segments, corresponding with the six pair of natatory legs; the anterior
segments are much plainer (even the first being distinctly separated by
a fold from the mouth), than the posterior segments, which is exactly
the reverse of what takes place in the mature Cirripede; in the latter,
the first segment is confounded with the part bearing the mouth. The
epimeral elements of the thorax are distinguishable; the sternal surface
is very narrow, and is covered with complicated folds and ridges. The
six pair of legs are all close, one behind the other, and all are alike
in having a haunch or pedicel of two segments, directed forwards,
bearing two arms or rami, each composed of two segments, the outer
ramus being a little longer than the inner one. On the lower segments
in both rami of all the limbs, there is a single spine. In all the
limbs, the obliquely truncated summit of the terminal segment of the
inner ramus bears three very long, beautifully plumose spines: in the
first pair, the summit of the outer ramus bears four, and in the five
succeeding pair, six similar spines. This difference, small as it is, is
interesting, as recalling the much greater difference between the first
and succeeding pairs, in the first and second stage of development. The
terminal segments of all the rami, bearing the long plumose spines, are
directed backwards. The limbs and thorax are well furnished with
striated muscles. The animal, according to Mr. King, swims with great
rapidity, back downwards. The limbs can be withdrawn within the
carapace.
_Abdomen and Caudal Appendages._--The abdomen is small, and its
structure might easily be overlooked without careful dissection of the
different parts: it consists of three segments; the first can be seen to
be distinct from the last thoracic segment, bearing the sixth pair of
limbs, only from the fold of the epimeral element, and from its
difference in shape; the second segment is very short, but quite
distinct; the third is four or five times as long as the second, and
bears at the end two little appendages, each consisting of two segments,
the lower one with a single spine, and the upper one with three, very
long, plumose spines, like those on the rami of the thoracic limbs. The
abdomen contains only the rectum and two delicate muscles running into
the two appendages, between the bases of which the anus is seated.
_Internal Viscera._--Within the body, in front of the mouth, it was easy
to find the stomach (with two pear-shaped cæca at the upper end),
running first anteriorly, and then curving back and reaching the anus by
a long rectum, difficult to be followed: it appeared, however, to me,
that this stomach had more relation to the young Cirripede, of which
every part could now generally be traced, than to the larva, with its
closed and rudimentary mouth: the fact, however, of its being prolonged
to the anus, which is in a different position in the larva and mature
state, shows that the stomach serves, at least, as an excretory channel.
Besides the stomach, the several muscles already alluded to, and much
pulpy and oily matter, the only other internal organs consist of two
long, rather thick, gut-formed masses, into the anterior ends of which
the cement-ducts running from the prehensile antennæ could be traced.
These masses are formed of irregular orange balls, about .001 of an inch
in diameter, made up of rather large cells, so to have a grape-like
appearance, held together by a transparent pale yellowish substance, but
apparently not enclosed in a membrane: these masses lie rather
obliquely, and approach each other at their anterior ends; they extend
from above the compound eyes, to the cæca of the stomach to which they
cohere, but in young specimens, they extend some way beyond the cæca,
between the folds of the carapace. The two cement-ducts, at the points
where they enter these bodies, expand and are lost; at this point, also,
the little orange-coloured masses of cells have the appearance of being
broken down into a finer substance. Within the cement-ducts I saw a
distinct chord of rather opaque cellular matter. We shall presently see,
that these gut-formed masses are the incipient ovaria.
_The Young Cirripede within the Larva._--Several times I succeeded in
dissecting off the integuments of the lately-attached larva, and in
displaying the young _Lepas australis_ entire. The following description
applies to the Cirripede in this state; but for convenience sake, I
shall occasionally refer to its condition when a little more advanced. I
may premise, and the fact in itself is curious, that the bivalve-like
shell of the larva, together with the compound eyes, is first moulted,
and some time afterwards, the inner lining of the sack, together with
the integuments of the thorax and of the natatory legs: hence, I often
found specimens, which externally seemed to have perfected their
metamorphoses, but which, within their sacks, retained all the
characters of the natatory larva. According to Mr. King, the larva of
Lepas throws off its external shell five days after becoming attached.
Whilst the young Lepas is closely packed within the larva, the
capitulum, as known by the five valves, about equals in length the
peduncle. The peduncle occupies the anterior half of the larva; when
fully stretched, it becomes narrower and slightly longer than the
capitulum; the separation between the capitulum and peduncle is almost
arbitrary in the mature animal, and corresponds with no particular line
in the larva. Even at this early period, the muscles of the peduncle are
quite distinct. No vestige is preserved in the outer integument, of the
sternal and dorsal sutures of the larval carapace; but in the corium of
the peduncle, three coloured marks which occur near the eyes, and two
little curled marks which occur near the acoustic orifices of the larva,
are all preserved for some time after maturity. The compound eyes, as we
have seen, are attached to apodemes, springing from the sternal surface
of the larval carapace, and are consequently cast off with it: whilst
the young Cirripede is packed within the larva, the outer integument of
its peduncle necessarily forms a deep transverse fold passing over the
eyes and apodemes, and this, as we shall presently see, plays an
important part in the future position of the animal. The antennæ are not
moulted with the carapace, but left cemented to the surface of
attachment; their muscles are converted into sinewy fibres, the corium
after a short period is absorbed, and they are then preserved in a
functionless condition. No trace of the two acoustic sacks can be
perceived in the corium of the young Cirripede, excepting the coloured
marks above alluded to.
In the young capitulum, the five valves stand some way apart from each
other; they are elegant objects under the microscope; they are not
calcified, but consist exclusively of chitine; they are rather thick,
composed of an outer membrane lined by hexagonal prisms, quite unlike
any other membrane in the animal. These valves, which I have called
_primordial_ valves, resemble pretty closely in shape the valves of the
mature animal; the fork of the carina, however, is indicated only by a
slight constriction above the lower end. After the exuviation of the
larval integuments, and when calcification commences, the first layer of
shell is deposited under, and then round these primordial valves. The
latter, in well preserved old specimens, may often be detected on the
umbones of the scuta, terga, and carina, but not on the umbones of any
other valves.
The _mouth_ seems one of the earliest parts developed: in the youngest
larva dissected, I could make out at least points corresponding with
each organ; and, at the period when the young Cirripede could be
dissected out of its larval envelopes, their general details were quite
plain. The labrum, however, had not become bullate. The mouth, as we
have seen, is formed under the rudimentary mouth of the larva, and at
the same relative spot occupied by the probosciformed mouth of the larva
in the second stage. Thus far, in the young Cirripede and larva, there
has been no great change in the relative positions of the parts: the
rudimentary eyes, however, of the former are developed posteriorly to
(or above, as applied to a Cirripede,) the cast-off compound eyes of the
larva; but the position of the mouth, of the antennæ, and of the several
coloured marks in the corium, prove to demonstration, the correspondence
in both of part to part. The case is rather different with what follows.
The _Cirri_ are developed at first of considerable length, so that the
young animal may soon provide itself with food; in _Lepas australis_
they are of great length, the sixth pair consisting of seventeen or
eighteen obscure segments. The extreme tips of the twenty-four rami of
the six pair of cirri, are formed within the twenty-four, corresponding,
little, bi-segmental rami of the six pair of natatory legs; but as the
cirri are many times longer than these legs, they occupy in a bundle
the whole thorax of the larva; no part whatever of the thorax of the
Cirripede is formed within the thorax of the larva, but (together with
the pedicels of the anterior cirri) within the cephalic cavity. As a
consequence of this, the longitudinal axis of the thorax of the young
Cirripede lies almost transversely to the longitudinal axis of the
larva; and the Cirripede, from this transverse position of its thorax,
comes to be, as it were, internally, almost cut in twain, and the sack
thus produced. As soon as the young Cirripede is free and can move
itself, the cirri are curled up, and the thorax is advanced towards the
orifice of the capitulum, its longitudinal axis resuming the position of
approximate parallelism to the longitudinal axis of the whole body,
which it had in the larval condition. The reader will, perhaps,
understand what I mean, if he will look at the mature Cirripede, figured
in Pl. IX, fig. 4. In this, he will see that the body or thorax is
united to the peduncle only by a small part below the mouth; on the
other hand, if he imagines the whole bottom of the body (as high up as
the letter _h_) united and blended into the peduncle, he will see the
state in which these parts exist in the larva. Now, let him greatly
shorten the cirri, so as to resemble the natatory legs of the larva, and
then imagine a young Cirripede, with cirri _of full length_, formed
within the old one, he will see that the new thorax supporting the cirri
will have to be developed in an almost transverse position,--the animal
consequently being internally almost separated into twain.
Of the internal organs, whilst the Cirripede is still within the larva,
I have already mentioned the stomach with its pair of cæca: from the
retracted position of the thorax and rudimentary abdomen, and
consequently of the anus, compared with these parts in the larva, the
alimentary canal is not above half its former length. There is, as yet,
no trace of the filaments supposed by some to act as branchiæ, at the
base of the first pair of cirri. Nor could I perceive a trace of the
testes or vesiculæ seminales: the penis is represented by a minute,
apparently imperforate projection. I have already briefly described the
pair of large, gut-formed bodies in the larva, into the anterior ends of
which the cement-ducts ran, and evidently derived their slightly opaque,
cellular contents. At a very early age, before the young Cirripede can
be distinctly made out, the posterior ends of these gut-formed bodies
are absorbed, so as not to pass beyond the cæca of the stomach. When the
young Cirripede is plainly developed within the larva, these bodies in a
relatively reduced condition are still distinct near the cæca, and at
the opposite or anterior end (_i. e._ lower, in the position in which
Cirripedes are usually figured), they have branched out into a sheet of
delicate inosculating tubes; these could be traced by every stage,
until, in the young perfected Cirripede, they filled the peduncle as
ordinary ovarian tubes. In the larva, the two gut-formed bodies or
incipient ovaria keep of equal thickness from one to the other end, but
in the mature Cirripede, the ovarian tubes in the peduncle and the
small, glandular, grape-like masses, near the stomach-cæca, are
connected only by a delicate tube; this I failed in tracing in specimens
in the very immature condition of those now under description.
The larva fixes itself with its sternal surface parallel and close to
the surface of attachment, and the antennæ become cemented to it: if the
Cirripede, after its metamorphosis had remained in this position, the
cirri could not have been exserted, or only against the surface of
attachment; but there is a special provision, that the young Cirripede
shall immediately assume its proper position at right angles to the
position which it held whilst within the larva, namely with its
posterior end upwards. This is effected in a singular manner by the
exuviation of the great compound eyes, which we have seen are fastened
to the outer arms of the double =°UU°=-like, sternal apodemes: these
together with the eyes stretch transversely across, and internally far
up into, the body of the larva; and, as the whole has to be rejected or
moulted, the membrane of the peduncle of the young Cirripede has
necessarily to be formed with a wide and deep inward fold, extending
transversely across it; this when stretched open, after the exuviation
of the larval carapace and apodemes, necessarily causes the sternal side
of the peduncle to be longer than the dorsal, and, as a consequence,
gives to the young Cirripede its normal position, at right angles to
that of the larva when first attached.
* * * * *
I may here state, that I have examined the larvæ in this the final or
perfect stage in four species of Lepas, in _Conchodermavirgata_, _Ibla
quadrivalvis_, and, though rather less minutely, in _Balanus
balanoides_, and I find all essential points of organisation similar.
With the exception of diversities in the proportional sizes of the
different parts, and in the patterns on the carapace, the differences,
even in the arrangement of the spines on the limbs and antennæ, are less
than I should have anticipated.
I have in this abstract treated the metamorphoses at greater length than
I should otherwise have done, on account of the great importance of
arriving at a correct homological interpretation of the different parts
of the mature animal. In Crustacea, according to the ordinary view,
there are twenty-one segments; of these I can recognise in the
Cirripede, on evidence as good as can generally be obtained, all with
the exception of the four terminal abdominal segments; these do not
occur in any species known to me, in any stage of its development. If
that part of the larva in front of the mouth, bearing the eyes, the
prehensile antennæ, and in an earlier stage two pair of antennæ, be
formed, as is admitted in all other Crustacea, of three segments, then
beyond a doubt, from the absolute correspondence of every part, and even
every coloured mark, the peduncle of the Lepadidæ is likewise thus
formed. The peduncle being filled by the branching ovarian tubes is no
objection to this view, for I am informed on the high authority of Mr.
J. D. Dana,[12] that this is the case with the cephalo-thorax in some
true Crustaceans, for instance, in Sapphirina. To proceed, the mouth,
formed of mandibles, maxillæ, and outer maxillæ, correspond with the
fourth, fifth, and sixth segments of the archetype Crustacean.
Posteriorly to the mouth, we come, in the larva, to a rather wide
interspace without any apparent articulation or organ, and then to the
thorax, formed of six segments, bearing the six pair of limbs, of which
the first pair differs slightly from the others. The thorax is succeeded
by three small segments, differently shaped, with the posterior one
alone bearing appendages; these segments, I cannot doubt, from their
appearance alone, and from their apparent function of steering the body,
are abdominal segments. If this latter view be correct, the thoracic
segments are the six posterior ones of the normal seven segments, and
there must be two segments missing between the outer maxillæ and first
thoracic pair of legs, which latter on this view springs from the ninth
segment. Now, in a very singular Cirripede, already alluded to under the
name of Proteolepas, the two missing segments are present, the mouth
being actually succeeded by eight segments, and these by the three usual
abdominal segments,--every segment in the body being as distinct as in
an Annelid: hence in Proteolepas, adding the three segments for the
mouth and three for the carapace, we have altogether seventeen
segments, which, as I stated, is the full number ever observed in any
Cirripede, the four missing ones being abdominal, and, I presume, the
four terminal segments. That the cavity in which the thorax is lodged,
in the larva and therefore in the mature Cirripede, is simply formed by
the backward production of the carapace, does not require any
discussion. The valves have no homological signification.
[12] This distinguished naturalist has given his opinion in the
‘American Journal of Science,’ March, 1846, that “the pedicel of
Anatifa corresponds to a pair of antennæ in the young;” although
the peduncle or pedicel is undoubtedly thus terminated, even in
mature individuals, I think it has been shown that it is the
whole of the anterior part of the larva in front of the mouth,
which is directly converted into the peduncle. Professor E.
Forbes, in his Lectures, and Professor Steenstrup, in his
‘Untersuchungen über das vorkommen des Hermaphroditismus in der
Natur,’ ch. v, have considered the peduncle as a pair of fused
legs. Lovén has taken, judging from a single sentence, the same
view of the homologies of the external parts as I have done; in
his description of _Alepas squalicola_, (Ofversigt of Kongl.
Vetens., &c., Stockholm, 1844, pp. 192-4), he uses the following
words: “Capitis reliquæ partes, ut in Lepadibus semper, in
_pedunculum mutatæ et involucrum_,” &c.; his involucrum is the
same as the capitulum of this work.
As we have just seen that the first pair of natatory legs is borne on
the ninth segment of the body, so it must be with the first pair of
cirri, which consequently correspond to the outer maxillipods (the two
inner pair of maxillipods or pied-machoires being here aborted) of the
higher Crustacea, and hence their difference from the five posterior
pair, which correspond with the five, ordinary pair of ambulatory legs
in these same Crustacea. The part of the body, which I have called the
prosoma, that is the protuberant, non-articulated, lower part of the
thorax (Pl. IX, fig. 4 _n_), is a special development, either of the
ninth segment, bearing the first pair of cirri, or of the segments
corresponding with the organs of the mouth. The three abdominal segments
of the larva are represented in the mature Cirripede, in the Order
containing the Lepadidæ, only by a minute, triangular gusset, let in
between the V-shaped tergal arches of the last thoracic segment: in this
gusset, small as it is, is seated the anus, and on each side the caudal
appendages, often rudimentary and sometimes absent. In another order, I
may remark, (including, probably, the Alcippe of Mr. Hancock,) the
cirri, of which there are only three pair, are abdominal.
I feel much confidence, that the homologies here given are correct. The
cause of their having been generally overlooked arises, I believe, from
the peculiar manner, already described, in which the animal, during its
last metamorphosis, is internally almost intersected: even for some
little time after discovering that the larval antennæ were always
embedded in the centre of the surface of attachment, I did not perceive,
that this was the anterior end of the whole animal. The accompanying
woodcut gives at a glance, a view of the homologies of the external
parts: the upper figure (from Milne Edwards) is a Stomapod Crustacean,
Leucifer of Vaughan Thompson, and the abdomen, which we know becomes in
Cirripedes, after the metamorphosis, rudimentary, and therefore does not
fairly enter into the comparison, is given only in faint lines: the
lower figure is a mature Lepas, with the antennæ and eyes, which are
actually present in the larva, retained and supposed to have gone on
growing. All that we externally see of a Cirripede, whether pedunculated
or sessile, is the three anterior segments of the head of a Crustacean,
with its anterior end permanently cemented to a surface of attachment,
and with its posterior end projecting vertically from it.
[Illustration: [_m._--Mouth.]]
CAPITULUM.