a single row; where the number exceeds five, several of the teeth are
small; the inferior angle is generally pectinated with fine spines; in
Lithotrya (fig. 2), the interspaces between the teeth are also
pectinated. In the same individual there is not unfrequently one tooth,
more or less, on opposite sides of the mouth. Internally, the mandibles
are furnished on their outer and inner sides with several ligamentous
apodemes, in Lithotrya roughened with points (Pl. X, fig. 2), for the
attachment of the muscles; of these (fig. 1), there is a chief depressor
and elevator, attached at their lower ends to near the basal fold of the
mouth, and a lateral muscle, attached to the broad basal end of the
palpi, and serving, apparently, to oppose the edge of mandible to
mandible. The _Maxillæ_ in the different genera (Pl. X, figs. 9 to 15)
differ considerably in outline; they are generally about half the size
of the mandibles; at the upper corner, there are always two or three
spines larger than the others, and often separated from them by a notch;
the rest of the spinose edge is straight, or irregular, or step-formed,
or with the lowest part projecting, or with one or two narrow
prominences bearing fine spines. All these spines, quite differently
from the teeth of the mandibles, are articulated on the edge of the
organ, and stand in a double row. At a point corresponding with the
upper articulation of the mandibles, a long, thin, narrow, rigid
apodeme, projects inwards (fig. 10), and running down nearly parallel
to the thin, outer, flexible membrane of the mouth, is attached to the
corium, and thus serves as a support to the whole organ. This apodeme is
embedded in muscles (Pl. X, fig. 10); there are other large muscles
attached to the inner side of the organ, and again others running
laterally towards the mandibles. The apodeme, of course, is moulted with
the integuments of the mouth. The _Outer Maxillæ_ (Pl. X, figs. 16, 17)
serve as a lower lip; they are thicker than the other trophi; they have
their inner surfaces clothed with spines, sometimes divided into an
upper and lower group, and occasionally separated by a deep notch: there
are often long bristles outside. They are furnished with at least two
muscles; in sessile Cirripedes I have seen that they are capable of a
rapid to and fro movement, and I have no doubt that their function is to
brush any small creature, caught by the cirri, towards the maxillæ,
which are well adapted to aid in securing the prey, and to hand it over
to the mandibles, by them to be forced down the oesophagus. On the
exterior face of the outer maxillæ, above a trace of an upper
articulation, either two small orifices or two large tubular projections
can always be discovered; and these, as will presently be mentioned, I
believe to be olfactory organs.
_Cirri._--The five posterior pair are seated close to each other and
equidistant; the first pair is generally seated at a little distance,
and sometimes at a considerable distance from the second pair. The first
pair is the shortest; the others, proceeding backwards, increase
gradually in length. The rami of each pair are either equal in length or
slightly unequal: those of the first pair are oftenest unequal. The
number of segments in the posterior cirri is sometimes very great; in
one species of Alepas, there were above sixty segments in one ramus, the
other ramus being in this unique case (Pl. X, fig. 28) small and
rudimentary. The pedicels consist of two segments, a lower, longer, and
upper short one (fig. 18, _c_, _d_.) In the usual arrangement of the
spines on the segments of the three posterior pair of cirri, there are
(figs. 26, 27) from three to six pair of long spines on the anterior
face, with generally some minute spines (occasionally forming a tuft)
intermediate between them: on the dorsal surface, in the uppermost part
of each segment, there is a tuft of short spines generally mingled with
some longer, finer ones: on the inner side of each segment, on the upper
rim, there are generally a few extremely minute and short spines. From
the increase of these latter and of the intermediate spines, the
antero-lateral faces of the segments of the first cirrus, and of the
lower segments of the anterior ramus of the second cirrus (Pl. X, fig.
25), are almost always thickly paved with brush-like masses of spines.
The lower segments of the anterior ramus of the third cirrus is
generally, though not always, thus paved: these paved segments are much
broader than the others. The posterior rami of the second and third
cirri are often in some slight degree paved, though in other cases they
resemble the three posterior pair of cirri. The two segments of the
pedicels have bristles on their anterior faces, essentially arranged on
the same plan as on the segments of the rami: the bristles are generally
not so symmetrically arranged on the pedicels of the second and third
cirri, as on the three posterior pair. There are some exceptions to the
foregoing general rules: in the posterior cirri of _Alepas cornuta_,
there is only one pair of long spines to each segment (fig. 28); in
_Dichelaspis Lowei_, there are eight pair; in _Lepas fascicularis_, in
old specimens, the segments are paved with a triangular brush of spines;
the upper segments in _Pæcilasma eburnea_ support small oblong brushes;
and, lastly, in _Pæcilasma fissa_ (fig. 29), and _crassa_, the spines
form a single circle round each segment, interrupted on the two sides.
These spines are often doubly serrated or plumose: many of them on the
protuberant segments of the first three pair of cirri, are sometimes
coarsely and doubly pectinated.
_Caudal Appendages._--These are present (Pl. X, figs. 18 to 24) seated
on each side of the anus, in all the genera, except in Conchoderma,
Anelasma, and _Scalpellum villosum_; they consist of a very small
single segment, destitute of spines in Lepas, and spinose in Pæcilasma,
Dichelaspis, Oxynaspis, Scalpellum, and some species of Pollicipes; they
consist of several segments in Alepas, Ibla, Lithotrya, and in some
species of Pollicipes. In the latter genus, some species have their
caudal appendages multiarticulate, though so obscurely articulated, that
the passage (fig. 22) from several to one segment is seen to be easily
effected. When the appendage consists of many articulations, it is
generally about as long as the pedicel of the sixth cirrus; but in _Ibla
quadrivalvis_, it is four times as long. The segments are narrow,
slightly flattened, much tapering; each (fig. 24) is surmounted by a
ring of short spines, which are generally longest on the apex of the
terminal segment. I could never trace muscles into these appendages.
_Alimentary Canal._--The oesophagus is of considerable length: it is
formed of strong, transparent, much folded membrane, continuous with the
outer integuments, and moulted with them: it is surrounded by corium,
and as already stated, by numerous muscles: at its lower end it expands
into a bell, with the edges reflexed, and sometimes sinuous: this bell
lies within the stomach, and keeps the upper broad end expanded.
According to the less or greater distance of the mouth from the adductor
muscle, the oesophagus runs in a more or less parallel course to the
abdominal surface between the first and succeeding pairs of cirri, and
enters the stomach more or less obliquely. In Ibla alone, it passes
exteriorly to, and over the adductor scutorum muscle. The stomach lies
in a much curved, almost doubled course; it is often a little
constricted where most bent; it is broadest at the upper end, and here,
in Lepas and Conchoderma, there are some deep branching cæca; in the
latter of these two genera, the whole surface is, in addition, pitted in
transverse lines. The stomach is coated by small, opaque, pulpy,
slightly arborescent glands, believed to be hepatic; these are arranged
in longitudinal lines, in all the genera, except in Alepas, in which
they are transverse and reticulated: the whole stomach is thus coated.
There is, also, a coating of excessively delicate, longitudinal and
transverse muscles without striæ. The rectum varies in length, extending
inwards from the anus to between the bases of the second and fifth pair
of cirri: it is narrow, and formed of much folded transparent membrane,
resembling the oesophagus, continuous with the outer integuments, with
which it is periodically moulted. The anus is a small longitudinal slit,
in the triangular piece of membrane representing the abdomen, let in
between the last thoracic tergal arches, as already mentioned under the
head of the Metamorphoses; it lies almost between the caudal appendages,
and opens on the dorsal surface. Within the stomach, there can generally
be plainly seen, in accordance with the period of digestion when the
specimen was taken, a thin, yet strong, perfectly transparent epithelial
membrane, not exhibiting under the highest power of the microscope any
structure: it enters the branching cæca, and extends from the edge of
the bell of the oesophagus to the commencement of the closed rectum, and
consequently terminates in a point: it consists of chitine, like the
outer integuments of the animal, and by placing the whole body in
caustic potash, I have dissolved the outer coats of the stomach, and
seen the bag open at its upper end, perfectly preserved, floating in the
middle of the body, and full of the debris of the food. In most of the
specimens which I have examined, preserved in spirits of wine, this
epithelial lining was some little way distant and separate from the
coats of the stomach; and hence was thought by M. Martin St. Ange to be
a distinct organ, like the closed tube in certain Annelids.
Occasionally, I have seen one imperfect epithelial bag or tube within
another and later-formed one. Digestion seems to go on at the same rate
throughout the whole length of the stomach; if there be any difference,
the least digested portions lie in the lower and narrower part. The
prey, consisting generally of crustacea, infusoria, minute spiral
univalves, and often of the larvæ of Cirripedes, is not triturated: when
the nutritious juices have been absorbed, the rejectamenta are cast out
through the anus, all kept together in the epithelial bag, which is
excluded like a model of the whole stomach, with the exception of that
part coated by the bell of the oesophagus. I have sometimes thought that
the bag was formed so strong, for the sake of thus carrying out the
excrement entire, so as not to befoul the sack. I believe Lepas can
throw up food by its oesophagus; at least, I found in one case, many
_half-digested_ small Crustaceans in the sack, and others of the same
kind in the stomach.
_Circulatory System._--I can add hardly anything to what little has been
given by M. Martin St. Ange: like others, I have failed, as yet, in
discovering a heart. The whole body is permeated by channels, which have
not any proper coat: there is one main channel along the ventral surface
of the thorax, dividing and surrounding the mouth, and giving out
branches which enter the inner of the two channels in each cirrus: as
Burmeister has shown, there are also two channels in the penis. There
are two dorso-lateral channels in the prosoma, which are in direct
connection with the great main channel, running down the rostral (_i.
e._, ventral) side of the peduncle. This latter main channel branches
out in the lower part, and transmits the fluid through the ovarian
tubes, whence, I believe, it flows upwards and round the sack,
re-entering the body near the sides of the adductor scutorum muscle. The
main rostral channel (or artery?) in the uppermost part of the peduncle,
has a depending curtain, which, I think, must act as a valve, so as to
prevent the circulating fluid regurgitating into the animal’s body
during the contractions of the peduncle.
_Nervous System and Organs of Sense._--In most of the genera, there are
six _main_ ganglia, namely, the supra-oesophageal, and five thoracic
ganglia; but in _Pollicipes mitella_ there are only four thoracic
ganglia. Of these, the first thoracic or infra-oesophageal ganglion is
considerably the largest and most massive; it is squarish, or oval, or
heart-shaped; it presents no trace of being formed by the union of two
lateral ganglia. Two great nerves spring from its under side (A),
represented in the woodcut on page 49, by dotted lines, and run straight
down amongst the viscera in the prosoma: these nerves are about as large
as those forming the collar and those running to the second ganglion;
hence, six great nerves meet here, two in front, two behind, and two on
the under side. At the anterior end, over the junction with the collar
chord, three equal-sized nerves rise on each side, with a fourth,
smaller one, outside; these go to the trophi and to the two olfactory
sacks. At the posterior end, on each side, a pair of nerves branch out
rectangularly, one of which (_a_,) goes to the first cirrus, and there
divides into two branches; of these, the upper runs up the cirrus, and
the lower one downwards. The other nerve (_b_), proceeding on each side
from this first thoracic ganglion, runs to the muscles beneath the basal
articulation of the first cirrus. The collar surrounding the oesophagus
is generally very long, sometimes equalling the whole thoracic chord; at
a middle point, a small branch is sent off, and at the anterior end
(_e_, _e_), close to the supra-oesophageal ganglia, double or treble
fine branches run to the true ovaria, lying close to the upper end of
the stomach. The four (or only three) other thoracic ganglia, when
viewed as transparent bodies, are seen to be solid; but in some of the
genera, as in Conchoderma, the outline plainly shows, that each consists
of a lateral pair fused together. The second thoracic ganglion (B) is
rather small; it is either close to the first, as in _Pollicipes
mitella_ and _Lepas fascicularis_, or far distant, as in Ibla. The third
(C) and fourth are of about the same size with the second: these three
ganglia send large branches to the second, third, and fourth pair of
cirri: other minute branches spring from their under sides, and from the
intermediate double chords. The fifth ganglion is larger and longer than
the three preceding ones, and gives off nerves to the fifth and sixth
pair of cirri; it is clearly formed by the union of the fifth, with what
ought to have formed a sixth ganglion. The two nerves going to the sixth
cirrus give off on their inner sides, each a great branch to the penis.
In _Pollicipes mitella_, in which there are only four instead of five
thoracic ganglia, it is evident from the outline and position of the
nerves going to the fourth pair of cirri, that the fourth ganglion is
fused into the fifth, itself, as we have just seen, normally composed of
two consecutive ganglia. In this Pollicipes there is other evidence of
concentration in the nervous system, for none of the ganglia show signs
of being formed of lateral pairs; the second is close to the first; and
the abdominal double chord is in part separated by a mere cleft; lastly,
as we shall immediately see, the same remark is applicable to the
supra-oesophageal ganglia.
The latter (D) alone remain to be described; they present far more
diversity in shape than do the thoracic ganglia; they are almost always
seen in outline to be laterally distinct, and usually resemble two pears
with their tapering ends cut off and united; in a transverse line they
are as long as the infra-oesophageal ganglion, but are much less
massive. In _Lepas fascicularis_ (D), they are pear-shaped; in
_Pollicipes mitella_ they are globular, and separated by a third
globular ganglion, which I believe is the ophthalmic ganglion, presently
to be described; in _Pollicipes spinosus_, however, the ophthalmic
ganglion is, as usual, placed in advance of the supra-oesophageal
ganglion, which latter, in this one species, shows no sign of being
formed of a lateral pair fused together. In _Alepas cornuta_ the
supra-oesophageal ganglion consists of two quite distinct ganglia,
elongated in the longitudinal axis of the body, and separated from each
other by the whole width of the mouth; the chord which unites them is of
the same thickness as the rest of the collar. In all the genera, from
the front of each of the two supra-oesophageal ganglia, a pair of
nerves, (_f_, _f_,) united and together as large as the collar nerve,
rises, and can be traced running unbranched, in a nearly straight line,
for a length equalling the whole rest of the nervous chord, so as to
supply the peduncle and the inside of the capitulum or sack. At the
inner ends of these two same ganglia, from a central point where they
are united, a little central branch runs in front to the adductor
scutorum and other adjoining muscles; and still smaller fibrils run
behind to the oesophageal muscles.
[Illustration: Diagram of the anterior portion of the nervous system in
_Lepas fascicularis_. A. First thoracic or infra-oesophageal ganglion.
B. Second thoracic. C. Third thoracic ganglion. D. Supra-oesophageal
ganglion. E. The two ophthalmic ganglia. F. Double eye. _a_. Nerve going
to first cirrus; _b_, to the muscles below the first cirrus; _c_, to the
second cirrus; _d_, to the third; _e_, nerves running to the ovaria;
_f_, double nerves supplying the sack and peduncle.]
_Ophthalmic Ganglia and Eyes._--Owing to Professor Leidy’s[16] discovery
of eyes in a Balanus, I was led to look for them in the Lepadidæ.
Extending from the front of the two supra-oesophageal ganglia, two
chords may be seen in _Lepas fascicularis_ (of which a rude diagram is
here given), to run into two small, perfectly distinct oval ganglia
(E), which are not united by any transverse commissure. From the
opposite ends of these two ganglia smaller nerves run, and, bending
inwards at right angles, enter, beyond the middle, an elongated (F),
almost black, eye, composed of two eyes united together. Although in
outline the eye appears single, two lenses can be distinctly seen at the
end, directed upwards and towards the ganglia; two pigment-capsules can
also be distinguished; these are deep and cup-formed, and of a dark
reddish-purple. The following measurements will show the proportions of
the parts in a specimen of the _Lepas fascicularis_ having a capitulum
4/10ths of an inch in length.
Double eye { length 26/6000
{ width 13/6000
Diameter of single lens 6/6000
Ophthalmic ganglion { length 16/6000
{ breadth 11/6000
Supra-oesophageal ganglion, }
transverse or longest axis } 126/6000
of both together }
Supra-oesophageal ganglion, }
longitudinal axis of } 45/6000
Infra-oesophageal ganglion, }
transverse axis of } 120/6000
Infra-oesophageal ganglion, }
longitudinal axis of } 114/6000
[16] Proceedings of the Academy of Natural Sciences,
Philadelphia. No. i, vol. iv, Jan. 1848.
In _Conchoderma aurita_ the ophthalmic ganglia are much smaller, and
nearer to the supra-oesophageal ganglion, than in _L. fascicularis_. In
_Alepas cornuta_ the ophthalmic chords run towards each other from the
two distant and separate supra-oesophageal ganglia; and the ophthalmic
ganglia, (instead of being quite separate, as in _L. fascicularis_,) are
united by their front ends, and the two eyes instead of standing some
way in front, with nerves running to them, are embedded on the double
ophthalmic ganglion; the pigment-capsules here, also, have the shape of
mere saucers, and are joined back to back, with the two lenses
projecting far out of them. In neither sex of Ibla could I perceive that
the eye was double. In _Pollicipes spinosus_ the ophthalmic ganglion
stands in front of the single supra-oesophageal ganglion, and shows no
signs of being formed of a lateral pair; the eyes themselves, however,
differently from, in all the foregoing cases, are, though approximate,
quite distinct. In _Pollicipes mitella_ I did not see the eyes; but the
ophthalmic ganglion consists, as I believe, of a single globular one,
placed exactly between the two globular, supra-oesophageal ganglia, all
three being of nearly equal size. Professor Leidy does not mention the
ophthalmic ganglia; hence I infer that in Balanus, which is a more
highly organised Cirripede, they are fused into the supra-oesophageal
ganglion.
In all the genera, the double eye is seated deep within the body; it is
attached by fibrous tissue to the radiating muscles of the lowest part
of the oesophagus, and lies actually on the upper part of the stomach;
consequently, a ray of light, to reach the eye, has to pass through the
exterior membrane and underlying corium connecting the two scuta, and to
penetrate deeply into the body. In living sessile Cirripedes, vision
seems confined to the perception of the shadow of an object passing
between them and the light; they instantly perceived a hand passed
quickly at the distance of several feet between a candle and the basin
in which they were placed.
As the infra-oesophageal ganglion sends nerves to the trophi and to the
first pair of cirri, it must correspond to the segments, from the fourth
to the ninth inclusive, of the archetype crustacean. The state of the
supra-oesophageal and ophthalmic ganglia appears to me very interesting:
I do not believe that in any _mature_ ordinary crustacean, the first or
ophthalmic ganglion can be shown to be distinct from the two succeeding
ganglia, or to be itself composed of a pair laterally distinct. The
ganglia, corresponding with the second and third segments of the body,
which should normally support two pair of antennæ, are in the Lepadidæ
united together; but laterally they are generally distinct in outline,
and are actually separate in Alepas: the supra-oesophageal ganglion
shows also its double nature, by giving rise to a pair of large double
nerves, evidently corresponding with the two pair of antennular nerves
in ordinary crustaceans. The embryonic condition of the whole
supra-oesophageal portion of the nervous system in the Lepadidæ,
corresponds with the rudimentary state of the only organ of sense
supplied by it, namely, the eye, which in size and general appearance
has retrograded to the state in which it was in, during the first stage
of development of the larva;--I have used the term embryonic, because,
in the embryos of ordinary crustacea, all the ganglia are at first
longitudinally distinct, and laterally quite separate. The conclusion at
which we before arrived from studying the metamorphoses, namely, that
the whole peduncle and capitulum consisted of the first three segments
of the head, is beautifully supported by the structure of the nervous
system, in which these parts are seen to be supplied with nerves
exclusively from the supra-oesophageal ganglion: now in ordinary
crustacea the supra-oesophageal ganglion sends nerves to the eyes and
the two pair of antennæ corresponding, as is known by embryological
dissections, to the first three segments of the body. Moreover, it is
asserted that the carapace which covers the thorax in crustacea, is not
formed by the development of the first segment; and this, likewise, may
be inferred to be the case with the peduncle and capitulum in the
Lepadidæ, as the nerves of the ophthalmic ganglia go exclusively to the
eyes. Finally, I may remark that in Pollicipes, looking to the whole
nervous system, the state of concentration nearly equals that in certain
macrourous decapod crustaceans, for instance the _Astacus marinus_, of
which a figure is given by Milne Edwards.