with the second pair, but does not otherwise differ from the type
of its sub-family: in Coronula and its allies, on the other hand,
the third pair is very short and broad, as may be seen (Pl. 29, fig.
5) in Xenobalanus: in this latter genus, the front surfaces of the
segments of the pedicels (fig. 6) of the posterior cirri, are extremely
protuberant, almost as in _Scalpellum vulgare_.
The last peculiarity in the cirri at all worth mentioning, is in
the sub-genus Acasta, in which, differently from in all other known
Cirripedes, the anterior ramus of the fourth pair does not absolutely
resemble the rami of the fifth and sixth pairs; in most of the species,
the spines on this anterior ramus are more crowded together, are
larger, and are mingled with some short thick points; and the spines
in the dorsal tufts are also longer than in the two posterior pairs of
cirri; but in _A. sulcata_ (Pl. 29, fig. 2), and in a lesser degree
in _A. cyathus_ and _A. purpurata_, the front margins of the lower
segments of this anterior ramus, and of the upper segment of the
pedicel, are developed into strong, downwardly curved teeth: it is very
remarkable that so beautiful a structure should be extremely variable,
as it certainly is in _Acasta sulcata_.
_Caudal Appendages._
With extremely few exceptions, these are present in all the Lepadidæ
and Verrucidæ; whereas amongst the Balanidæ they occur only in the
two species of Pachylasma, and in one species of Catophragmus;
these being the genera most closely allied to the Lepadidæ, and
where, consequently, their presence might have been anticipated.
These appendages are seated close together over the anus; they are
multiarticulate, each segment being sub-cylindrical, with a few small
bristles round its upper edge.
_Alimentary Canal._
I have not much on this head to add to what I have said under the
Lepadidæ. As in that family, the strong internal membrane of the
œsophagus terminates in a remarkable, bell-shaped expansion (Pl. 26,
fig. 3, _g′_), which, as observed by M. St. Ange, serves to keep
the upper broad end of the stomach expanded. The œsophagus is well
furnished with constrictor and radiating muscles for closing and
opening it; and it is thus capable of a strong swallowing movement.
The stomach runs down to the lower end of the prosoma, and then
doubling back on itself extends to the anus. As the prosoma is much
elongated in Tubicinella and Xenobalanus, so is the stomach of
unusual length in these genera. In several species of Balanus, the
upper edge of the stomach is surrounded by from six to eight cæca;
these cæca I ascertained, in _Balanus perforatus_, are branched, and
penetrate a considerable way into the body; and some of them at least
expand a little at their extremities. Each cæcum, from the manner in
which it retained fluid, must, I think, be furnished, at the point
where it enters the stomach, with a sphincter muscle. In Tetraclita,
Chthamalus, Tubicinella, Coronula, and Xenobalanus, there are no cæca;
but in Xenobalanus and _Coronula balænaris_, there are longitudinal,
approximate folds in the upper, broad end of the stomach, which would
serve to expose the food to a greater extent of digesting surface.[37]
[37] The presence and absence of these cæca in genera so closely
allied as Balanus and Tetraclita, shows, I think, that these
cavities are not of high importance; and I must doubt whether Von
Siebold's view ('Anatomie Comparée,' tom. i, p. 445), that these
cæca form a passage to a true or isolated liver, such as exists in
the higher Crustacea, can be admitted. Cæca are said by Von Siebold
to occur in some of the Entomostraca, as Daphnia, Argulus, &c.
As in the case of the Lepadidæ, a transparent, structureless,
epithelial tube, composed of chitine, containing more or less digested
food, is found, in specimens preserved in spirits, occupying the whole
length of the stomach, and where there are cæca, sending branched
prolongations into them. It does not extend into the œsophagus or into
the rectum. This epithelial tube or model of the stomach, filled with
excrement, is expelled by the rectum, whole, that is in a single piece,
as I observed in some living specimens of _Balanus balanoides_: in
some specimens, however, of _Chthamalus stellatus_, the excrement was
ejected, perhaps from the animal being confined, in fragments, and the
sack thus became befouled. Beneath the epithelial layer, the stomach is
lined by a delicate, pulpy and cellular mucus layer, which easily peels
off in flakes: this is surrounded by a muscular layer with the fibres
closely approximated and transverse; and this by a layer of stronger,
longitudinal muscles, but more distant from each other. Lastly,
outside this double muscular layer, there is a rather thick, somewhat
laminated, pulpy layer, abounding with cells, often nucleated, and
frequently containing much oily matter. This structure agrees closely
with Dr. C. H. Jones's[38] account of the external covering of the
stomach in Daphnia, and which he believes to be hepatic: as in Daphnia,
there does not seem to be any ducts. I may here observe, that within
the upper part of the prosoma, but not immediately connected with the
stomach, I have often observed much white pulpy substance, permeated by
lacunal passages, and exhibiting no structure except some excessively
minute cells.
[38] 'Philosophical Transactions,' 1849, p. 116. Karsten ('Nov.
Actorum Acad. Nat. Cur.,' 1845, tab. xx) has excellently figured
the testes, as the hepatic glands; and has indicated the ovaria
as salivary glands; it is singular that this anatomist overlooked
the ducts which lead from his supposed hepatic glands, into the
vesiculæ seminales, within which he observed spermatozoa.
The rectum, lined by membrane continuous with that investing the
thorax (and seen through it, in Pl. 26, fig. 8, _c_), extends inwards
to about opposite the bases of the third or fourth pairs of cirri.
It is longitudinally plaited; the ends of the folds forming a sort
of valve where joined on to the stomach. It is coated by circular,
transverse muscular fibres: judging from the movements, the anus
itself is surrounded by a strong sphincter muscle. The anus opens on
the dorsal surface of the thorax (fig. 8, _b_); but as in the genera,
in which caudal appendages occur, it opens under them, the orifice, I
believe, is homologically terminal, and owes its dorsal aspect to the
aborted state of the whole abdomen, and to the great development of the
probosciformed penis; for the anus may be said to be situated on the
dorsal base of this organ.
Altogether we see that the alimentary canal is of a very simple
structure. The food, judging from the contents of the stomach, seems
generally to be composed of infusoria and minute animals: but in the
case of Tetraclita, I have been surprised at the size and number of the
included amphipod, isopod, and entomostracan Crustaceans, in one case,
together with an annelid. I have, also, sometimes seen some confervoid
matter within the stomach.
_Circulatory System._
On this subject I can add nothing, except to express my conviction that
there is no heart, or true vessels; the circulation being strictly
lacunal. A passage has often been quoted from Poli, in which he states
that he saw a pulsating organ, close above the anus; but I have seen
this movement, which appeared to me to be a convulsive twitching of the
sphincter muscle of this orifice. The largest lacunal channel extends
down the middle of the rostral compartment of the shell: and this
answers to the rostral channel down the peduncle in the Lepadidæ. Large
nerves and the main pair of unbranched ovarian tubes (Pl. 25, fig. 1,
leading into _g_) extend along this channel. At the basis (at least in
Coronula) this channel joins on to a large circular one, running all
round the sack, and sending off branches into the mass of ovarian tubes
and cæca.
_Nervous System._
It has been shown in my former volume, that in Lepas and in some
other genera of the Lepadidæ, there are six main ganglions; one
supra-œsophageal, and five infra-œsophageal, or thoracic. In
Pollicipes, however, there are only four thoracic ganglions, the
last ganglion supplying the three posterior pairs of cirri with
nerves, whereas in the other genera, the last ganglion supplies only
the fifth and sixth pairs of cirri. In this genus, moreover, the
lateral fusion of the ganglions has been so complete, that there is
no evidence of their having been formed by the union of two. Amongst
sessile cirripedes, we discover evidence of much higher concentration
even than in Pollicipes. My chief examination has been confined to
_Coronula diadema_, and to _Balanus tintinnabulum_: and in these
genera we find (and the fact appears to me highly remarkable) as high
a degree of concentration in the infra-œsophageal ganglion as in any
decapod Crustacean, for instance, as in Maia, judging from the figure
given by Milne Edwards; for all the nerves, with the exception of
those connected with the supra-œsophageal ganglions, radiate from a
single great ganglion.[39] The nervous system is, moreover, otherwise
complicated.
[39] It must, however, be observed that, according to Mr. Dana,
there is in certain suctorial Entomostracans, as in Caligus,
only one infra-œsophageal ganglion. Mr. Dana speaks of this as
resulting from reduction. In Cirripedes, from the gradation which
may be observed from Lepas through Pollicipes into Balanus, the
ganglions are certainly not reduced but concentrated. In Van de
Hoeven's figure of the nervous system in Limulus, there is seen to
be no chain of thoracic ganglions; all the nerves rising from the
circa-œsophageal collar; but this, on the other hand, seems hardly
developed into a ganglion.
To begin with _Coronula diadema_ the great infra-œsophageal ganglion
(Pl. 27, fig. 1, A) is seated nearly opposite to the anterior margin of
the second pair of cirri, which are homologous with the first pair of
legs in the decapod Crustaceans. This ganglion shows no trace of any
longitudinal medial suture; its shape is hardly discoverable, for it
is formed by the union of eleven principal pairs of nerves, besides
several arising from its under surface; in outline, however, it may be
said to be divided into a posterior and anterior half; the latter being
somewhat heart-shaped, and the posterior half elongated. The nerves
going to the five posterior pairs of cirri rise from the posterior
margin of the ganglion, and run for some distance in a sheet, parallel
and close together; the pair, however, going to the second pair of
cirri soon branches off from the others. Each of these nerves enters at
the inner and posterior margin of the cirrus to which it belongs, and,
at least in the case of the first pair, divides into two branches as it
enters. The nerves (Pl. 27, fig. 1, _r^5_, _r^6_) going to the fifth
and sixth pairs of cirri are more closely united together than are the
others, and appear, till they branch off, like a single large nerve.
That which belongs to the sixth cirrus gives off, opposite to the
fifth cirrus, a branch (_s_) nearly as large as itself, which enters
the probosciformed penis. I may remark, that homologically this is the
only abdominal nerve in any cirripede of the Order. From the under side
of the nerves which run to the five posterior pairs of cirri, small
branches are given off, extending dorsally into the thorax.
The anterior end of the great infra-œsophageal ganglion is formed by
the union of a set of nerves, extending parallel in a bundle in a
directly opposite direction to those running to the five posterior
pairs of cirri. These nerves consist of an outer larger pair (_r^1_)
entering the first pair of cirri; and within these, and rather dorsally
to their roots, we have the circa-œsophageal chord (_c_, _c_), or
collar nerve; between the roots of the latter, and on the ventral
surface (or near side of the figure), there are three closely united,
small pairs, running to the gnathites, and, as I believe, to the
olfactory sacks. From the under (or dorsal) surface of the anterior
end of the ganglion, two nerves, larger even than the circa-œsophageal
chord, and which I shall call the splanchnic pair (_d_, _d_) arise;
and the singular course of these nerves will presently be described;
between this great pair, there is a single (_b_) medial nerve, which
runs down and branches into that large diverging muscle, which is
attached to the upper ventral surface of the stomach. Posteriorly to
these three nerves, we have two pairs of much smaller nerves (not
figured), running dorsally into the body, so that we have seven nerves
rising from the under surface of the infra-œsophageal ganglion. I
need only further add, that on each side of this ganglion, between
the nerves going to the first and second pairs of cirri, there is a
moderately sized nerve (_k_), which appeared to run into the muscles of
the thorax: a nerve in a similar position is figured by Milne Edwards
in Maia.
The circa-œsophageal chord (_c_, _c_) nearly equals in length the whole
distance from the centre of the main ganglion to the posterior end
of the thorax. This collar bows out on each side, where passing the
œsophagus (_œ_), which is seated at its anterior end. From the collar a
branch is given off on each side, which I traced as far as between the
mandibles and maxillæ; from analogy with other Crustaceans, it perhaps
runs to the mandibles. The collar has not a transverse commissure, such
as described by Milne Edwards in the Podophthalmia, and as figured by
Van de Hoeven in Limulus.
The supra-œsophageal ganglions (B) present a singular contrast with
the infra-œsophageal ganglion in their little development, size, or
degree of confluence. They lie directly under the basal edge of the
labrum. They are laterally quite distinct, and consist merely of a
slight enlargement of the circa-œsophageal chord. From the anterior
edge of each ganglion, a broad nerve (_f_) extends for some distance in
a straight line, and, on close examination, can be seen to be formed of
two nerves closely united, of which the inner and smaller one, after
a space, appears to cross over the larger nerve: both become at this
point tortuous, and, giving off branches (_m_, _m_), form a plexus. The
two nerves (_f_) then bend inwards, and almost touching each other, run
down, together with the two ovarian simple ducts, along the rostral
compartment of the shell. No doubt, if the smaller branches from these
nerves could be traced, they would be seen to form a network over the
whole sack; and would therefore enclose, as in a cage, the rest of the
nervous system. These nerves correspond, I believe, to the two pair
of antennular nerves of ordinary Crustaceans, and hence I will call
them by this name. Just in front, at the outside corners of the two
supra-œsophageal ganglions (B), a branch (_e′_) arises, which I traced
to the ends of the adductor scutorum muscle, and to those several
muscles which serve to retract the interspace of membrane between the
mouth and the adductor.
The pair of great splanchnic nerves above alluded to, which arise from
the anterior and dorsal surface of the infra-œsophageal ganglion, are
in Pl. 27, fig. 1, _d_ _d_, (and in fig. 2), laid flat; but in nature
they first bow outwards, and then, penetrating deeper into the body,
approach each other, and running nearly parallel, pass round the lower
end of the œsophagus: their course consequently is nearly similar
to that of the circa-œsophageal chord, with this difference, that
the outwardly bowed portion is situated near the infra-œsophageal,
instead of near the supra-œsophageal ganglion. The splanchnic nerve,
a little beyond the supra-œsophageal ganglion, joins a plexus (_d′_);
and into this plexus another large nerve (_e_) which I will call the
supra-splanchnic nerve, sends branches; this nerve takes an almost
semicircular bend over the ovarian glands. The supra-splanchnic nerves
(_e_, _e_), though appearing to spring from the supra-œsophageal
ganglions, do really arise, as may be seen by tracing the constituent
fibres, from the circa-œsophageal chord. The plexus (_d′_) lies close
to the coats of the upper end of the stomach: several branches,
proceeding from it, run further on, but I was able to trace only a few
of them: one went (at least in the case of _Balanus perforatus_), to
the adductor scutorum muscle: another branch spread out on the flanks
of the prosoma: I strongly suspect that one branch goes to the acoustic
sack: it appeared, also, as if some of the small branches entered the
second plexus (_m_), where the inner antennular nerve and ophthalmic
nerve cross over the outer antennular nerve.
I have called the nerves (_dd_, _ee_) splanchnic and supra-splanchnic,
from their course and apparent function in supplying the viscera. In
the descriptions of the nervous system of other Crustaceans I can find
nothing analogous to my great splanchnic nerve (_dd_); the so-called
supra-splanchnic nerves (_ee_), which arise from the circa-œsophageal
chord, seem to be the analogues of the ordinary splanchnic nerves,
though these latter are always described as uniting into a single
medial branch. The plexus (_d′_) is the cervical ganglion of M.
Martin St. Ange,[40] who has likewise indicated the course of my
splanchnic and supra-splanchnic nerves; but the plexus, when viewed
as a transparent object, hardly appears to me to be ganglionic in its
nature. In my former volume on the Lepadidæ, I quite misunderstood the
course of these splanchnic nerves.
[40] 'Mémoire sur l'Organisation des Cirripèdes,' p. 19.
From the commissure between the two supra-œsophageal ganglions, a
straight chord (Pl. 27, fig. 1, _g_) arises, which terminates in
a small ganglion (C), barely exhibiting traces of being formed of
two laterally confluent ganglions. This is the ophthalmic ganglion.
The chord connecting it with the two supra-œsophageal ganglions is
accompanied by a small nerve (_h_) which runs on to the muscles round
the adductor scutorum muscle; the chord is encased by much fibrous
tissue, and its dissection is thereby rendered difficult. From the
ophthalmic ganglion, on each side, a nerve (_i_) goes forth and crosses
the antennular nerve; these, if I could have traced them, would have
been found to run, as may be safely inferred from what is known in
_Balanus tintinnabulum_, to a pair (D, D) of eyes.
In _Balanus tintinnabulum_, the structure of the great infra-œsophageal
ganglion (Pl. 27, fig. 2, A) is essentially the same as described under
Coronula. The great pair of splanchnic nerves springing from its under
side, are here actually twice as large as the circa-œsophageal chord.
The plexus (_d′_) formed by the splanchnic nerve (_d_), on each side,
with the supra-splanchnic nerve (_e_), which arises close posteriorly
to the supra-œsophageal ganglion, is here much less complicated, but
is perfectly distinct; and there was no appearance of the cervical
ganglion of M. Martin St. Ange. The chord (_g_) running from between
the two supra-œsophageal ganglions to the ophthalmic ganglion, is
nearly as large as the double antennular nerve (_f_) on each side of
it. The ophthalmic chord (which is accompanied in its whole course by a
small branch running to the adductor scutorum muscle) terminates in a
small ophthalmic ganglion (C), which seems to be formed by the almost
complete fusion of two ganglions. This ganglion is hardly larger than
the chord which it terminates: it appeared to me to give rise to more
than one pair of nerves, and a single nerve (to my surprise) joined the
branch just mentioned, which goes to the adductor scutorum muscle.
From each supra-œsophageal ganglion, two closely united antennular
nerves (_f_) extend, of which the inner one crosses over the main
or exterior nerve, nearly opposite to the ophthalmic ganglion, and
here forms (_m_) a plexus. The structure of this plexus I was not
able, any more than in Coronula, to make out thoroughly, but I traced
quite distinctly a long nerve (_i_) running from it into what must be
considered as the eye. As in the case of Coronula, I traced a nerve
on each side from the ophthalmic ganglion into the plexus, where I
lost it; and as here in Balanus, I saw on each side of the ophthalmic
ganglion a cut off nerve, of about the size of that which runs from
the plexus on each side into the eye, I think we may safely conclude
that the latter or optic nerve does really arise from the ganglion here
called ophthalmic. I may add that the analogy of the nervous system in
the Lepadidæ most strongly confirms the view of this latter being the
ophthalmic ganglion.
_Eyes and Vision._
The optic nerve (_i_), running from the plexus to the eye, is of
considerable size; it runs nearly parallel to the main antennular
nerve, diverging from it a little. It retains nearly the same diameter
throughout; and gives off only one single, small, inner branch. It can
be traced beyond the basal edges of the scuta, to just under the upper
edge of the transparent opercular membrane, which unites the scuta
to the sheath of the rostrum. The nerve itself, at a little distance
from its further end, was, in a full-sized specimen, 5/1000ths of an
inch in diameter; widening a little, it expands slightly, and abruptly
terminates in a circular disc, about 8/1000ths of an inch in diameter,
(see Pl. 27, fig. 5). The nerve just beneath this slight expansion,
is coated all round by pellets of dark purple pigment-cells, but not
actually united into a continuous layer. These pigment-cells are
the more conspicuous from the surrounding parts being colourless. I
could not make out distinctly any cornea; and I suppose the external
transparent membrane, to which the above slight circular expansion is
attached, acts as such. This description very closely agrees with that
given of these organs in _Bal. rugosus_ of Gould, (_B. crenatus_?) by
Dr. Leidy,[41] who first discovered the eye in the adult cirripede,
but he did not observe the ophthalmic ganglion. These eyes differ from
those in some of the genera of the Lepadidæ, only in the greater length
of the optic nerve, and by standing laterally further apart from each
other.
[41] 'Proceedings of the Acad. Nat. Sciences of Philadelphia,'
vol. iv, 1848, p. 1. I may add that I have, also, observed the
supra-œsophageal and ophthalmic ganglions in _Bal. perforatus_.
I may here mention that I tried a few simple experiments on the senses
of _Balanus balanoides_, _B. crenatus_, and _Chthamalus stellatus_.
I found these three species very sensitive to shadows, that is, to
an object like my hand passing even quickly, and at the distance of
about a foot, between them and the source of the light.[42] They were
indifferent to a gradual change from bright to obscure light; but
instantly perceived and drew in their cirri, when my hand was passed
between the basin in which they were kept and the window, even when
this was tried rather late on a dusky evening; and likewise when my
hand was passed between them and a single candle. I took, of course,
the precaution of passing my hand in other directions, but this
never produced any effect. These species are moderately sensible to
any vibration in the vessel in which they were kept, but they were
indifferent to noises made in the air, or in the water. I found it
impossible to touch, under water, an individual shell ever so lightly
with a needle, without all the immediately surrounding individuals,
when several adhered together, perceiving it, and retracting their
cirri: it made no difference whether the one touched had already
withdrawn its cirri and was motionless: from this fact, and from seeing
that a similar but slighter effect was produced by touching the rock
on which the specimens adhered, I infer that the perception by the
others of the one being touched, is communicated by vibration. When an
individual was touched under water, not by a needle, but by a pointed
camel-hair brush, it generally withdrew its cirri, but the neighbouring
specimens took no notice: when touched by a single hair of the brush,
no notice was taken, unless the skin of the orifice leading into
the sack was so touched. In these trials, it is of course necessary
carefully to avoid intercepting the light. I could not make out that
cirripedes perceived odours diffused in the water.
[42] I find that this fact was long ago observed by Von Siebold,
'Anatomie Comparée,' tom. i, p. 434.
_Acoustic Organs._
These are situated in the same position as in the Lepadidæ, namely,
in a slight swelling on the sides of the thorax (Pl. 25, fig. 1,
_d′_) just beneath the basal articulation of the first pair of cirri.
The orifice in Tubicinella and Xenobalanus is slightly produced, or
is tubular; the free part in the former genus projecting 5/100ths
of an inch. The structure of all the parts is essentially the same
as in the Lepadidæ, but I think all are proportionally larger. The
external membrane of the body is turned inwards at the orifice,
as a short flattened tube, which widens considerably (being, in a
middle-sized specimen of Coronula, 4/100ths of an inch in width) before
it abruptly terminates. The meatus, as I have called the sack-like
cavity which encloses the true acoustic sack or vesicle, is formed
of pulpy membrane, and is apparently continuous with the corium of
the whole body, but by dissection it can be separated entire. The
acoustic vesicle is of various shapes, as we shall immediately see; but
in all essential respects it is identical with the same part in the
Lepadidæ; it is formed of the same peculiar, soft, elastic, brownish,
transparent tissue, which seems to be composed of fine, transverse
pillars, becoming towards the outside fibrous, and at their inner ends
appearing when viewed vertically from above, like hyaline points. In
_Coronula diadema_, I observed on the outside of the acoustic vesicle,
some excessively minute bristles, only 1/3000ths of an inch in length,
seated on little eminences. I examined carefully the contents of the
vesicle in this species, in specimens well preserved in spirits, and
there was nothing within but a very little, thin, pulpy fluid, and a
few yellowish nucleated cells, here and there aggregated into small
groups. In Coronula, the flattened acoustic vesicle is elongated, with
a somewhat sinuous, but not very irregular margin (Pl. 27, fig. 4),
and is without any ridges on the surface; its neck or orifice projects
at right angles to the elongated portion, which stands obliquely to
the tubular orifice of the meatus. In a moderately-sized specimen of
_Coronula diadema_, the elongated portion of the acoustic vesicle was,
6/100ths of an inch in length. In Tubicinella, the acoustic vesicle
is heart-shaped, with the neck attached to its broader end; and the
surface is covered by zig-zag ridges. In _Balanus tintinnabulum_ (fig.
3), the acoustic vesicle is almost square at the lower end, with the
neck placed at one of the upper corners; on the external surface,
there is an oblique prominent ridge or fold, which sends off downwards
another ridge; its length, in a large individual, was 5/100ths of an
inch.
In all these cases, the acoustic vesicle is mainly attached by its
neck, to the upper end of the sack-like meatus; but there is likewise
a layer of soft, pulpy, cellular matter, slightly connecting that side
of the vesicle which is opposite to the neck, with the walls of the
meatus or outer sack. The mouth or orifice of the vesicle is closed
by a delicate lid or diaphragm, which can easily be separated; and
this diaphragm is formed by the expansion of a large nerve, which here
abruptly terminates. In a very large specimen of _Coronula diadema_ I
clearly made out the existence of this nerve, and traced its course
for some distance from the point where the summit of the meatus and
the neck of the vesicle are joined together; the nerve first runs
posteriorly, and then turns inwards and doubles back or anteriorly;
and I clearly followed it to the antero-lateral sides of the uppermost
end of the stomach, where it seemed to enter a ganglion, so that I
unfortunately cut it off, but found only a slight plexus, with the cut
off nerve apparently running onwards with nearly the same diameter.
The diameter is great, fully equalling, in its widest part, that of
the circa-œsophageal chord; but it is very much flattened, and so
has not nearly so much bulk as that nerve. Before it reached the
stomach, it gave off one branch, which ran towards the mouth. The only
nerves which, from their size, could, I think, be continuous with
this from the acoustic sack, are the main branches proceeding from
that plexus (_d′_) formed by the interbranching of the splanchnic and
supra-splanchnic nerves.[43]
[43] I have always feared that anatomists would reject my view of
these organs being acoustic, owing to the absence of otolithes;
but I observe that so high an authority as Von Siebold ('Anatomie
Comparée,' tom. i, p. 433) does not believe that otolithes occur
in the acoustic organs even of the highest Crustacea. He considers
an "ampoule volumineuse, a parois mince, remplie d'un liquide
transparent," and a "membrane tympanique," though having a fissure
in the centre, as sufficient. I may here remark, that the nerve
proceeding from the acoustic vesicle in Cirripedes, and apparently
running to the splanchnic nerve, may easily be placed in connexion
with the antennular nerves, by the second plexus (_m_) in figs. 1
and 2, pl. 27. I should infer from Von Siebold's remarks on his
ampoule volumineuse in the higher Crustacea, that my acoustic
vesicle answered to the labyrinth in higher animals.
_Olfactory Sacks._
I can add nothing to the account given of these organs under the
Lepadidæ: I saw them in all the genera which I examined for this
object. In _Coronula diadema_ the orifices are large; they are seated
in the usual position (Pl. 26, fig. 4, _n_), in the confluent segments,
beneath the free part of the outer maxillæ, and somewhat exteriorly,
or as near as possible to the inner maxillæ. In no sessile cirripede
are the orifices produced or tubular, as is the case with several
genera amongst the Lepadidæ. I failed, as heretofore, in tracing with
certainty the nerve, which appears to enter the base of the sack, to
its ganglion.
_Male Organs of Generation._
All the Cirripedes of the family we are now describing, are bisexual
or hermaphrodite; and no instance has been observed of the presence
of males or complemental males. I have very little to add to the
observations made by M. Martin St. Ange and R. Wagner,[44] and to
those given in my former volume. The testes seem always to be confined
to within the thorax, including the prosoma. With their ducts, they
resemble club-moss or stag's horns, with the extremities a little
enlarged: a figure[45] of a small portion from _Balanus perforatus_ is
given in Pl. 25, fig. 2. It is quite surprising how like in structure
and appearance the branching ovarian tubes often are to the testes with
their ducts; but the latter are of smaller diameter. Two main ducts
generally unite just before entering the broad, often reflexed, end of
the vesicula seminalis: in _Coronula balænaris_, however, I observed
four ducts entering this receptacle. The two vesiculæ seminales, lying
within the thorax and prosoma, are usually very long and tortuous: they
are formed of a thin inner tunic, which is strengthened by thicker
reticulated lines, and of an outer layer of transverse fibres, which
are either elastic, or probably muscular, as they serve to expel
the contents with force when the end is cut off. The inner tunic is
prolonged up the probosciformed penis, at the base of which the two
vesiculæ unite.[46] The contents of the vesiculæ are commonly pulpy and
cellular; and from the cells the spermatozoa are developed; soon after
their development, they are, as it appears, expelled.
[44] The 'Report' on M. Martin St. Ange's memoir was laid before
the Academy of Sciences, July 14, 1834, so that I suppose it was
read previously to this date. R. Wagner's paper was published
in 'Müller's Archiv,' 1834, p. 467. Burmeister's 'Beiträge
zur Naturgeschichte der Rankenfüsser,' was published this
same year, 1834; so that these three authors published almost
contemporaneously.
[45] A far better figure is given by Karsten ('Nov. Act. Acad. Cæs.
Nat. Cur.,' 1845, Pl. 20, figs. 2, 3, 4), but under the erroneous
supposition that these organs were hepatic.
[46] In _Conchoderma aurita_, the ducts, as shown by Burmeister
('Beiträge,' &c. tab. ii, fig. 17), unite half way up the
probosciformed penis.
I have seen the spermatozoa in _Balanus crenatus_, _perforatus_, and
_balanoides_, and in _Chthamalus stellatus_. The cells, from which the
spermatozoa are developed, and which are often found in vast numbers
within the vesiculæ, are on an average about 1/5000th of an inch in
diameter. The spermatozoa differ remarkably within the vesicula of
the same individual, according to their state of development. I have
observed in _B. perforatus_ and in the Chthamalus, that the shortest,
and therefore, I presume, the youngest (Pl. 29, fig. 7, _a_), had a
globular head with no projection in front: as they increased in size,
this head became less in diameter, and a short tapering filament, (_a_,
_b_,) like the tail, projected out of it. This anterior filament does
not lie in exactly the same line with the posterior filament, which is
straight as an arrow. In _Bal. crenatus_, the anterior filament was
1/2000th of an inch in length, and the posterior filament 4/2000th,
giving a total length of 5/2000th: in the longest and best developed
specimens of _Chthamalus stellatus_, the nodular enlargement was
much elongated and spindle-shaped, and not above half the diameter
it had in the earliest stage; the posterior filament (measured from
the front of the enlargement, this consequently being included) was
5/2000th in length, and the front part only 1/4000th, giving a total
length of 11/4000ths of an inch. These observations agree pretty well
with Kölliker's;[47] but this author states, that perfectly developed
spermatozoa are absolutely without any nodular enlargement: if this be
the case, I have never chanced to see the spermatozoa in their perfect
condition. Mr. Bate, also, figures some (Pl. 29, fig. 7, _c_) in this
state, without any enlargement.
[47] 'Annales des Sciences Naturelles,' (2d series), tom. xix, p.
348. Kölliker refers to Wagner's paper on the same subject, in
Wiegmann's 'Archiv,' 1835, part ii, pl. iii, fig. 9. He also refers
to Von Siebold's observations. Mr. C. Spence Bate has figured, in
the 'Annals and Magazine of Natural History' (vol. viii, 2d series,
1851), the spermatozoa of _Balanus balanoides_, _perforatus_, and
of _Verruca (Clitia) Strömia_, and of these I have given copies,
Pl. 29, fig. 7.
The probosciformed penis lies adpressed on the under side of the
thorax, with its apex generally projecting between the first and second
pairs of cirri. It presents the same ringed or articulated structure as
in the Lepadidæ: it arises from an unarticulated projection or support,
which also forms the posterior border to the anus. This support
often terminates, as first observed by Poli, in a very sharp point;
but this point cannot be of much functional importance, for though
present in _Balanus balanoides_, it is absent in the closely allied
_B. crenatus_; in Tubicinella there is only a rudiment of this point;
I have not observed it in any member of the Chthamalinæ. The strong,
transverse and longitudinal muscles with which the penis is furnished,
are attached to this support. The apex or orifice of the penis is, I
believe, invariably surrounded by some bristles. Its length varies
much, according to its state of contraction or relaxation; and this
again, I believe, is dependent on the condition of the male secreting
organs. In a small specimen of _Elminius modestus_, the penis was
actually thrice as long as the whole thorax, including the prosoma:
in Pachylasma and in _Octomeris angulosa_, the penis is very short,
being equal only to once and a half the length of the pedicel of the
sixth cirrus: in _Octomeris brunnea_, the unarticulated support is
much elongated, being as long as the pedicel of the sixth cirrus, in
which respect this organ resembles that of _Ibla quadrivalvis_, and of
no other Cirripede. From the attachment of the penis at the posterior
end and on the under side of the anus--from the position of the caudal
appendages (where such occur) over the anus--from the position of
these same appendages in the pupa--and lastly, from the position of
the papilla-like penis in the abnormal _Proteolepas_, I infer that,
homologically, the penis is situated at the apex of the abdomen, on
its ventral surface; and that, consequently, this organ cannot be
considered as the abdomen itself in a modified condition.
_Female Organs of Generation._
I have scarcely anything to add to the statements in my former volume.
These organs consist of the true ovaria, or glandular bodies seated
on each side, not far from the basal edge of the labrum; of the main
or unbranched ovarian ducts; and of the (Pl. 25, fig. 1, _g_) ovarian
branching tubes and cæca. I traced distinctly in Balanus, Tetraclita,
and Coronula, the two main ovarian ducts, running from within the
prosoma to the layer of inosculating, branching, ovarian cæca[48] which
overlie the basis. In _Coronula diadema_ one of these main ducts was
1/100th of an inch in diameter. Though I traced these ducts near to the
grape-like, glandular masses,[49] which I cannot doubt are the true
ovaria, I did not succeed in tracing them into actual connection. As
in the Lepadidæ, these ovarian glands lie on the sides, near the basal
margin of the labrum, and almost under, but rather to the outside of
the antennular nerves. The branching and inosculating ovarian cæca
form a layer, which corresponds with the mass filling up the peduncle
in the Lepadidæ. In Tetraclita they do not cover the whole basis, but
are confined to the circumference; they, however, likewise extend
up between the two layers of corium round the walls of the shell,
and chiefly in the interspaces between the depressor muscles of the
opercular valves. In Chelonobia, they enter between the radiating septa
in the thickness of the walls: in _Coronula diadema_, they extend from
over the basal membrane into the six large square chambers (Pl. 16,
fig. 7, _v_) separating the radii and alæ: in Tubicinella they are
confined to the basis: in Xenobalanus, they form a layer over the basis
and likewise round the upper part of the peduncle-like body, which
answers to the shell of other sessile cirripedes.
[48] These are well described in Lepas, by R. Wagner, in 'Müller's
Archiv,' 1834, p. 467. Von Siebold, I observe, refers to Burmeister
as the first author who discovered the ovarian cæca within the
peduncle; I had thought that M. Martin St. Ange had a prior claim.
[49] These are obscurely figured by Karsten ('Nov. Act. Acad. Cæs.
Nat. Cur.,' 1845, Pl. 20, fig. 1_d_) as salivary glands; they were
so considered by Cuvier and M. Martin St. Ange: I may observe
that salivary glands have not been positively recognised in any
Crustacean.
As after the most careful and repeated examinations of various
Lepadidæ, I was convinced that there were no oviducts, so I have
come to a similar conclusion in regard to the Balanidæ; the ova
being brought to the surface, by the formation of a new membrane
round the sack underneath them, and by the subsequent exuviation of
the old membrane. The ova are united together by a most delicate
tunic investing each egg; the ovigerous lamellæ being thus formed,
as in the Lepadidæ. In the cases of _Chthamalus stellatus_, _Balanus
balanoides_, and _Platylepas decorata_, I saw a pair of very distinct
but fragile lamellæ. In Xenobalanus, the two ovigerous lamellæ form
two sub-cylindrical packets, pointed at their lower ends and often
cohering. There are no ovigerous fræna, for the attachment of the
lamellæ; the ova being sufficiently well retained, as it would appear,
by the well-closed shell. I have elsewhere stated my full belief that
it is the ovigerous fræna which have been metamorphosed into the
branchiæ of the Balanidæ. Most sessile cirripedes breed when very
young; and I have every reason to believe that they breed several times
in the year. The ova are ovate, and vary in length from 14/2000th of
an inch in Chthamalus, to 19/2000th in some species of Balanus, in
which this greater length was owing to a more elongated shape,--up to
25/2000th in some other species of Balanus. The ova are wonderfully
numerous, especially in the genus Coronula.
I may here mention the singular case of some elongated specimens
of _Balanus balanoides_, from Tenby, in South Wales: some of these
presented nothing abnormal; but in no less than seven specimens, the
two, three, or four posterior pairs of cirri, either on one or both
sides, were in an almost rudimentary condition, being of small size
and having a shrunk and wasted appearance. In six out of these seven
specimens, the probosciformed penis was quite short and abruptly
truncated, as if from abortion. By cutting off the truncated apex, and
cleaning the external tissue, I ascertained that it was imperforate,
apparently in all the cases, and I am certain of this fact in several
of the cases. In three of the specimens, I examined the vesiculæ
seminales; in one, I found some spermatozoa, but cohering together
in a peculiar manner; in the second, there were no spermatozoa; and
in the third, the vesiculæ were shrunk, empty, and quite rudimentary
in size. So that these three individuals certainly could not have
impregnated their own eggs; nevertheless, within the shell of these
very three, there were perfectly developed larvæ: I am led to conclude
from this fact, that adjoining specimens in a perfect condition had,
by means of their long probosciformed penis, effected the fecundation
of their imperfect neighbours. I need only further add, that some
out of the above six specimens, with more or less aborted cirri and
imperforate male organs, were infested by a peculiar parasite, allied
to Bopyrus,[50] and that these specimens did not contain ova.
[50] I have given a short notice on this parasite, in my former
volume on the Lepadidæ, in a foot-note to p. 55.
_Metamorphoses and Homologies, throughout the Order of Thoracica._
In my former volume, the metamorphoses were described under three
principal stages or heads; but whether these three included all
the main changes, I was then hardly able to conjecture. But now
I have reason to believe that such is the case, for in the genus
Cryptophialus, belonging to the Abdominalia, the whole course of the
metamorphosis, from the egg to the pupa, takes place within the sack of
the parent; and I found, when having, on the coast of South America,
numerous specimens to examine, that the egg-like larvæ (Pl. 24, fig.
15-18) could be naturally grouped into two main stages, but with many
transitional intermediate grades (answering to the successive moults in
the first stage of ordinary larvæ), before they passed into the third
or pupal stage. And the first two stages in these egg-like larvæ of
Cryptophialus, clearly seem to correspond with the first two stages in
ordinary larvæ; for in both the chief changes are, the shortening of
the terminal projection--the increase in size and approximation on the
ventral surface of the anterior horns or cases for the antennæ--and the
compression of the whole body. In all members of the Thoracica, the
metamorphosis seems to run a remarkably uniform course. The larva in
the first stage undergoes several moults and slighter changes--how many
is not known--before arriving at its second main stage, which has been
observed only in one single instance; and judging from Cryptophialus,
this second stage passes abruptly by one moult into the pupal stage;
and this, certainly, passes abruptly into the Cirripedial or mature
stage.
_Larva, First Stage._
The larvæ in this stage are known, amongst the Balanidæ, in Balanus,
Pyrgoma, Coronula, Platylepas, and Chthamalus; and these genera include
all the principal forms. Amongst the Verrucidæ they are known in its
one genus, Verruca. Amongst the Lepadidæ, in Scalpellum, Ibla, Alcippe,
Lepas, Conchoderma, &c.; and in all these genera the larvæ present no
important difference--hardly any difference which could be viewed as
generic, were these larvæ independent animals,--as may be inferred,
chiefly, from Mr. C. S. Bate's descriptions.[51] The abstract given in
my former volume was not accompanied by any illustrations, and I have
consequently here given (Pl. 29, fig. 8), a view of the larva, in the
first stage before moulting, of _Scalpellum vulgare_: the natatory legs
are not drawn with accuracy, only the relative position of the several
organs having been carefully attended to. I have also had copied from
Mr. Bate's memoir, a figure of the larva (Pl. 29, fig. 9) of _Balanus
balanoides_, in its first stage, _before_ moulting, with its ventral
surface exhibited; and another figure (with a few trifling alterations
made after examining specimens most kindly sent me by Mr. Bate) of
the larva of _Chthamalus stellatus_ (fig. 10), in its first stage, but
_after moulting once_. It should be observed that Mr. Bate has given
a drawing of the larva of this latter cirripede, in the first stage,
_before moulting_; and it does not differ essentially from that just
referred to (fig. 9), of _B. balanoides_, but is rather more fully
developed. These drawings suffice to show the character of the larvæ
in the first stage, both before and after the first moult, and even
after the second moult, throughout the Order of Thoracica. The larvæ
sometimes undergo their first moult within the sack of their parent, as
I have been informed by Mr. Bate, and as I have observed in Coronula.
[51] 'Annals and Magazine of Natural History,' vol. viii (2d
series), 1851, Plates 6, 7, and 8.
I will now make a few remarks on these larvæ in the first stage, before
and after the first moult, supplemental to those in my former volume.
Their shape is oval, and the whole dorsal surface is evidently covered
by a carapace. It is remarkable that the body exhibits no distinct
articulations; those given by Goodsir[52] being certainly erroneous.
Commencing at the anterior extremity, the eye varies considerably in
the state of its development; in _Platylepas decorata_ it is nearly
circular, and in most of the specimens very distinct; whereas in
the allied _Coronula balænaris_, before the first moult, it is very
imperfect, but afterwards square and of considerable size. In _Balanus
galeatus_, in the immature larvæ dissected out of the egg, the cellular
matter which was in process of conversion into the eye, formed a
transverse band, obscurely divided into two portions, and this seems
to indicate that the single eye is in fact formed by the confluence of
two eyes. In _Scalpellum vulgare_, this heart-shaped eye lies between
a V-shaped muscle, the nature of which I cannot understand, and which
has not been represented in (Pl. 29, fig. 8, _a_). I need only further
add, that in _Chthamalus stellatus_, after the first moult, the eye
exhibits, in specimens sent me by Mr. Bate, some appearance of tending
to become double.
[52] 'Edinburgh New Philosophical Journal,' July, 1843, Pl. 3, 4.
Arising posteriorly to the eye, we see, in _Scalpellum vulgare_, a
pair of minute curved horns (_b′_), directed backwards; and within
these horns I distinctly saw an articulated organ. These horns are
difficult to be distinguished, and probably could not be made out
previously to the first moult, in any larva of less size than that of
_Scalpellum vulgare_. But after the first moult, Mr. Bate has seen, in
two species of Balanus, in Verruca and in Chthamalus (fig. 10, _b_),
a pair of articulated organs, in this same position, evidently now
forming antennæ, and directed anteriorly, and free from any envelope.
It is somewhat important, as we shall presently see, to bear in mind
that these antennæ first appear within an envelope or horn; and that I
detected that they included an articulated organ, before I had heard
of Mr. Bate's observations. These antennæ, from their small size, from
being seated internally with respect to the horns containing the other
pair of antennæ, and from the position which the latter assume in the
later stages of the larva, I believe to be the first or anterior pair.
Their position in appearance posteriorly to the large lateral horns,
containing the second pair of antennæ, is probably due to the anterior
cephalic segments having been driven inwards, the truncated outline
of the front of the head, and likewise, probably, the position of the
mouth between the bases of the natatory legs being thus caused.
In this same larva of _Scalpellum vulgare_, within the great lateral
horns just alluded to (fig. 8, _c_), filiform organs, supporting rows
of spines, could be distinguished; and these appeared to me to be
antennæ. These horns or cases resemble in structure the smaller pair
just described; they arise from the ventral surface, and can hardly,
therefore, be considered as prolongations of the carapace. After
the first moult (fig. 10, _c_) they are seen to have increased much
in length: in some cases they are of considerable length before the
first moult, as in Lepas: in the Balanidæ they seem to be generally
shorter than in the Lepadidæ; but in _Balanus galeatus_ I found them
one third of the entire length of the animal. Whilst within the
egg, these horns are adpressed laterally to the body, and so point
posteriorly; afterwards they project rectangularly from the sides,
or, as in _Scalpellum vulgare_, are directed somewhat anteriorly. As
in the larvæ of all ordinary Crustaceans, as yet known, the antennæ
are amongst the earliest developed organs; and as the first pair of
natatory legs (Pl. 29, figs. 8-10, _e_) in these Cirripedial larvæ,
might so very naturally be thought to be antennæ (as has been remarked
to me by Mr. Dana), both from their structure and from their position a
little anteriorly to the mouth, I am well aware that to prove my view
correct, namely, that these horns are the second pair of antennæ in
process of formation, it is not sufficient merely to have seen organs
resembling antennæ within them; nor is it sufficient to advance the
strictly analogical fact of the first-mentioned pair of antennæ, which
in Scalpellum indisputably appear in their earliest condition within an
envelope or horn. Further evidence is required, and this is presented
in Cryptophialus, in which the lateral horns of the egg-like larva, in
its first stage (Pl. 24, fig. 16), can be actually followed step by
step until, in the second stage (fig. 17), just before passing into
the pupa, the horns are seen to have become larger and more nearly
approximated to each other on the ventral surface; and whilst in this
condition, I several times dissected out the prehensile antennæ of the
future pupa with _every character perfectly recognisable_. Hence I
cannot doubt that in the larvæ of Cirripedes the law of development is,
that in their very earliest condition, the small first pair of antennæ
are enclosed in cases; and that the large second pair remains thus
enveloped until the pupal stage. This conclusion, we shall immediately
see, is in harmony with the late development of the succeeding
appendages or organs of the mouth, which certainly do not appear in the
first larval stage, and are not known to appear even till after the
final metamorphosis.[53]
[53] According to M. Joly, ('Annales des Sciences Naturelles,' 2d
series, tom. xix, p. 59) in the larva of the macrourous Caridina,
the natatory legs appear before the gnathites or parts of the
mouth; so that in ordinary Crustaceans there is no invariable order
of development from the anterior towards the posterior end of the
body, as has sometimes been supposed.
The mouth is more or less probosciformed (Pl. 29, figs. 8-10, _d_),
differing considerably in this respect in different species of the
Lepadidæ; and this, probably, is due to the larva being born in a more
or less mature condition. Its exact position likewise varies, for it
arises either between the first or second pairs of natatory legs. It is
known, from Mr. Bate's observations, to have the power of movement. It
is directed posteriorly, the œsophagus extending anteriorly; both these
directions being the same as in the mature cirripede. Certainly during
these early stages there are no jaws or gnathites; but the margin,
answering to the labrum, is furnished with some short, thick, sharp
spines, and with hairs. In _Scalpellum vulgare_ the orifice of the
œsophagus seems to lie rather beneath the upper prominent spinose edge,
which, as just remarked, probably answers to the labrum; but this is
one of the species in which the probosciformed mouth, at least before
the first moult, is not much developed.
We come, now, to the three pairs of natatory legs: the first (Pl. 29,
figs. 8-10, _e_) has throughout the order only one ramus, whereas the
two succeeding pairs (_f_, _g_) are biramous. I must here remark that
the straight and strong, and the curved plumose spines, with which
these limbs, after the first moult, become furnished, now appear to me
as more probably prehensile, rather than masticatory as I imagined in
my former volume. That these spines are important organs to the larvæ
I do not doubt. With regard to the homologies of these three pairs of
limbs, my first impression was that they were the mandibles and the
two pairs of maxillæ in their earliest condition; but I consider this
view as quite untenable, for several reasons; viz., the wide interval
between their bases and the mouth itself,--the somewhat variable
position of the mouth with respect to the legs,--and the position
which the latter occupy in the _second_ larval stage.[54] A far more
tenable view is that these three pairs of legs are the three pairs
of maxillipeds, in their earliest condition, in accordance with the
view of M. Joly[55] on the nature of the three very similar pairs
of natatory legs in the larva of Caridina, a macrourous Crustacean.
But, in Cirripedes, the three pairs of natatory legs, in the larva
in the first stage, are apparently the very same as the first three
pairs, in the larva in the second stage, and in the pupa. And in the
pupa the first three pairs, which certainly correspond with the first
three pairs of cirri in the mature animal, seem to me, for reasons
presently to be assigned, to be the second, third, and fourth thoracic
limbs. Hence I am led to the conclusion that the first pair of legs in
the larva in the first stage, are homologically the second thoracic
(answering to the third pair of maxillipeds in the higher Crustaceans),
and that the two succeeding pairs are the third and fourth thoracic
limbs; to be succeeded, in the _pupal_ stage, by the fifth, sixth, and
seventh thoracic appendages.
[54] Mr. Dana, moreover, has remarked, ('Crustacea: United States
Exploring Expedition,' p. 1386), "that he knows of no instance of
a mandible becoming so completely a leg, as to lose wholly the
mandibular function even of its basal portion."
[55] 'Annales des Sciences Naturelles,' 2d series, tom. xix, 1843,
p. 34. M. Joly's observations were made on the Caridina. I owe to
the great kindness of Mr. C. Spence Bate, an examination of some
larvæ of the allied genus _Hippolyte varians_, and I found, on
dissection, the view of M. Joly, that the three pairs of natatory
legs are the maxillipeds, so far strongly confirmed, that they
followed closely, with equal intervals, the mandibles and two pairs
of maxillæ. The first pair of natatory legs in Caradina, in its
earliest condition within the egg, is uniramous, like the first
pair in the larvæ of Cirripedes. There is one fact which seems
rather strongly opposed to the view of these three pairs of legs
in the larvæ of the macrourous Crustaceans being the maxillipeds,
which is that Capt. Du Cane ('Annals of Nat. Hist.,' 1838, vol.
ii, pl. 6, and 7) observed only three pairs of limbs in process
of formation posteriorly to the first three pairs, whereas there
should be found, in accordance with M. Joly's view, five pairs, _i.
e._ all five pairs of ambulatory legs. This one fact countenances
the view, which I hold on the nature of the legs in the larvæ of
Cirripedes during their early stages, namely, that they are the
second, third, and fourth thoracic limbs, to be succeeded by only
three additional pairs.
Lastly, behind the natatory legs, on the ventral surface, (Pl. 29,
figs. 8, 9, _i_), the body is much produced, and terminates in a
horny fork, which, after the first moult (fig. 10, _i_), becomes much
elongated. Anteriorly to this fork, on the ventral surface, there
is another fork (_l_), and again above this I could distinguish, in
_Chthamalus stellatus_, after the first moult, another fork (_m_),
or at least a pair of short thick spines. From the structure of the
forked abdomen in the known larvæ of the Podophthalmia, I presume
that this portion of the body is the abdomen of the young Cirripede,
but it is not at all plainly articulated. After the first moult, the
posterior end of the carapace (_h_), which is always pointed, becomes
much elongated and serrated on both sides;[56] reminding one of the
structure of the carapace of the so-called Zoea, or larva of certain
Podophthalmia. Situated under this posterior prolongation of the
carapace, there is a swelling (_n_, with long hairs on both sides),
which apparently lies on the dorsal surface of the spinose and forked
abdomen; here, when the larva is compressed, the cellular and oily
contents of the body burst forth; and I suspect that this swelling
is the anus, for it is known from the researches of Rathke,[57] that
the anus in the higher Crustaceans opens during the earliest periods
dorsally.
[56] I suspect that the account given by Goodsir ('Edinburgh New
Phil. Journal,' 1848) of the posterior points of the carapace and
abdomen in the larva of a Balanus, is not quite accurate.
[57] 'Annales des Scienc. Nat.,' tom. xx, p. 451.
_Larva, Second Stage._
I have given, from Burmeister,[58] a lateral view (Pl. 30, fig. 1)
of the one single specimen, ever observed of a larva in this stage,
belonging, as is supposed, to the genus Lepas. The carapace has now
greatly altered its character. The two fleshy projections, as so
called by Burmeister, by which the larva adhered to the sea-weed,
were supposed by this author to include the great prehensile antennæ
of the pupa; from my observations, already alluded to, on the two
projections (Pl. 24, fig. 17) in the closely analogous egg-like larva,
in the second stage, of Cryptophialus, by which it also adheres, I
have not the least doubt that this is the case. The small, internal,
and anterior pairs of antennæ, are, as it would appear, now aborted.
The eye, according to Burmeister, has commenced becoming double;
but the two approximate eyes are not as yet compound. The mouth is
probosciformed (_m_), and does not differ much from its condition in
the first stage; no gnathites were observed by Burmeister, and they
could not be expected to be present, for they are not found even in
the pupa. The mouth, which in the larva in the first stage differs in
different genera, in being more or less advanced forward, here stands
some way anteriorly to the natatory legs, as in the pupal condition.
The first pair of legs is uniramous, and the two other pairs biramous;
this fact, together with the number of the legs in this second stage
being still three, and their structure being not very different,
leaves little doubt on my mind that we here have the same three pairs
as during the first stage. The abdomen has become much shortened, but
still space is left for the development, in the pupa, of the three
posterior pairs of legs. I may here remark that in the pupa the
anterior natatory legs have become, like the others, biramous; but yet,
as it were for the purpose of showing their metamorphosis from the
uniramous legs of the earlier stages, they have their bristles arranged
rather differently from those on the succeeding five pairs of legs.
[58] 'Beiträge zur Naturgeschichte der Rankenfüsser,' tab. 1, figs.
3, 4.
_Larva in the Last or Pupal Stage._
I have given a lateral view of the pupa of _Lepas australis_ (Pl. 30,
fig. 2), illustrative of the description in my former volume: the
specimen is drawn as if transparent, and it was to a certain extent
thus rendered by boiling in caustic potash. A sketch of the position
of the young Cirripede within the pupa, was made by the camera. At
first the drawing will perhaps hardly be comprehended: the darker
shaded portion to the left of the letter (_b_) shows the extent of the
sack, with the included thorax and natatory legs of the pupa: to the
right of the same letter, if we do not consider the young included
Cirripede, the only organs distinguishable in the mass of cellular and
oily matter, are the alimentary canal, the cement-glands (_t_), _i.
e._ the incipient ovaria, and the cement-ducts (_t′_) which enter the
antennæ. A view is also given (fig. 4) of the ventral surface of the
pupa; and a transverse section (fig. 7) of the carapace, taken close
to the eye-apodemes. On comparison with the larva in the second stage,
the changes in external appearance and structure are not very great;
the prehensile antennæ are freed from their cases; the two eyes stand
further apart; the three posterior pairs of legs have been developed,
and a small abdomen has become distinctly separated from the thorax.
Before proceeding to make a few additional remarks and corrections to
my former description of the pupa, it will be advisable, on account of
the importance of the subject, to discuss the homologies of the limbs.
From the presence of eyes and of two pairs of antennæ in the larva,
during its earlier stages, the front of the head consists, in
accordance with all analogy, of three segments; the mouth, likewise,
from being formed of three gnathites (which can be detected by
dissection in the pupal state), consists, also in accordance with all
analogy, of three segments, making altogether six segments--on the
nature of which I apprehend no objection will be raised. In two out
of the three orders into which Cirripedes may be divided, the mouth
is succeeded, in the adult animal, by eleven most distinct segments;
of which the first (_i. e._ the seventh cephalic) differs from the
succeeding seven thoracic segments; and these seven again differ
from the three abdominal and terminal segments. Hence it must be
admitted that, as far as the cephalo-thorax of the archetype Cirripede
is concerned, it consists, like that of the archetype Crustacean,
of fourteen segments, of which eight succeed the first-named six
that form the mouth and front of the head; and that, with the three
abdominal segments, there are altogether seventeen segments. In the
order Thoracica, however, which includes all common Cirripedes, both
in the pupa and in the mature animal, only six thoracic segments
with their appendages, succeed the mouth, two having been lost; and
the question arises which are these two, whether the seventh and
eighth, or the thirteenth and fourteenth (_i. e._ the two terminal
thoracic) segments; for there is no reason to suspect any other
segments of having disappeared. In my former volume, I inferred,
without sufficiently entering into my reasons, that it was the seventh
and eighth, _i. e._ the last cephalic and first thoracic segments,
which had disappeared; but I now find that Mr. Dana[59] believes
that, in ordinary Crustaceans, the abortion of the segments with
their appendages almost always takes place at the posterior end of
the cephalo-thorax. Nevertheless, after due deliberation and fresh
examination of the pupa, I must retain my former opinion, that it
is the last cephalic and first thoracic segments which have either
coalesced with the others, or wholly disappeared. In the pupa, the
mouth, although functionless, has its place most plainly marked by
being slightly prominent, and by the presence of a sort of labrum and
of a shrivelled œsophagus, round which latter the gnathites and the new
œsophagus of the future young cirripede are in process of formation.
Now between the mouth of the pupa and the first pair of natatory legs,
there is a space of membrane, equalling, when stretched out, the three
succeeding thoracic segments in length and breadth: this interspace, I
conceive, must have some homological signification; here then we have
at least an appearance of the abortion of appendages; whereas, at the
posterior end of the cephalo-thorax, no such appearance is presented.
Moreover this interspace of membrane is divided nearly in the middle by
a most conspicuous fold, which, on the view here adopted, would mark
the separation of the seventh (cephalic) from the eighth (thoracic)
segment; and the interspace and fold are thus simply explained. Lastly,
I have shown, in the Introduction (p. 18), that the first and five
succeeding pairs of cirri of the mature Cirripede present certain
small, but significant, resemblances in structure and in the origin
of their nerves, with the outer pair of maxillipeds and with the five
pairs of ambulatory legs in the Podophthalmia; which resemblances are
all futile, if the cirri belong to the 7th, 8th, 9th, 10th, 11th, and
12th segments of the cephalo-thorax, or those immediately succeeding
the mouth; but are full of meaning, if the six pairs of cirri belong,
as I believe, to the 9th, 10th, 11th, 12th, 13th, and 14th segments, or
the six posterior segments of the cephalo-thorax.
[59] 'Crustacea: United States Exploring Expedition,' p. 22.
Before commencing on details, I may premise that I have examined
the pupa of _Lepas australis_, _pectinata_, _fascicularis_, and
_anatifera_, of _Conchoderma virgata_, partially of _Dichelaspis
Warwickii_, of _Ibla quadrivalvis_, and of _Alcippe lampas_; and in
the Balanidæ, of _Balanus balanoides_ and _Hameri_. In the pupæ of all
these genera there is a most close general agreement in structure,
excepting in minute details: I was surprised to find exactly the same
slight differences in the spines on the first pair of natatory legs,
as compared with the succeeding pairs, in _Balanus Hameri_, as in
Lepas. The abdomen and caudal appendages of the pupa in the abnormal
Alcippe, as we shall presently see, offer the only marked exception
to this uniformity of character throughout the Thoracica. The outline
of the carapace or shell is usually not so blunt at the anterior end,
as in the pupa of _Lepas australis_ (Pl. 30, fig. 2); more commonly
the shape is that of the pupa of Alcippe (Pl. 23, fig. 16). In _Lepas
pectinata_ the two posterior points of the carapace are produced into
two short spines. The surface of the carapace in _L. australis_ is
lined, as represented in fig. 4: the colour of this species when alive
was blue:[60] in _L. fascicularis_ the surface is punctured: in _L.
pectinata_ it is marked with curious points of various shapes, often
star-shaped, in parts reticulated, and confluent along the dorsal
margin, and in parts lined: in _B. balanoides_ it is very obscurely
punctured, and in _B. Hameri_ the punctures pass into lines. The whole
of what is externally visible consists of the carapace, for this is
produced not only backwards, so as to enclose the thorax and abdomen
with their appendages, but also forwards, so as to overhang the whole
front of the animal; and the prehensile antennæ, in Lepas, Ibla,
Balanus, and probably in all the genera, can be retracted within its
lower edge. The protection afforded by the carapace to the antennæ
is aided by two crests (Pl. 30, fig. 7, _c_) parallel to this lower
edge. The whole sternal surface is very narrow (fig. 4), and is
likewise protected by the carapace; that is, when the two sides are
drawn together by the adductor muscle. The shell, however, when thus
drawn together, gapes a little at the two ends, at least in the case
of _Lepas australis_. The adductor muscle, if introduced in fig. 4,
would have crossed close anteriorly to the basal margin of the mouth;
and in fig. 2, its end on the near side would have been attached under
the dark cæca, which enter the upper end of the stomach. The adductor
is shaped almost like an hour-glass, and so differs from this muscle
in the mature Lepas, in which it is of the same thickness throughout.
I may here add that the pupa of _Lepas australis_ could swim very
rapidly, and often on one side in a circle; it could walk by the aid
of its antennæ, but often fell over; being thus locomotive, and, as
we shall immediately see, well provided with senses, it cannot be
considered as very lowly organised.
[60] I took this species alive in the Southern Atlantic Ocean; and,
mistaking it for an independent Crustacean, was much perplexed
where to class it. I had overlooked these specimens when publishing
my former volume.
_Acoustic Organs._--Commencing at the anterior end, two small elongated
orifices, 10/6000th of an inch in diameter, (_e_, fig. 4, Pl. 30),
may be seen; these lead, as described in my former volume, into a
sack, with a bag suspended in it, which is provided with a large
nerve, and which I believe to be the acoustic vesicle. These orifices
occur in the carapace, either in the same position, or a little more
posteriorly, in the pupæ of all Cirripedes. In _Balanus balanoides_
they are minute, being only 2/6000th in diameter, but are surrounded
with a border: in _Conchoderma virgata_ they are also surrounded
by a border: in _Lepas pectinata_, the orifices are 3/6000th of an
inch in diameter, and are very singular from being seated on rounded
prominences, causing the carapace to have two short, blunt horns in
front. In _Lepas australis_, and I believe in the other species, the
corium round the acoustic orifices is darkly coloured; and these
coloured marks can be distinguished for some little time on the
peduncle of the young Cirripede, after the metamorphosis, and after
the entire organ, together with the whole pupal carapace and eyes, has
been moulted. Knowing the connection in the higher Crustacea, of the
acoustic organs and the antennæ, and seeing the very backward position
(figs. 2 and 4) of the one great pair of antennæ, I have always
imagined that these orifices probably marked the normal position of the
anterior pair of antennæ, which, since the earlier larval stages, have
disappeared. And I now find[61] that Schödler affirms, that in most,
if not in all Daphnidæ, there is a black spot in front of the eye,
which is connected with an opening in the basal portion of the anterior
antennæ, and he concludes that it is an organ of hearing.
[61] Quoted by Dana, 'Crustacea of United States Exploring
Expedition,' p. 1264.
_Antennæ._--These, from their present position, and from standing, in
their earlier stages whilst within their envelopes or horns, exteriorly
to the small medial pair (since aborted), I believe to be the second
pair; and this is Mr. Dana's opinion. In my former description of these
very singular and important organs (Pl. 30, figs. 4 and 8), I have
fallen into some considerable mistakes: the two plates or segments
(fig. 4, N), of which the posterior margins are inflected as apodemes
(_n_), carrying the eyes, are certainly, as may be clearly seen in the
pupa of Alcippe, Pl. 23, fig. 16, and as affirmed by Burmeister,[62]
the basal segments of the antennæ. The second or main segment
(formerly called by me the basal segment) has in some species an upper
portion of the membrane of which it is composed, next to the body,
excessively thin, and separated from the rest of the membrane composing
the segment, by an oblique line (fig. 8, _o_), which I mistook for
its articulation with the body.[63] We then come to the disc or third
segment; and lastly to the fourth, or ultimate segment. This ultimate
segment, generally, has its external corner projecting up, as a
step; and this sometimes, as in _Dichelaspis Warwickii_, gives the
appearance of its consisting of two segments; but a careful examination
of this part in Ibla, in which the step-like structure is carried to
an extreme, makes me believe that there is only one segment.[64] The
prehensile antennæ, therefore, like the natatory legs, are formed of
four consecutive segments, of which the basal segments give rise to the
singular apodemes, presently to be noticed (fig. 7), that carry the
great compound eyes. This basal segment, in all Cirripedes, is moulted
with the eyes, the three other segments invariably remaining cemented
to the surface of attachment.
[62] 'Beiträge zur Naturgeschichte der Rankenfüsser,' p. 19.
In tab. 1 of this work there are good drawings of the general
structure of the pupa of a species of Lepas, probably _L.
australis_. I believe this author was the first who made out the
structure of the abdomen of the pupa.
[63] In the table of measurements of the antennæ of the several
genera and species of the Lepadidæ (p. 286) of my former volume,
the articulation, called by me _basal_, I now know to be really
the articulation between the basal and second segment. In the
fourth column, headed "Length from end of the disc to the inner
margin of the basal articulation," the term inner margin really
applies to the oblique curved line separating the thin and scarcely
visible membrane from the thicker membrane of that segment. These
corrections do not in the least affect the object for which the
table was given.
[64] In a sketch, sent me by Mr. Dana, of this organ in the pupa
of a Lepas from the Antarctic Ocean, I observe that he divides my
ultimate segment into two segments.
In the Southern Atlantic I took some specimens of the pupa of
_Lepas australis_, not yet attached, and therefore with the muscles
of the antennæ, not having suffered any of that absorption, which
they undergo, as soon as the pupa is permanently cemented to some
floating object. In my former volume I noticed a pair of strong
muscles, attached to the tips of the middle forks (Pl. 30, fig. 7)
of the apodemes, and I now find two pairs attached to the bases of
the two outer forks, and directed dorso-anteriorly; and two other
pairs, also attached to their bases, but directed dorso-posteriorly,
so that altogether there are five pairs of muscles attached to the
apodemes; their chief function, I should think, was to draw the antennæ
posteriorly and upwards within the carapace; but as the apodemes cannot
be moved without the great compound eyes being likewise moved, the
muscles probably serve a double purpose. When the pupæ were alive, I
noticed that their eyes were constantly kept in a state of vibratory
movement. Flexor and extensor muscles are attached at one end to the
posterior margin of the basal segment, and at the other end to the
second or main segment; other powerful muscles attached to this latter
segment, are prolonged by ligaments into the disc. In Cryptophialus I
observed that the disc-segment had a movement almost like that of the
wrist. Whether any muscles enter the small terminal segment, I know not.
The drawing in Pl. 30, fig. 8, of part of the second segment, of the
third or disc segment, and of the fourth or ultimate segment, in
_Lepas australis_, is, I think, very accurate. The second segment
articulates on the upper or dorsal surface of the disc, and has the
articulation on one side constricted and formed of thin and flexible
membrane; the little terminal segment, which is turned outwards at
right angles, also, articulates on the disc. That the disc forms a
true segment is shown clearly in Cryptophialus (Pl. 24, fig. 18),
where the articulation with the second segment is not in such close
contact. The disc is either circular, as in Lepas, or hoof-shaped, as
in Ibla: in _B. balanoides_ the disc is rather hollowed out on the
inner side. It has the power of adhering even to so smooth a substance
as glass, placed vertically. It is surrounded by a rim of transparent
membrane. On the hinder margin some spines arise from the central and
more opaque part: in _Lepas australis_, there are no less than seven of
these spines (fig. 8): in _Conchoderma virgata_ there are only four,
in _Scalpellum_ and _Ibla_ only one. When the disc is placed on the
surface of attachment, these spines lie parallel to it. The middle
part of the disc is, almost always, nearly opaque; and in tracing
the cement-ducts from within the body of the pupa, or of the young
Cirripede, I in many cases traced them as far as this point, but here
lost them. From this same obscure central part of the disc, in most,
if not in all species of the Lepadidæ, spokes radiate, which sometimes
are branched, and are not regular, not always even resembling each
other on the opposite sides of the same individual. Round the proper
membranous border of the disc, a second one may be observed (fig. 8,
_p_), which differs in shape and extent in different specimens: under
favorable circumstances, and very high powers, it may be seen to
have a reticulated structure, and to be of a very pale brown colour;
towards the exterior margin, the reticulations become finer, and are
blended together and lost; on the inner margin, the substance forming
this membrane may be seen to come out of the spokes. This substance
is the cement, which has the power of adhering to whatever substance
it grows against; and thus the disc of each antenna becomes cemented
down, and soon both the antennæ are surrounded by a common border of
cement, which gradually increases, after the metamorphosis, in extent.
Occasionally the cement forms little projections, like short spines, on
the edges opposite to the orifices of the spokes.
The small terminal segment usually bears on its truncated extremity
six spines, some of which are occasionally hooked; in Scalpellum, two
spines, rather longer than the others, are borne on a step some way
down on the inner side of this segment; but in Lepas, two spines (fig.
8), very much longer than the others, arise from the outer corner
of the extremity. These two are very different from the other four
borne by this segment, or indeed any other spines on the body; for
they are quite flexible, and are furnished with a double row of very
long, straight, excessively fine hairs, which seem to be articulated
on them--the whole presenting a very beautiful appearance. These
spines are of considerable length, and in _Conchoderma virgata_ they
even equal in length the whole antenna. I can hardly doubt that these
beautiful, plumose, flexible spines, into the thick bases of which the
coloured corium could sometimes be seen to enter, serve as feelers.
Owing to the facts immediately to be mentioned, I erroneously stated,
in my former volume, that there were three long spines.
In three species of Lepas, in _Dichelaspis Warwickii_, and in
_Scalpellum Peronii_, after having torn the lately-cemented antennæ
from the surface of attachment, I observed proceeding from the end of
the terminal segment, from between the above two groups of spines, what
appeared to be a long narrow ribbon with its end torn off; and which,
in the case of _Lepas australis_, I fancied was one of the plumose,
long, flexible spines ripped open. But now that I have examined some
of the pupæ of this species before their attachment, I find (as
represented in Pl. 30, fig. 8, _v′_) a flattened tube, ending in a
blunt point, and having a peculiar ringed structure. I have noticed
similar appendages to the antennæ of specimens just attached of _Lepas
anatifera_. In the Dichelaspis and the Scalpellum, the tube was very
long, and seemed, from its torn appearance, to have been firmly
attached to the supporting surface. In both these cases, the tube came
out from within another slightly larger tube, which had been broken
off close to the extremity of the terminal segment of the antenna. In
the case of the _Lepas anatifera_, the tube expanded a little after
leaving the antenna. In the Dichelaspis, it had exactly the same
diameter as the cement-duct, which could be clearly distinguished
within the two lower segments. From these several facts, and from the
peculiar appearance of the tube itself, I believe it to be a tube of
cement-tissue which thus, sometimes even before the pupa is attached,
independently grows outwards. That the cement-tissue can grow outwards
and assume definite forms, we know from the singular case of _Lepas
fascicularis_, in which the cement proceeding from several apertures,
forms a vesicular float round the peduncle of not only a single
individual, but often of a group of specimens: we shall presently find
a somewhat analogous fact in the case of Coronula. It is possible
that this tube, proceeding from the extremity of the antenna, may be
the channel through which cement continues to be poured forth during
the continued growth of the above Cirripedes; and the manner in which
this is effected, considering how firmly the end of the peduncle is
cemented down, has always appeared to me a difficulty. In those pupæ
of _Lepas australis_, which I caught swimming about unattached, it is
surprising that the disc should have been edged with cement, and that a
tube, similarly formed, should have grown out of the ultimate segment:
it shows, I presume, that the cement-tissue will grow out, whether or
no the pupa has succeeded in finding a proper object for attachment.
Lastly, I have felt some surprise, in two or three instances in
observing some dark purple pigment-cells, like those in the corium,
within the terminal tube of cement; and likewise within the spokes of
cement in the disc: this is the only fact which causes me the least
doubt, whether I have rightly determined the nature of the terminal
tube, as wholly formed of cement tissue; or whether it may not be
covered by an outer integument, itself lined by true corium, coloured
purple.
Finally, I may add, that, excepting in small details, the prehensile
antennæ present no difference throughout the Order: I have minutely
examined them in several genera of the Lepadidæ; and in the Balanidæ,
I have seen them in Coronula, and in several species of Balanus. In
_B. balanoides_ I have examined them carefully; they are smaller and
thicker than in Lepas, with the second or main segment bowed outwards,
carrying its usual single spine; with the disc excised on its inner
margin and apparently without the spoke-like vessels for the cement;
and with the ultimate segment proportionably longer, and carrying,
I believe, six spines, of which two appeared to be longer and more
flexible than the other four shorter and somewhat hooked spines. In
_Coronula balænaris_, also, the terminal segment is, proportionably
to the others, of large size. Not only throughout the order, but
throughout the whole Class, the antennæ are singularly uniform in
structure, as will be seen, when the last two orders are described.
_Eyes._--These present no difference, except in size, throughout the
class; and have been sufficiently described in my former volume. The
true basal segments of the antennæ (incorrectly designated formerly as
sternal plates or segments) are separated from each other by a deep
fold; and are separated from the edges of the carapace on each side
by a crest and slight fold (Pl. 30, fig. 7, _c_; and 4); these folds
and crests die out posteriorly, and disappear. The hinder, rounded
margins of the basal segments are inflected inwards, and their corners
are produced far up into the body, thus forming the curious UU-like
apodemes. These apodemes exist throughout the whole class; and the
outer arms always carry the great compound eyes. I noticed, in _Lepas
pectinata_, that the two middle arms are proportionably longer than in
_L. australis_. Owing to the presence of these apodemes, and to certain
coloured marks on the adjoining corium, the eyes, though enclosed
fairly within the carapace, yet deceptively appear pedunculated,
so that even J. Vaughan Thompson was thus deceived. I have already
described the several muscles attached to these apodemes, and the
constant vibratory movement of the eyes. Whilst the pupa remains a
freely swimming animal, the eyes are included, not only within the
shell or carapace, but (as would naturally happen) within the corium or
true skin lining the carapace; but after the pupa has become attached,
preparatory to its final metamorphosis (in the state represented at Pl.
30, fig. 2.), not only are the muscles, as before remarked, which are
attached to the apodemes, absorbed, but so is the corium investing the
apodemes and the immediately adjoining parts of the carapace. Hence it
comes that the new corium of the young Cirripede within, is formed in
a deep transverse fold across the whole lower half of the animal, and
the apodemes with the eyes are thus, as it were, rejected from within
the corium, though still remaining within the carapace. Consequently in
this final stage, the eyes and apodemes are very conspicuous from the
outside, being seen only through the transparent carapace. I presume
that the eyes at this period have become functionless, with the optic
nerve divided and absorbed. The eyes, apodemes, and carapace soon
afterwards are all moulted together.
The eyes of Cirripedes certainly undergo a remarkable series of
changes: in the larvæ in the first stage, there is a single eye,
perhaps formed by the confluence of two eyes, occupying the normal
position in the front of the head: in the second stage, according to
Burmeister, the eye has become double, but the two are as yet simple;
they are now situated posteriorly to the second pair of antennæ: in
the third or pupal stage, they remain in the same situation, but have
become compound, of great size, and are attached to the apodemes of
the antennæ: in the mature and fourth stage, they have moved someway
posteriorly, and again have become simple, of minute size, and are
either confluent, as in the Lepadidæ, or tolerably far apart, as in the
Balanidæ. It must not be supposed that the eye of the mature Cirripede
is metamorphosed from the eye of the pupa, for such is not the case;
the new eyes and old eyes being formed someway apart, and frequently
both can be seen within the pupa (as in Alcippe, Pl. 23, fig. 16) at
the same time. It is scarcely possible that the eye of the larva in the
first stage, can be changed into the double eyes of the second stage;
though these latter may possibly be multiplied into the eyes of the
pupa, as both continue to occupy nearly the same position.[65]
[65] Zenker, in his 'Physiological Remarks on the Daphnidæ,'
('Journal of the Microscopical Society,' 1853, p. 274), speaks of
a "tripartite azygous eye" as common amongst Crustacea, and as
occurring "in conjunction with the aggregated eyes in Artemia,
Argulus, &c.; but as appearing regularly in all the Branchiopoda
and Siphonostomata as the _earliest_ visual organ." Hence I
conclude that this azygous eye is the homologue of that single eye
which appears in the earliest larval stage of Cirripedes; and that
the compound eyes of the cirripedial pupa, answer to the aggregated
eyes of Artemia and Argulus, &c., with the difference, that in
these latter genera the single eye is retained. See, also, Von
Siebold, 'Anatomie Comparée,' tom. i, p. 435.
_Mouth_, _thorax_, _limbs_, _abdomen_.--I have nothing to add regarding
the mouth, except to confirm my former account; viz., that it is
functionless, consisting merely of crests, which project inwardly
between the gnathites of the young Cirripede, and of a shrivelled
closed tube representing the œsophagus. In fact the mouth is a model
of the outside of the mouth of the young Cirripede. I may remark that
some little way beneath the membrane answering to the labrum, a pair
of ligamentous apodemes, the use of which I do not know, slightly
penetrate the body. The degree of prominence of the mouth varies, but
it is far less than in the mature animal. On the limbs I have nothing
particular to add: the drawing of the first pair of legs (Pl. 30, fig.
5) is, I think, very accurate: I observed all the spines here figured,
on the corresponding leg of the pupa of _Balanus Hameri_. The five
posterior pairs of legs differ only in the outer ramus having five
plumose spines, instead of four, and one short simple spine at the
exterior angle, making six altogether. The legs, in their natural
position (fig. 2), have only the terminal segments of their two rami
directed posteriorly; and as a consequence the spine (close to _i_
in fig. 5), borne on the penultimate segment of the outer ramus, is
directed in the same line with that segment and with the pedicel,
namely, anteriorly, and at right angles to the natatory plumose spines.
This short spine acts, I imagine, as a defensive weapon; it has been
omitted in fig. 2. Of the thorax I need not give, from my notes, any
more details. The abdomen (fig. 6) is similarly constructed, as far as
I have seen, throughout the order, with the exception of Alcippe (Pl.
23, fig. 17), in which it is composed of only one segment instead of
three. In this genus the caudal appendages likewise consist of only one
segment, with very short spines. In the pupa of _Balanus balanoides_,
the three spines borne on each caudal appendage are very much more
unequal in size than in the pupa of _Lepas australis_, although in the
latter (fig. 6) the inner spine is considerably thicker than the two
outer. Whether the three segments of which the abdomen is composed, are
the three anterior or three posterior, of the normal seven segments,
I know not: on the view that they are the three posterior segments, I
presume, according to analogy, that the caudal appendages are borne on
the penultimate segment, and that the ultimate segment is here quite
aborted.
On the _internal viscera_ I have nothing to add. The cement-duct is
represented in Pl. 30, fig. 2, _t′_, on the near side, running into
the antennæ; and I repeatedly traced it, for the duct is very strong,
as far as the disc segment; at the other end it joins the cement-gland
(_t_) on the same side of the body. This cement-gland is proved, by
the clearest series of facts, to be converted into the incipient
ovaria and ovarian cæca. The cement-glands in the older pupæ could be
traced as far as the cæca of the stomach, exactly where the ovaria lie
in the mature animal; but in some young pupæ, they extended further
posteriorly, past the mouth, between the outer and inner membranes
of the overlapping carapace. I have faintly shown the course of the
stomach, with its two cæca at the upper end; the anus lies between the
caudal appendages, at the extremity (above _b′_) of the abdomen. At
this age there is no trace of the vesiculæ seminales, so conspicuous in
the mature Cirripede.
_Young Cirripede, whilst within the pupa._--I repeatedly succeeded in
dissecting the young _Lepas australis_ out of the pupa; and by the
previous action of boiling potash, and by a strong light, I was enabled
to make a camera sketch (Pl. 30, fig. 2) of the relative positions of
their several parts. The young Cirripede is drawn very faintly, and is
best seen by holding the plate in the same position with the mature
animal, of which a section is given in my volume on the Lepadidæ, Pl.
9, fig. 4. I may just notice how complicated are the membranes in a
longitudinal section taken at this period: we have, 1st, beginning at
the back, the horny tissue of the carapace or bivalve shell of the
pupa; 2d, the primordial valve (_z_, in fig. 3) of the young Cirripede;
3d and 4th, two folds of corium; 5th, the membrane of the sack of the
Cirripede; 6th, the membrane of the sack of the pupa; 7th, the outer
tunic of the thorax of the pupa; 8th, the outer tunic of the thorax
of the young Cirripede; 9th, the corium lining the latter membrane;
and these nine membranes would be repeated on the opposite side of the
section, if it were taken through either side of the shell or carapace,
bordering the orifice.
After the exuviation of the outer membranes of the pupa, certain
pre-existing coloured marks in the corium, such as those round the eyes
and round the acoustic orifices, along the ridge of the back and on the
borders of the orifice, &c., are still retained by the young Cirripede,
either temporarily or permanently; so that the correspondence of part
with part of the external surface admits of no doubt. Moreover, the
three terminal segments of the antennæ are invariably retained by
the young Cirripede, though in a functionless condition, and into
them the outer membrane of the body, and an important organ, viz.,
the cement-ducts are still prolonged; hence these prolongations must
be considered as aborted antennæ. Again, we have seen that the mouth
of the young Cirripede is formed under the rudimentary mouth of the
pupa, with the new œsophagus, round the old œsophagus, leading into
the same alimentary canal. The twenty-four extreme tips, likewise,
of the six pairs of biramous cirri of the Cirripede are formed
within the twenty-four extremities of the six pairs of biramous,
natatory legs of the pupa. Consequently, in the Cirripede and pupa,
thus far, part corresponds with part, notwithstanding that new eyes
are formed posteriorly to the old eyes, and new acoustic organs in a
quite different position from the old ones; but now we come to a most
important diversity in the metamorphosis, or rather to follow Professor
Owen,[66] in the metagenesis, of the young Cirripede. Although, as
just stated, the extremities of the cirri are formed within the legs
of the pupa, yet, from the great length of the cirri, they occupy more
than the whole of the thorax of the pupa; so that the thorax of the
young Cirripede is not formed within the pre-existing thorax of the
pupa, but within that part of the pupa, (homologically a portion of the
first three cephalic segments), which lies anteriorly to the thorax.
As a consequence of this, the pedicels and lower portions of the
cirri, the segments of the thorax and its dorsal surface, all come to
occupy a position at nearly right angles to that of the corresponding
parts in the pupa: this is shown in Pl. 30, fig. 2. And as a further
consequence, (and this is the more important point), the sack, which
both in the young Cirripede and pupa is formed by the overhanging and
produced portion of the carapace, and which is _internally lined by a
reduplication of the membrane of the thorax_, is necessarily, owing to
the changed position of the thorax, altered in extent and carried much
further; namely, from extending merely parallel to the longitudinal
axis of the pupa (from _b_ to _b′_), it is now in the young Cirripede,
in addition, carried (to _s′_) almost quite across the inside of the
animal. Hence it comes that the young Cirripede is, as I have said in
my former volume, internally almost intersected; and its body remains
attached only by a small space, (see the broken line, round _a_ and _b_
in Pl. 25, fig. 1, of a Balanus with the shell, &c., removed from one
side), to the sternal or ventral, inner surface of the carapace,--the
carapace being modified either into the capitulum and peduncle, or into
the shell with its operculum and basis. As a still further consequence
of this change of position of the body of the young Cirripede within
the body of the pupa, the alimentary canal becomes shortened to fully
half its former length. At the same time, the interspace between the
mouth and first pair of legs of the pupa, (consisting of the seventh
and eighth segments of the archetype), is quite lost in the Cirripede
by coalescence. The final cause of the thorax of the young Cirripede
not being developed within the thorax of the pupa, probably is, that
the cirri may be formed of considerable length, so as to be immediately
enabled to seize prey; and that the thorax, which supports the cirri
(and this probably is even more important) should be as free as
possible within the sack, so as to aid the captorial action of the
cirri.
[66] 'Parthenogenesis,' pp. 13 and 26.
After these remarks, more especially with regard to the formation
of the sack, if any one will look at the sectional drawing of a
pedunculated Cirripede in my former volume, or of a sessile Cirripede
(Pl. 25, fig. 1) in this present volume, in which latter the shell adds
to the complexity, he will perceive the cause of the extreme difficulty
in understanding the relative position of the parts throughout the
whole class. Even after I had discovered that the prehensile antennæ
of the pupa might always be found in the centre of the basis or
surface of attachment, and which fact, it might have been thought,
should have convinced me that this was the anterior end of the whole
animal, yet still I fancied that the prominent mouth represented the
entire head, and that the shell was something quite distinct. It is
clear that others have been equally perplexed; for that which is the
anterior end in the eyes of one naturalist, is the posterior end in
the eyes of another; so with the dorsal and ventral surfaces: one
naturalist considers the peduncle of the Lepas as the abdomen; another
considers it as a pair of metamorphosed, thoracic limbs, &c.! The
probable position of the segments of the body of a mature Cirripede,
in relation to the three anterior cephalic segments, or carapace, is
shown in the diagram (Pl. 25, fig. 6) of the supposed position of the
mature Proteolepas within its pupal envelopes. Here, in the diagram,
the two segments immediately succeeding the mouth (_c_), which are the
seventh and eighth of the archetype, (for the mouth consists of three
segments, and all in front of the mouth of three other segments),
have come to adhere by their dorsal surfaces to the internal surface
of the carapace,--that is, of the first three segments, which ought
of course to have stood quite in advance of these two segments, and
these two segments again ought to have stood in advance of the mouth.
The mouth is directed posteriorly, instead of from the body; and the
three segments of which it is formed (closed at their anterior end by
the labrum), and are very small compared to the relatively monstrously
great, three anterior cephalic segments, composing the carapace. To
place the segments of the body of Proteolepas in proper sequence, in
respect to those of the carapace, and in accordance with the sequence
of the archetype Crustacean, it would be necessary, by seizing the
extremity of the abdomen (_a_), to tear the two segments succeeding
the mouth from their dorsal attachment, as far back as the basal
margin of the labrum; and then pull them till they stood posteriorly
to (or in the diagram, above) the mouth; which latter part would,
by the same movement, be made to project out at right angles to the
ventral surface, and would then be preceded only by the first three,
great, confluent segments of the head, which being produced backwards,
form the carapace. All that has just been said on the position, in
Proteolepas, of the segments of the body in relation to those forming
the carapace, I believe to be applicable to all ordinary Cirripedes,
with this difference, that in the latter, after the metamorphosis,
the two segments succeeding the mouth quite disappear on the ventral
surface, and dorsally are either aborted or have coalesced with the
adjoining segments.
_Act of Metamorphosis._
When the due time for the act of metamorphosis has arrived, the pupal
carapace splits along the dorsal ridge, and is cast off, together with
the acoustic sacks, the basal segments of the two antennæ, and the
great, black, compound eyes, hanging to the UU-like apodemes. The three
terminal segments of the antennæ invariably remain cemented to the
surface of attachment. The exuviæ usually continue for a time united to
the cemented antennæ, but are finally washed away. Besides the split
along the dorsal ridge, the carapace separates, all round the orifice,
from the delicate tunic lining the sack and investing the thorax and
natatory legs of the pupa; for these membranes are not moulted for some
considerable time afterwards. Hence all these inner parts retain for a
period the appearance and structure of the natatory pupa, whilst the
exterior resembles, in every respect, a fixed and perfect Cirripede.
In my former volume, I have insisted on the important and curious
results which ensue from the eye-apodemes penetrating so deeply into
the body (see Pl. 30, fig. 7, in which the proportions are more correct
than in fig. 2), with the eyes attached exteriorly to their outer
arms; for as these apodemes have to be ejected, the external membrane
of the young Cirripede (Pl. 30, fig. 2) has to be formed in a deep
fold or arch over them, and consequently the membrane on the sternal
surface is formed considerably longer than on the dorsal surface. From
this it follows, when all the membranes are free and are stretched
fully out after the moult, that the whole animal, posteriorly to the
cemented-down surface, turns vertically up, and assumes its normal
position at right angles to the surface of attachment, and to that
which it held in its pupal condition; for the pupa always adheres with
its sternal surface parallel to the surface of attachment. A young
Lepas, which has just moulted its pupal carapace, and has assumed its
proper vertical position, with the cemented antennæ and the surface of
attachment remaining as before, is shown at fig. 3, but is drawn on a
smaller scale than the pupa fig. 2, out of which it may be supposed
to have been excluded. In this fig. 3, it may be observed that the
natatory legs and caudal appendages of the pupa have not as yet been
moulted. The fact of the stretching out, in the young Cirripede, of the
fold of membrane, which in pupa, just before the metamorphosis passes
over the apodemes and eyes, is well shown by three darkly-coloured
bands in the corium, which in the pupa are curled, but after the moult,
are stretched straight out on the peduncle of the young Lepas.
The pupa, and consequently the young Cirripede, from being attached
at first by the antennæ, does not adhere by the actual anterior
extremity, but by the sternal surface near it; the anterior extremity,
however, soon becomes cemented down, and afterwards, in ordinary
cases, ceases to grow. In Cryptophialus, however, and in certain
genera of the Lepadidæ, as Alcippe, Lithotrya, and Anelasma, the
anterior or basal extremity does continue to grow, and is not cemented
down, and therefore comes to be prolonged beyond the original point
of attachment; in order to allow of this, the surface to which the
Cirripede is attached has to yield, apparently simply to the pressure
exerted in the case of Anelasma, but in the three other genera, to
the rasping action of the roughened surface of the extremity of the
peduncle.
When after a period the pupal membranes of the sack, thorax, and
natatory legs are moulted, the cirri of the young Cirripede are
curled up, and its thorax is raised towards the orifice, and we have
the animal in its ordinary position, and perfect with the exception
of a few parts to be further developed or modified. For, instead of
calcareous valves, we have at this period only the so-called primordial
valves, composed of chitine; and in the case of _Lepas australis_,
some minute spines and some coloured marks on the peduncle, which soon
disappear. The muscles, which enter the three terminal segments of the
antennæ in the pupa, have to be absorbed and converted into ligamentous
threads. In Lepas, the labrum has to become bullate; and the cæca have
to increase in number round the upper end of the stomach, and their
dark colour and that of the whole alimentary canal has to disappear or
be much weakened. The filamentary appendages at the bases of the cirri,
which generally contain some of the testes, have to be developed.
The probosciformed penis, which at first equals only the pedicels of
the posterior cirri in length, and is apparently imperforate, has to
increase greatly in length. The testes and vesiculæ seminales have to
be formed. And lastly, and this is a more important point, the two
gut-formed cement-glands (or incipient ovaria, _t_, fig. 2, Pl. 30)
which, at the period of the moulting of the carapace and eye-apodemes,
and when the whole animal was upturned, came to occupy, together
with the cement-ducts (_t′_), their normal position, _i. e._ nearly
parallel to the sternal surface, now undergo further changes. Their
upper and posterior ends lying near the cæca of the stomach, increase
in size, but retain nearly the same character, and thus form the two
true ovaria; their middle parts become emptied of their cellular
contents, and are converted into the two simple ovarian tubes; and
their lower ends branch out, inosculate, and form the inextricable mass
of ovarian tubes and cæca. The points of junction on each side between
the two cement-ducts and the newly branched ovarian tubes, become now
developed into the two cement-glands. The cement-ducts, which continue
throughout life growing, either still enter the old antennæ and there
pour out the cement-tissue, or they pour it out through special
orifices formed for this purpose in the lower part of the peduncle. The
changes, supervening during the metamorphosis, in the ovaria and in
the cementing apparatus, here described, I have no doubt are general
throughout the Order.
I have above alluded to the _primordial valves_; these are beautiful
objects when viewed under a high power: they are composed of chitine
without a trace of calcareous matter, but prefigure in shape, size,
and direction of growth, the shelly valves soon to be formed under
and round them. They are composed of an outer membrane, with its
margins separated by yellow thickened rims from the membrane uniting
the several primordial valves together; and this outer membrane is
underlaid by a single layer of generally hexagonal, thickish cells (Pl.
30, fig. 3 _a_), varying from 1 to 2/6000th of an inch in diameter.
These cells seem to contain a nucleus; and they are at first separated
from each other by clear interspaces. If a specimen be taken, only
a little before the formation of the calcareous valves, one or more
layers of membrane, marked by an hexagonal reticulation, can be
separated from the lower surface of the main hexagonal network. It
is a singular fact, that in those genera in which there are several
valves, the primordial valves occur only on five, namely, on the two
scuta, two terga, and the carina; and these are the most persistent
valves in the several genera. The other valves are prefigured only by
brownish membrane, without the hexagonal tissue. In the mature _Lepas_,
the membrane connecting the several shelly valves is not moulted, but
disintegrates; in the primordial valves, however, which stand far
separate from each other, this membrane is moulted; and immediately
after the first moult, the first layer of shell appears under and
a little way beyond each primordial valve; shelly matter likewise
appears, at least in some cases, between the cells of the hexagonal
tissue. The young shelly valves are connected together, at each
successive moult, by narrower strips of membrane, till, in the case of
Lepas, the valves when mature come to touch each other (Lepadidæ, Pl.
1, fig. 5). The primordial valves are often preserved for a long time
on the umbones, or centres of growth of the five valves, on which they
occur, in the same manner as the larva-shell is sometimes preserved on
the apex of certain spiral molluscs. Had not Cirripedes gone through
so many and such complicated metamorphoses, this last state, when
furnished only with primordial valves and with several internal organs
only partially or not at all developed, would have deserved to have
ranked as a special stage, and not as merely subordinate to the last or
pupal condition.