and are added to at their bases, like the valves, at each period of
growth. In Lithotrya alone the scales of the peduncle are moulted
together with the connecting membrane. These scales on the peduncle are
generally placed symmetrically in whorls, with each scale corresponding
with the junctions of two scales, both above and below. Except in
_Scalpellum ornatum_ and the fossil _Loricula pulchella_, they are very
small compared with the valves of the capitulum. When the scales are
symmetrical, new ones are first formed only round the summit of the
peduncle, and only those in the few uppermost whorls continue to grow or
to be added to at their bases; afterwards membrane is deposited under
them. The shelly matter of the scales resembles that of the valves, and
the manner of growth is the same; tubuli generally run to and through
them from the corium. From the continued enlargement of the membrane of
the peduncle, the scales come to stand, in the lower portion, some way
apart. In Ibla, new horny styles are formed indifferently in all parts
of the peduncle. In some species of Pollicipes, the calcareous styles
are not symmetrical or symmetrically arranged; and besides those first
formed round the top of the peduncle, there are other and larger ones
formed near its base. Lastly, in Lithotrya we have a row of calcareous
discs or an irregular, basal cup, formed in the same manner as the
valves of the capitulum: in this genus alone (as already stated,) the
calcified scales are moulted, and here alone their edges are serrated.
The peduncle is lined within by three layers of muscles, longitudinal,
transverse, and oblique, all destitute of the transverse striæ,
characteristic of voluntary muscles; they run from the bottom of the
peduncle to the base of the capitulum, as in Lepas, or half way up it,
as in Conchoderma; in Alepas alone they surround the whole capitulum up
to its summit. In Lithotrya there are two little, fan-like, transverse
muscles (involuntary), extending from the basal points of the terga to a
central line on the under side of the carina. The gentle swaying
to and fro movements, and the great power of longitudinal
contraction,--movements apparently common, as I infer from facts
communicated to me by Mr. Peach, to all the Pedunculata,--are produced
by these muscles. The interior of the peduncle is filled up with a great
mass of branching ovarian tubes; but in Ibla and Lithotrya, the upper
part of the peduncle is occupied by the animal’s body.
_Means of Attachment._--If the peduncle be very carefully removed (Tab.
IX, fig. 7 and Tab. I, fig. 6 _b_), from the surface of attachment,
quite close to the end, but not at the actual apex, the larval
prehensile antennæ can always be found: these have been sufficiently
described for our present purpose under the head of the Metamorphoses;
but I may add, that the diagnostic differences between them in the
several genera are briefly given, for a special purpose, in a discussion
on the sexes of Scalpellum at the end of that genus. We have seen in the
larva, that the cement-ducts, with their opaque cellular contents, can
be traced from within the discs of the antennæ to the anterior or lower
ends of the two gut-formed bodies, which it can be demonstrated are the
incipient ovaria.
In mature Cirripedes these ducts can be followed, in a slightly sinuous
course, along the muscles on each side within the peduncle, till they
expand into two small organs, which I have called cement-glands. These
glands are found with great difficulty, except in _Conchoderma aurita_,
where they are placed on each side under the inner layer of corium, at
the bottom of the sack, so as to be just above the top of the peduncle;
they resemble in shape a retort, (Pl. IX, fig. 3.). In _Pollicipes
mitella_ and _polymerus_ they lie half way down the peduncle, close
together, and apparently enclosed within a common membrane; in these two
species the broad end of the gland is bent towards the neck of the
retort. In Scalpellum the position is the same, but the shape is more
globular. In Ibla the structure is more simple, namely, a tube slightly
enlarged, running downwards, bent a little upwards, and then resuming
its former downward course, the lower portion forming the duct. The
gland contains a strongly coherent, pulpy, opaque, cellular mass, like
that in the cement-ducts; but in some instances, presently to be
mentioned, this cellular mass becomes converted within either the ducts
or gland, or within both, into transparent, yellow, tough cement.
Generally in Conchoderma, Pollicipes, and Scalpellum, two ovarian tubes,
but in one specimen of _Conchoderma aurita_, three tubes, and in Ibla
one tube could be seen running into or forming the gland; of the nature
of the tubes there could not be the least doubt, for at a little
distance from the glands they gave out branches (Pl. IX, fig. 3),
containing ova in every state of development. In some specimens as in
that figured of _Conchoderma aurita_, the ovarian tube on one side of
the gland is larger than on the other, and has rather the appearance of
being deeply embedded in the gland than of forming it; but, in other
specimens, the two ovarian tubes first formed a little pouch, into which
their cellular contents could be clearly seen to enter; and then this
pouch expanded into the gland; thus quite removing a doubt which I had
sometimes felt, whether the ovarian tube was not simply attached to or
embedded in the gland, without any further connection. By dissection
the multiple external coats of the gland and ovarian tubes could be seen
to be continuous. The cellular contents of the tubes passed into the
more opaque cellular contents of the gland, by a layer of transparent,
pulpy, pale, yellowish substance. There appeared in several instances to
be a relation, between the state of fulness and condition of the
contents of the gland, and of the immediately adjoining portions of the
ovarian tubes. In one specimen of _Pollicipes mitella_ it was clear that
the altered, tough, yellow, transparent, non-cellular contents of the
two glands and ducts, had actually invaded for some little distance, the
two ovarian tubes which ran into them, thus showing the continuity of
the whole. From these facts I conclude, without hesitation, that the
gland itself is a part of an ovarian tube specially modified; and
further, that the cellular matter, which in the ovarian tubes serves for
the development of the ova, is, by the special action of the walls of
the gland, changed into the opaquer cellular matter in the ducts, and
this again subsequently into that tissue or substance, which cements the
Cirripede to its surface of attachment.
As the individuals grow and increase in size, so do the glands and
cement-ducts; but it seems often to happen, that when a specimen is
immovably attached, the cementing apparatus ceases to act, and the
cellular contents of the duct become converted into a thread of
transparent tough cement; the investing membrane, also, of the ducts, in
Conchoderma sometimes becomes hard and mamillated. I have already
alluded to the case of a Pollicipes, in which both glands and ducts, and
even a small portion of the two adjoining ovarian tubes, had become thus
filled up. As in sessile Cirripedes, at every fresh period of growth a
new cement gland is formed, it has occurred to me, that possibly in
Pollicipes something similar may take place. In sessile Cirripedes, the
old cement-glands are all preserved in a functionless condition,
adhering to the membranous or calcareous basis, each new larger one
attached to that last formed, and each giving out cement-ducts, which,
bifurcating in the most complicated manner, pass outside the shell and
thus attach it to some foreign body.
The cement, removed from the outside of a Cirripede, consists of a thin
layer of very tough, bright-brown, transparent, laminated substance,
exhibiting no structure under the highest powers, or at most a very fine
dotted appearance, like a mezzotinto drawing. It is of the nature of
chitine; but boiling caustic potash has rather more effect on it than on
true chitine; and I think boiling nitric acid rather less effect. In one
single instance, namely, in Coronula, the cement comes out of the four
orifices of the two bifurcating ducts, in the shape of distinct cells,
which, between the whale’s skin and the basal membrane, arrange
themselves so as to make a circular, continuous slip of cement; then the
cells blend together, and are converted into transparent, structureless
cement. Cementing tissue or membrane would, perhaps, have been a more
correct title than cement; but, in ordinary cases, its appearance is so
little like that of an organised tissue, that I have for this reason,
and for brevity-sake, preferred the simple term of Cement.
In the larva the cement always escapes through the prehensile antennæ;
and it thus continues to do throughout life in most or all of the
species of Lepas, Conchoderma, Dichelaspis and Ibla. In the first two of
these genera, the cement escapes from the borders of the lower side of
the disc or penultimate segment of the antennæ, and can be there seen
radiating out like spokes, which at their ends divide into finer and
finer branches, till a uniform sheet of cement is formed, fastening the
antennæ and the adjoining part of the peduncle down to the surface of
attachment. In _Dichelaspis Warwickii_ and _Scalpellum Peronii_, the
cement, or part at least, comes out of the ultimate segment of the
antennæ, in the shape of one tube, within another tube of considerable
diameter and length. In _Scalpellum vulgare_, and probably in some of
the other species, which live attached to corallines, the cement soon
ceases to debouch from the antennæ, but instead, bursts through a row of
orifices on the rostral margin of the peduncle (Pl. IX, fig. 7), by
which means this margin is symmetrically fastened down to the delicate,
horny branches of the zoophyte. In Pollicipes, the two cement-ducts,
either together or separately (Pl. IX, fig. 2, 2 _a´_), wind about the
bottom of the peduncle in the most tortuous course, at each bend pouring
out cement through a hole in the membrane of the peduncle. In Ibla the
lower part of the peduncle is internally filled by cement, and thus
rendered rigid. In _Lepas fascicularis_ a vesicular ball of cement
surrounding the peduncle is thus formed (Pl. I, fig. 6), and serves as a
float! All these curious, special adaptations are described under the
respective genera. How the cement forces its way through the antennæ,
and often through apertures in the thick membrane of the peduncle, I do
not understand. I do not believe, though some appearances favoured the
notion, that the duct itself debouches and divides, at least this is not
the case in Coronula, but only that the internal chord of cellular
matter thus acts and spreads itself out; nor do I understand how, when
the antennæ and immediately adjoining parts are once cemented down, any
more cement can escape; yet this must take place, as may be inferred
from the breadth of the cemented, terminal portion of the peduncle in
Lepas and Conchoderma; and from the often active condition in old
individuals of the cementing organs.
I have entered on this subject at some length, (and I wish I had space
for more illustrations,) from its offering, perhaps, the most curious
point in the natural history of the Cirripedia. It is the one chief
character of the Sub-class. I am well aware how extremely improbable it
must appear, that part of an ovarian tube should be converted into a
gland, in which cellular matter is modified, so that instead of aiding
in the development of new beings, it forms itself into a tissue or
substance, which leaves the body[15] in order to fasten it to a foreign
support. But on no other view can the structure, clearly seen by me both
in the mature Cirripede and in the larva, be explained, and I feel no
hesitation in advancing it. I may here venture to quote the substance of
a remark made by Professor Owen, when I communicated to him the
foregoing facts, namely, that there was a new problem to solve,--new
work to perform,--to attach permanently a crustacean to a foreign body;
and that hence no one could, _a priori_, tell by what singular and novel
means this would be effected.
[15] The protrusion of the egg-bearing pouches in Cyclops and its
kindred genera, outside the body, offers a feeble analogy with
what takes place in Cirripedes. Professor Allman (‘Annals of
Natural History,’ vol. xx, p. 7,) who has attended to the
subject, says that the external egg-bearing pouches are “a
portion of the membrane of the true ovaries:” if the membrane of
these pouches had been specially made adhesive, the analogy would
have been closer.
_Filamentary Appendages._--These have generally been considered to act
as branchiæ; they occur at the bases of the first pair of cirri in
Lepas, Alepas, Conchoderma, and in three species of Pollicipes: in
Conchoderma there are similar appendages attached to the pedicels of the
cirri (Pl. IX, fig. 4, _g-k_); and in the above three species of
Pollicipes there is a double row of them on the prosoma: their numbers
differ in different species (in some there being none) of the same
genus, and even in different individuals of the same species; they are
entirely absent in the majority of the genera. These facts would
indicate that they are not of high functional importance; and they seem
so generally occupied by testes (Pl. iv, fig. 5), that I suspect their
function is quite as much to give room for the development of these
glands, as to serve for respiratory purposes. With the exception of the
four above-named genera, the mere surface of the body and of the sack
must be sufficient for respiration: in _Conchoderma aurita_ the two
great expansions of surface, afforded by the folded, tubular, ear-like
projections, aid, as I believe, towards this end.
The shape of the body varies, owing to the greater or less development
of the lower part of the prosoma, the greater or less distance of the
first from the second pair of cirri, and of the mouth from the adductor
scutorum muscle, (Pl. IX, fig. 4, and Pl. IV, 8 _a´_). In all the
genera, the body is much flattened. I may here mention a few particulars
about the muscular system. One of the largest muscular masses is formed
by the adductor scutorum, and by the muscles which surround in a double
layer (the fasciæ being oblique to each other) the whole of the upper
part of the prosoma. From under the adductor, a pair of delicate muscles
runs to the basal edge of the labrum, so as to retract the whole mouth,
and two other pair to the integument between the mouth and the adductor,
so as to fold it: again, there are other delicate muscles in some (for
instance in _Lepas Hillii_) if not in all the Lepadidæ, crossing each
other in the most singular loops, and serving apparently to fold the
membrane between the occludent edges of the scuta. Within the prosoma
there is a strong adductor muscle, running straight from side to side,
for the purpose, as it appears, of flattening the body. The thorax, on
the dorsal and ventral surfaces, is well furnished with straight and
oblique muscles (without striæ), which straighten and curl up this part
of the body. The muscles running into the pedicels of the cirri, cross
each other on the ventral surface of the thorax; the muscles within the
rami are attached to the upper segments of the pedicels. Finally, I may
remark that the whole of the body and the cirri are capable of many
diversified movements.
_Mouth._--This is prominent, and almost probosciformed (Pl. IX, fig. 4
_b_), and in the abnormal Anelasma (Pl. IV, fig. 2 _d_), quite
probosciformed,--such, also, was its character in the larval condition.
In outline, it is either sub-triangular, or oval with the longer axis
transverse; the whole is capable, as well as the separate organs, of
considerable movement, as I have seen in living sessile Cirripedes. It
is composed (Tab. V, fig. 2) of a labrum, swollen or bullate, often to
such an extent as to equal in its longitudinal axis the rest of the
mouth; of palpi soldered to the labrum; of mandibles, maxillæ, and outer
maxillæ, the latter serving as a lower lip. These organs have only their
upper segments free, but there are traces, clearly seen in the mandibles
(Pl. X, fig. 1, _a_, _b_), of their being formed of three segments. The
two lower segments are laterally united, and open into each other, the
prominence of the mouth being thus caused: this condition appears to me
curious, and is, to a certain limited extent, intermediate between those
articulated animals which have their trophi soldered into a proboscis,
and those furnished with entirely free masticatory or prehensile organs.
The palpi adhere to the corners of the labrum; and I call them palpi
only from seeing that they spring laterally from above the upper
articulation of the mandibles. The prominence of the mouth, measured
from the basal fold by which the whole is separated from the body, is
much greater on the half formed by the labrum and mandibles, than on the
other half facing the cirri. The trophi surround a cavity--the
supra-oesophageal cavity--in the middle of which, between the mandibles
is seated the orifice of the oesophagus. The oesophagus is surrounded by
long, fine, muscular fasciæ, radiating in all directions, opposing the
constrictor muscles, and is capable of violent swallowing
movements,--constriction after constriction being seen to run down its
whole course: there are also some fine muscles attached to the membrane
forming the supra-oesophageal cavity. The trophi serve merely for the
prehension of prey, and not for mastication.
The _Labrum_, as stated, is always bullate or swollen; and sometimes the
upper exterior part forms, as in Ibla (Pl. IV, fig. 8 _a_, _c_), and
Dichelaspis, an overhanging blunt point. The object, I suspect, of this
bullate form is to give, in the upper part, attachment to longer muscles
running to the lateral surfaces of the mandibles, and lower down to the
oesophagus. The crest close over the supra-oesophageal cavity, is
generally furnished with small, often bead-like teeth. The _Palpi_ are
small, their apices never actually touching each other; they are more or
less blunt, not differing much in shape in the different genera (Pl. X,
figs. 6 to 8), and clothed with spines. They are not capable of
movement; their function seems to be to prevent prey, brought by the
cirri, escaping over the labrum; I infer this from finding in Anelasma
and in the male of Ibla, which have the cirri functionless, that the
palpi are rudimentary.