_Cirripedia having a flexible peduncle, provided with muscles: scuta
and terga, when present, not furnished with depressor muscles: other
valves, when present, not united into an immoveable ring._
This Family has been fully treated of in my former volume, published
by the Ray Society,[141] and I should here only have alluded to its
existence, had it not been for the genus Alcippe, which differs in
so many important characters from the other members of the Lepadidæ,
that formerly I did not even suspect that it could belong to this
Family, and therefore deferred its examination. The genus Alcippe was
discovered, well described and illustrated, in 1849, by Mr. Hancock;
to whose very great kindness I am indebted for permission to dissect
and examine his entire stock of this truly remarkable Cirripede. In the
classification of the whole class I have not felt so much doubt, as
whether I ought to institute a family for the reception of this genus.
Alcippe differs from all other Cirripedes (putting on one side for the
instant, the males and complemental males of Ibla and Scalpellum) in
the very singular fact of being destitute of a rectum and anus;--in
the three segments of the thorax, which usually support the second,
third, and fourth pairs of cirri, being without any appendages;--in the
fifth and sixth pairs of cirri having their inner or posterior rami
metamorphosed into very singular roughened cushions or buttons, which
apparently serve to triturate the food;--in the caudal appendages being
muscular, and being used conjointly with the cirri;--and lastly, in
the pupa having a lesser number of segments in its abdomen and caudal
appendages than in (as far as I have seen) any other Cirripede. It will
be thought that these characters are amply sufficient to justify the
placing Alcippe in a separate family, more especially when the close
general resemblance in the animal's body in most of the other members
of the Balanidæ, Verrucidæ, and Lepadidæ, is borne in mind. On the
other hand, the males and complemental males of Scalpellum and Ibla
must indisputably be considered as members of the Lepadidæ; yet the
male of _Scalpellum vulgare_ and _ornatum_ has no stomach, anus, or
mouth, which is a far more abnormal structure than the absence only of
the anus in Alcippe: the cirri, also, in these same males, differ from
the ordinary cirripedial type decidedly more than in Alcippe. Again,
in the male of Ibla, all the cirri, excepting the fifth and sixth
pairs, are aborted, and these two pairs are usually only uniramous;
here, then, we have a decided resemblance to Alcippe. Hence, if we
might assume that the female Alcippe had partially assumed characters
confined to the males of the other genera, it would assuredly stand
amongst the Lepadidæ. Independently of this comparison with the
foregoing males, the affinities of Alcippe are so special to several
genera amongst the Lepadidæ, that it seems unnatural to force it out
of the position which it well occupies between Ibla and Anelasma, and
place it in another family by itself: thus, in being bisexual, and
in the general character of its very curious males, Alcippe shows an
affinity to Ibla and Scalpellum; and to the former of these genera it
is related in several particulars, such as in the body being lodged
within the peduncle, and in the structure of the larval antennæ, &c.:
to Anelasma and Alepas it is allied in the general character, and to a
certain extent in the muscles, of the capitulum; Anelasma, also, has
all its cirri to a certain degree rudimentary, and _Alepas cornuta_ has
the inner rami of the fifth and sixth pairs of cirri,--namely, the very
same rami which are so curiously modified in Alcippe,--small, destitute
of muscles, and functionless for their proper purpose: to Anelasma
and Lithotrya it is allied in the peculiarity of the lower end of the
peduncle becoming elongated by growth, and in being imbedded; and to
Lithotrya by its powers of excavation and manner of attachment. Now,
I believe it generally holds good that when a form is really distinct
from another group, its affinities are general, or only in a slight
degree special to the members of that group. Nor, indeed, can it be
asserted that Alcippe differs much more, somewhat more it certainly
does, from the other genera, than does Anelasma, with its more singular
mouth, spineless rudimentary cirri, and fimbriated peduncle; and I have
never regretted having included this genus amongst the Lepadidæ. Hence,
after much consideration, I have resolved to consider Alcippe as one of
the Lepadidæ, though so curiously modified,[142] and having characters
confined to the males of the other genera. Perhaps I have been in some
degree influenced by the difficulty of finding external characters by
which to separate Alcippe as a family from the other Lepadidæ.
[141] The fossil species have been described in a separate
Monograph published by the Palæontographical Society. Since
its publication, M. Bosquet has produced an excellent memoir,
containing descriptions, with the most beautiful illustrations, of
several new Cretacean species of Pollicipes and Scalpellum. The
memoir is entitled a 'Monographie des Crustacés Fossiles du Terrain
Crétacé, du D. de Limbourg.'
[142] Adrien de Jussieu, in his 'Memoir on the Malpighiaceæ,'
'Archives du Museum,' tom. 3, p. 86, when speaking of the
characters afforded by the degraded flowers, which in certain
genera are borne together with ordinary flowers, makes the
following observations bearing on the question here discussed,
viz., whether or not to include Alcippe amongst the Lepadidæ.
"Ces exemples peut-être aideront à comprendre comment à des
genres d'une organisation assez compliquée, viennent quelquefois
s'en rattacher d'autres d'une organisation beaucoup trop simple
en apparence, membres appauvris et dégradés d'une même famille,
qui lui appartiennent sans la représenter; comment le type,
s'y présente comme effacé, ne conservant plus pour se laisser
reconnaître que quelque trait isolé, mais caractéristique, dont la
valeur, essentiellement ordinale, peut être ainsi constatée." Under
the point of view, so strongly and admirably insisted on lately by
Milne Edwards ('Annales des Sciences Nat.,' 3d series, tom. 17), of
describing types without regarding whether the different members
blend together on their confines, perhaps Alcippe should be raised
to the rank of a Family: I feel quite unable to decide how properly
to act.
But we shall presently find, when we come to Cryptophialus, that all
the above difficulties, great as they are, are greatly enhanced,
for Cryptophialus is certainly allied in a very direct and curious
manner (in decided opposition to the remarks just made on special
affinities) to Alcippe, and yet in all the more important parts of its
organisation, and in its metamorphosis, it differs so fundamentally,
that I have felt myself obliged to form not merely a Family, but a
distinct Order for its reception.
_Genus_--ALCIPPE. Pl. 22, 23.
ALCIPPE. _Hancock._ Annals and Mag. of Nat. Hist., vol. 4, 1849, Pl.
8, 9.
_Fem.--Capitulum without valves, with the orifice spinose: peduncle
with the basal end added to during growth; its rostral surface
depressed and covered by a horny disc: capitulum and peduncle imbedded
in a cavity excavated in the shells of molluscs._
_Labrum very large, with a row of long hairs on each side: palpi
rudimentary: mandible one-toothed: second, third, and fourth cirri
absent: fifth and sixth cirri with the posterior ramus represented by a
button-like body: caudal appendages four jointed, muscular: anus none._
_Males,--several, adhering to the upper end of the horny disc of
the female: capitulum naked, transparent, elongated, with a small
orifice at the end: peduncle lobed, with the lower end extending far
beyond the pupal antennæ: eye, testis, and vesicula seminalis single;
probosciformed penis very long: mouth, stomach, thorax, abdomen, and
cirri none._
ALCIPPE LAMPAS, _Hancock_ ut suprà.
_Hab._--North-eastern shores of England, fifteen to twenty fathoms,
imbedded in dead shells of _Fusus antiquus_ and _Buccinum undatum_
(A. Hancock); south-eastern shores, off the Eddystone, Lighthouse (C.
S. Bate).
FEMALE. Pl. 22.
I may premise that after the sketch of the leading peculiarities of
Alcippe, and after the discussion on its affinities, just given under
the Family, I think it would be superfluous to institute a full generic
description, separately from the following detailed account of this
most anomalous cirripede.
_General Appearance._--The whole animal is from .2 to .3 of an inch in
length, of a soft texture, colourless or yellowish, and lives concealed
in a cavity of its own formation in the shells of certain Gasteropods.
This cavity communicates with the water by a narrow fissure-like
orifice (Pl. 22, fig. 4), broadest at the posterior end, where the
cirri are exserted; narrow, closed, and generally curved at the other
(_a_) end: the two sides of the fissure (_b_) are commonly bordered
by a calcareous inorganic deposit: the walls of the cavity are worn
so thin over the peduncle, at the narrow end of the fissure, that the
orange-coloured ovaria can generally be seen through the shell of the
mollusc, and hence there is here a distinct fan-shaped stain (fig. 3)
on the surface. The animal consists of a compressed capitulum, without
valves, and of a sort of peduncle depressed on its rostral face, and
covered with a broad, oval, thin, horny disc. We must remember that
in the Lepadidæ the peduncle does not essentially differ from the
capitulum, being only the flexible lower or anterior end of the animal,
and is separated from the capitulum only by shape, and generally by the
direction of the lines of growth. The disc, when most regular (fig.
1, H), lies in a plane at right angles to the sides of the capitulum,
and almost in a line with the orifice leading into the sack; but the
peduncle is often very irregular (fig. 2), and the disc comes even to
occupy a position nearly parallel to one or the other side of the
capitulum. On the carinal side, the capitulum is generally separated
from the peduncle by a rather deep fold (_f_, in the section fig. 5),
but this depends in some degree upon the state of distension of the
mass of ovarian cæca. I have given a drawing, fig. 1 (partly taken
from Mr. Hancock), of a very regular individual, and of an extremely
distorted specimen (fig. 2). The distortion, I believe, is generally
caused by the animal, during its excavation, breaking into some old
cavity.
_External Structure._--The orifice leading into the sack is about
one third of the total length of the animal: its edges or lips are
thickened, horny, and brownish: at the lower end, exactly where the
orifice ends, the lips are formed, from being deeply notched, into
two sharp projections (_a_, figs. 1, 5, 6), unlike anything occurring
in any other Cirripede. The external membrane (_c_, fig. 6) of the
lip supports an irregular but nearly straight band of sharp, thick
spines of chitine, about 1/1000th of an inch in length, together with
a few hairs: at the carinal or upper end of the orifice the spines are
largest and most numerous; at the other and lower end, they decrease in
size; and on the two projections (_a_, fig. 6), and on the adjoining
parts of the external membrane, they graduate into the small dentated
points which cover the whole surface of the animal. The inner tunic of
the sack (_b_), on each side along the upper half of the orifice, is
remarkable from having a moderately broad, curved band of short, sharp
spines, not quite so thick as those on the external surface, closely
adpressed together and pointing upwards, like the javelins of an
ancient phalanx, thus probably preventing the ingress of any intruding
animal. This band of spines curves at the upper end, conformably with
the shape of the orifice. The inner tunic of the sack in this upper
part is yellowish, and, what is very unusual, is thicker than the
external membrane. A little way down, within the orifice, and more
especially in front of an elegant row of hairs on the two sides of the
great labrum, there is a band of very fine but stiff hairs (5/1000ths
of an inch in length), pointing upwards, and making together with those
on the labrum a hedge, barring ingress into the sack.
The external membrane over the whole animal, excepting the horny
disc which covers the rostral face of the peduncle, is very thin and
transparent; it is periodically and often moulted, as may be inferred
from the many old lines of junction round the edges of the horny disc:
it is irregularly and pretty thickly (but not so thickly as in fig.
7) studded with star-headed, minute points, from 2 to 5/10,000ths
of an inch in diameter, composed of hard chitine, seated on a short
footstall, and this on a circular, yellowish, slightly thickened disc
of the general investing membrane, appearing like a halo surrounding
each little point. These points are directed obliquely upwards. There
are none on the horny disc, though particularly numerous close to its
margin. Their state varies much: just after a moult, when newly formed,
the spines are regularly star-headed, with quite sharp rays, from two
to six in number, with some of them occasionally bifid; but these
points or rays soon become blunted, and ultimately half the star is
worn away, so that the appearance then presented is that of a crescent
with a few blunt points on its convex side. At each exuviation, the
thickened membrane of the orifice with its strong external spines (the
condition of which also varies according to the period elapsed since
the last moult), and of course the whole internal tunic of the sack,
with its spines and hairs, are all moulted, together with the external
membrane and the little star-shaped points. In most specimens a barely
distinguishable band or bar of yellowish, slightly thickened membrane,
runs from the points (_a_), at the lower end of the orifice, for some
way obliquely downwards; and at the lower end of this bar the weak
adductor scutorum muscle (having transverse striæ) is attached. This
bar is often strengthened by a prominent external fold of membrane, but
yet it is so flexible, and as it is united only to the lower end of the
orifice, I can hardly believe that it can, by means of the adductor
muscle attached to its opposite extremity, have much power in closing
the orifice. I believe that this muscle acts simply in narrowing the
whole animal, so as to favour its movement within the cavity in which
it is imbedded. Owing to this position of the adductor muscle, and its
consequent little power in closing the orifice, we can understand the
necessity for the defence afforded by the bands of spines and hairs on
the inner tunic of the sack and on the labrum, which do not occur in
other Cirripedes.
_Horny disc._--The general shape of the disc, its irregularity and
position, have been already described. It never extends, as remarked
by Mr. Hancock, to the extreme lower point of the peduncle; upwards it
reaches to a little below the lower end of the orifice. It consists of
successive layers of membrane, either moderately thick and opaque, or
only a little thicker than the general membrane of the body, but never
furnished with the little sharp points; it increases in size, in like
manner as the calcareous valves of other Cirripedes, the undermost and
last formed layer extending beyond the others, with its edge united,
till the next exuviation, to the general membrane of the body. The disc
is attached, at its upper end, apparently in the usual way, by cement,
to the roof of the cavity of the shell in which it is imbedded; but
the lower parts of the disc are also slightly and partially attached,
chiefly along the lines of growth or exuviation; and this, I suspect,
is effected by an inorganic calcareous deposit; anyhow I could not
perceive here any cement or cement-ducts. Beyond the circumference of
the disc the whole animal lies free in its cavity. The lines of growth
in the middle part of the disc are generally obliterated by the decay
of the older and outer layers. These lines, though of course ordinarily
conformable with the general outline of the disc, are not always so,
for the disc sometimes becomes during growth slightly changed in form,
and the animal, consequently, slightly changed in position; sometimes
either one or the other side or the upper end of the disc is left
deserted by the new layers of the growing disc; these being formed on
the deserted side of less size or extension, instead of larger size, as
they normally should be all round the disc.
The upper end of the disc is always produced into a projection of not
regular shape, but generally hollowed out or embayed in front (fig.
1), and almost always hollowed out on the two sides. This projection
stands directly over the adductor muscle (_b_ in fig. 5), and on the
exterior surface is generally convex, being concave on the under
side for the attachment of several muscles presently to be described.
The horny layers are in this part usually thicker than elsewhere. The
disc is thus upwardly produced, owing apparently to the fissure which
leads into the cavity of the shell of the mollusc becoming, during the
process of excavation, considerably longer than is necessary,--that
is longer than the orifice leading into the sack; and consequently,
for the protection of the imbedded animal, the lower and narrow end
of the fissure is closed on its under side by this upward production
of the horny disc, formed of layers of membrane of unusual thickness.
A deposition, also, of lime, hereafter to be mentioned, gives further
protection.
In the bays on each side of the upward production of the horny disc,
and likewise a little lower down on its edges, and therefore somewhat
protected by lying within the narrow, pointed, lower end of the fissure
in the shell of the mollusc, the short-lived Males (Pl. 22, fig. 1,
_m_) are attached often in groups of two, three, or more together.
It may be asked to what part, in other Cirripedes, does the horny
disc answer? Not considering the upward prolongation, which has been
developed for a special purpose, the disc is irregularly circular,--is
added to all round,--serves for the attachment of the whole animal
to the supporting surface,--is covered on the under surface by a
conformable and parallel mass of ovarian cæca, and the latter by
the inner tunic of the sack; therefore in every character, and in
its relation to the other parts of the animal, the disc answers to
the end of the peduncle, or to the basal cup in Lithotrya, or still
more closely to the basis in sessile cirripedes, with the important
exception that it lies in a line with the longitudinal axis of the
whole animal instead of at right angles to this axis. We know that all
ordinary cirripedes become first permanently attached in their pupal
state by their antennæ, which are seated on the ventral or rostral
surface, near to the anterior end of the body; and that from the young
cirripede, after the act of metamorphosis, being turned vertically
upwards, and from the extreme anterior, now lower, end of the body not
being rapidly developed, the surface cemented down, or the basis,
encroaches almost equally on the dorsal, lateral, and ventral surfaces.
But if we were to suppose the extreme anterior point of the body to be
rapidly developed, the surface of attachment or basis, without it grew
still more rapidly, could not possibly reach the dorsal surface, and
would, consequently, be confined to the ventral or rostral surface. I
have not seen the young of the ordinary or female Alcippe soon after
its metamorphosis, but in the male the development of the extreme
anterior end of the body is extraordinarily rapid, and from analogy we
may fairly conclude that this is likewise the case with the female.
Hence I believe that the horny disc answers to the cemented down, lower
end of the peduncle, in other members of the Lepadidæ, and to the basis
in the Balanidæ, and that it is confined to the ventral or rostral
surface, owing to the anterior or lower end of the body having been
rapidly developed. To make all the parts, internal and external, of
Alcippe, correspond with those of other cirripedes, the main circular
part of the horny disc must be turned up at nearly right angles to its
present position (the dorsal or carinal integuments, to the right-hand
in fig. 5, being shortened), and then we should have a peduncle,
certainly very short and broad, but holding its proper relative
position.[143]
[143] In the genus Lithotrya, as long as the animal continues to
bore into the rock, the calcareous discs by which it is attached in
its cavity, stand, as in Alcippe (Pl. 8, fig. 2, 2_a′_, Darwin's
'Monograph on the Lepadidæ'), parallel to the longitudinal axis,
but as soon as the animal ceases to bore, and the discs become
converted into a cup, they occupy a normal position at right angles
to the peduncle. According to Reinhardt, these discs, in Lithotrya,
are situated on the carinal or dorsal surface of the peduncle, at
which statement I now feel considerable surprise, as undoubtedly
the pupa must first permanently attach itself by its prehensile
antennæ on its ventral or rostral surface. In Anelasma I failed to
discover any cement or cement-ducts; but I am now strongly inclined
to believe, considering that the extreme lower or anterior end goes
on growing, that the surface of attachment will be found to occur,
as in Alcippe, on the rostral surface, a little way below the
orifice.
_Sack and its Muscles._--I have already described the curious phalanx
of spines, the long fine hairs, and thickened condition of the inner
tunic of the sack along the sides of the orifice. This inner tunic
is a reflexion from that enveloping the body of the animal, in the
usual manner, as may be seen in the section (Pl. 22, fig. 5). Between
the external membrane and the inner tunic of the sack (_e_), there
is of course the usual double fold of corium, these two folds being
united by minute, transverse, ligamentous fibres, branched at the two
ends, as in other Lepadidæ. Imbedded in the corium there are numerous,
longitudinal, striæ-less muscles, which do not run quite up to the
orifice, but to an oblique line beneath it. Externally to these muscles
there are, as in the other Lepadidæ, fine transverse muscles, confined
to the middle part of the animal, and running from the carinal margin
more than half way round both sides. Attached to the upper notched or
folded end of the orifice (_g_, fig. 5, above the upper ends of the
longitudinal muscles), there is a fan of rather strong, striæ-less
muscles, expanding downwards, with their lower extremities attached to
the outer membrane of the capitulum; these muscles apparently serve to
open the orifice: there is a somewhat analogous muscle in Lithotrya,
but in no other member of the Family: in Cryptophialus, however, there
is a closely similar muscle. Owing to the action of these several
muscles, the tissues forming the capitulum and peduncle are, according
to Mr. Hancock, highly contractile.
I have stated that the under surface of the upper produced end of
the horny disc is concave, and serves for the attachment of several
muscles. Of these some run to the basal margin of the great labrum, and
no doubt, as usual, move the whole mouth; others, as usual, run to the
skin between the labrum and the lower end of the orifice,--_i. e._ in
fig. 5, between the lower end of the row of fine hairs (see fig. 11),
which shows where the basal margin of the labrum is situated, and the
lower side of the point (_a_), where the orifice terminates: others
run obliquely on both sides towards the point of attachment of the
small adductor scutorum muscle (_b_, fig. 5): others, of considerable
strength, and these are more peculiar, run and are attached to the
lower end of the orifice (_a_), and serve apparently to draw up the
orifice from within the fissure-like cavity, in which it lies lodged:
others, again, extend transversely on both sides, close beneath the
inner tunic of the sack, a little beyond the line whence the ovigerous
fræna or branchiæ arise. These transverse muscles lie within the
longitudinal muscles, and therefore are quite different from the
exterior transverse muscles, which are situated more towards the
carinal portion of the peduncle and capitulum, and which are common to
most Lepadidæ. The internal transverse muscles, and those running to
the lower end of the orifice, are peculiar, but we shall hereafter meet
with them even more developed in Cryptophialus.
Along the medial carinal line there is, between the two layers of
corium, the usual circulatory channel. On each side of this line, on
the inside of the sack, there are generally some slight irregular
swellings, and sometimes a large extent of the inner surface is
irregularly carunculated with little knobs. The sack (_e_ in fig. 5)
extends down almost to the basal point of the peduncle (_d_), more
especially when the ovarian cæca are not gorged with ova.
_Branchiæ, or ovigerous Fræna._--Within the sack, on each side of the
body, rising not far from the ends of the adductor muscle (_b_), there
is a large fillet or fold; the two occupy so exactly the position
of the ovigerous fræna that I cannot doubt such is their nature,
though, as happens in the case of some species of Pollicipes, they
are destitute of their proper glands, and so do not serve for the
attachment of the ovigerous lamellæ; this attachment probably is not
required, owing to the protected situation which the lamellæ hold in
the sack, under the animal's body, and over the ovarian cæca. From
the unusually large size of these so-called fræna, I cannot doubt
that they serve as branchiæ, equally well with the plicated folds of
membrane, believed to be homologous with the fræna, in the Balanidæ,
which have by every one been considered as branchiæ. The fræna are
broad and truncated at their upper ends; their margins are sinuous, and
their outer surfaces papillose; they run longitudinally down the sack,
narrowing as they extend, almost to the basal point of the peduncle,
and hence are of considerable length; they are hidden in the section
(fig. 5) by the medial, somewhat protuberant mass (_c_) of ovarian
cæca, and partly by the (_i_) prosoma.
_Body._--The body is constructed on the usual type, and indeed does
not differ greatly from that of Ibla. The labrum is very large, its
lower or basal margin is separated by an unusual space (capable of
being contracted or folded) from the lower end of the orifice of the
sack; hence the labrum and whole mouth is placed quite remarkably
near the upper (or carinal) end of the orifice. This upper end of the
orifice, I may remind the reader, is homologically the posterior end
of the general covering or carapace, and all that portion of the whole
animal (as the sectional figure, 5, stands) below the lower margin of
the labrum, on the rostral or ventral surface, is formed by the three
anterior segments of the head. The main part of the body, carrying the
mouth, is formed by the great development of that segment of the thorax
which bears the first pair of cirri (_h_), here closely adpressed, as
usual, to the sides of the mouth. The lower portion of this segment
forms the prosoma (_i_), and has the characteristic outline, but is not
much developed. On each side of the prosoma an oval space of membrane
is yellowish and is thickened, and so gives support to this part of
the body. The five succeeding thoracic segments, which ought to carry
the five succeeding and posterior pairs of cirri, are together of very
small size (as in Ibla), in comparison with either the prosoma, or
the whole anterior part of the animal. The segment (_k_) which should
have borne the second pair of cirri, is considerably longer than the
following segments, and is at the same time less distinct, owing to an
oval convex shield of thickened membrane on the sides, not extending
the whole length of the segment, thus causing two transverse creases,
which, when the thorax is contracted, appear like two additional
segments. Had this segment borne cirri, they would have stood, as in
Ibla, at a considerable distance from the first pair. The segments
(_l_, _m_) which should have borne the third and fourth pairs of cirri
are like each other, except that the former is rather the longest.
The membrane covering all the thorax is surprisingly thin; and at the
articulations, which are straight and transverse, is deeply folded, so
that the thorax must be highly extensible, to a degree which I have
not seen equalled in any Cirripede except in the males of _Scalpellum
vulgare_ and _ornatum_. The thorax is represented as somewhat extended
in fig. 5. This part of the thorax is amply furnished with striated and
striæ-less muscles for its retraction and protrusion, and for lateral
movements. The segment (_m_) which should have borne the fourth pair
of cirri, at first sight falsely appears like the terminal segment
of the thorax: in one monstrous specimen it bore a single cirrus,
showing (if there had been any doubt) that it was a true segment.
The three terminal pairs of articulated appendages, form together a
brush; they consist of the fifth and sixth pairs of cirri and of the
caudal appendages: my reasons for considering the last-named organs
as of caudal origin will be given hereafter. A moderately careful
inspection, especially of the ventral surface, will show that the fifth
pair of cirri are borne on a small segment (_n_, fig. 5, but plainer
in fig. 13), which is quite distinct from, but partially concealed
by, that which ought to bear (and did bear in the monstrous case) the
fourth pair of cirri: this segment is oblique, and cannot be traced
distinctly all round the dorsal surface. The segment (_o_, fig. 13)
bearing the sixth pair is much less distinct, and can only be seen by
a longitudinal section, or when the cirri are a little separated, but
it certainly exists,[144] as is likewise shown by the presence of small
apodemes dipping in amongst the muscles, between this and the last
segment. The posterior or caudal appendages are closely approximated;
they are not separated by any fold from the sixth thoracic segment;
but _appear_ as if they were articulated on the dorsal surface of the
sixth pair of cirri, in exactly the manner usual in the other Lepadidæ.
The segments bearing the fifth and sixth pairs of cirri are highly
oblique to the preceding segments, and consequently the cirri, which
they support, instead of projecting inwards, lie like a brush in a
line with the longitudinal axis of the main part of the thorax. The
membrane forming the two small oblique terminal segments of the thorax
is strengthened by irregularly shaped plates of thicker and yellowish
membrane.
[144] In the middle, the fold is slightly prominent and pointed,
and being most finely villose, I for some time looked at this
projection as a rudiment of the probosciformed penis.
_Mouth._--The mouth is constructed on the strictly normal type of the
Family, but is peculiar in every part; it is remarkable from being
situated so near the upper (or posterior) end of the capitulum, this
being caused by the great length of the labrum, and of the space of
body between the latter and the lower end of the orifice. The _labrum_
is a very singular part of the mouth from its vast size and outline:
in fig. 8, we have a front view of the mouth, of which the whole upper
pointed part consists of the labrum, and _h h_ is the first pair of
cirri; in fig. 11, we have a lateral view of the labrum, with the
surrounding thin membrane of the body, _a_--_a_, still adhering to its
edges; _h_ is the first cirrus on the near side; _m_ the mandible, a
little distorted in order to show its tooth, marking the position of
the transverse crest of the labrum and of the orifice of the œsophagus;
_b b_ is the medial longitudinal ridge of the labrum. In the Balaninæ
the labrum forms a mere rim to the back of the mouth, consisting of
an inner fold running down the œsophagus, and of an outer fold, both
close together: in the Lepadidæ the folds are separated, the outer one
being swollen or bullate; and here this structure is carried to even a
greater extreme than in Ibla and its allies. The distance between the
transverse crest over the œsophagus and the blunt projecting point on
the summit of the medial ridge, _b b_, equals twice the longitudinal
diameter of the rest of the mouth. The lower margin on each side of
the labrum is produced into two projections (fig. 11), the longer
one curling round to a point beneath the jaws, with its extremity
imbedded as an apodeme. Another very peculiar character in the labrum,
prominently noticed by Mr. Hancock, is caused by a longitudinal row,
on each side, of closely approximate, long, very finely pointed hairs,
which, as already stated, are fronted on the opposed internal surface
of the sack by an irregular band of still finer hairs. The surface
of the labrum is partially covered by minute toothed scales, and
these, seen on the longitudinal medial ridge, _b b_, give it a finely
denticulated structure. At each end of the transverse crest which
overhangs the œsophagus, there is a knob, such as occurs on the labrum
of every Cirripede. United to these two knobs, which are formed of
thick and yellowish membrane, and springing from the adjoining sides of
the mandibles, there are two swellings formed of thin membrane (fig.
8), which occupy the exact position of the palpi, and may be considered
as these organs in a rudimentary condition and destitute of bristles.
The _mandibles_ are simpler than in any other Cirripede; they are
minute; they consist of an oblong plate, with only one very strong
tooth at the upper end: the face towards the labrum is swollen: beneath
the upper free part there is a small, sub-triangular piece of thickened
membrane, let in and forming part of the general outer surface of
the mouth, and representing the large square plate found in other
Cirripedes. The _maxillæ_ (fig. 15) are smaller but broader than the
mandibles; they have an upper tooth and a smaller lower one, lying not
quite in the same plane with the upper one, but nearer the mandibles.
The apodeme (fig. 8, 15) is of remarkable length, extending beneath
the basal fold of the mouth: it does not arise from the ridge or outer
edge of the maxilla, but a little on one side, from the face directed
towards the mandible. Between maxillæ and mandibles there is a very
singular prominent fold of membrane (fig. 8, 15), which resembles,
but probably falsely, the supposed rudimentary palpus attached to the
mandible. Altogether the maxillæ differ considerably from the same
part in other Cirripedes. In structure they seem adapted to assume
the function of mandibles; but they do not stand directly over the
œsophagus. The _outer maxillæ_ (fig. 8) appear like a minute, deeply
notched lower lip: each consists of a simple, oblong, rounded plate,
with a few small bristles at its upper end. The basal fold of the
mouth in front, beneath the outer maxillæ is distinct, and runs in a
line with the basal articulation of the first pair of cirri. In the
rudimentary palpi, minute and little developed outer maxillæ; and in
the inner maxillæ, taking the function of the mandibles, the mouth of
Alcippe presents some resemblance with that of Anelasma.
_Cirri._--These consist of the first, fifth, and sixth pairs: the other
pairs are absent, except in one monstrous specimen, in which there was
a fourth cirrus quite like the fifth. _First pair_, fig. 14, these are
seated on each side of the mouth in the usual position. They are formed
of very thin and flexible membrane. The pedicel, as usual, consists of
two segments, the upper one is short and not very distinct; but when
viewed on the inner side can be seen to have the ordinary structure:
both segments are destitute of bristles. There are two short rami,
being about one third of the length of the pedicel: they are directed
either in the line of the pedicel, or more commonly posteriorly, that
is towards the other cirri, and therefore in an unusual direction. The
anterior ramus is generally rather longer and thinner (as is commonly
the case with other Cirripedes) than the posterior ramus; but there is
some variation in this respect. On neither ramus is there any trace
of the ordinary articulations: both are thickly clothed with fine
bristles, which are singular from being thickened in their lower parts,
and plumose, like a feather. These cirri have some resemblance, as
remarked by Mr. Hancock, to a pair of pincers; but they cannot act as
such; they serve, I believe, as brushes. Delicate muscles, transversely
striated, enter and are attached within both rami and within both
segments of the pedicel, on the usual type, showing that these organs
(if there had been any doubt) are truly cirri.
The _fifth_ and _sixth pairs_ of cirri (fig. 13, _n′_, _o′_) are almost
exactly alike: they are of very small size: each cirrus consists of
four segments: the lower or basal segment is broad, with a few minute
bristles scattered on its inner surface: the second segment is also
broad, but shorter, with a few, generally hooked bristles, in two short
irregular rows, in the upper part: these two segments answer to the
two segments of the pedicel of ordinary cirri. The third segment is
thinner and longer than the second; it bears two or three longitudinal
rows of bristles, most of which are neatly hooked at the point; its
upper end is surrounded with a circle of bristles. The fourth and
terminal segment is short, thin, and simple, with only a few bristles
at the apex. These two upper segments are bent a little inwards; they
answer to one of the two normal rami of ordinary cirri. The third
segment does not stand exactly on the middle of the summit of the
second segment,--the posterior corner of the latter being occupied by
a very curious, convex, oblong, rather hard (especially in the lower
part), protuberant cushion (as called by Mr. Hancock) or button (fig.
9, _c′_), transversely wrinkled by fine, distinctly crenated ridges.
This button presents a considerably different appearance according to
the point of view, fig. 9, 10: on one of its sides it projects beyond
the outline of the second segment, whence it arises; on the other
side it is prolonged, as a smooth ridge, on the top of the second
segment, embracing to a certain extent the base of the third segment.
On the face opposite to that which has been drawn (fig. 9) as most
characteristic, it is seen to be somewhat constricted round its base;
this constriction, representing, I believe, an articulation. When
viewed directly in front (fig. 10) its outline is oval, passing into
shield-shaped. Its longitudinal axis is 3/1000ths of an inch in length;
but it varies a little in shape and size. I shall presently assign my
reasons for believing that these buttons are the posterior or inner
rami of the fifth and sixth pairs of cirri in a rudimentary and much
modified condition.
_Caudal Appendages._--These (fig. 13, _p_) are placed close together,
being articulated between the bases of the sixth pair of cirri, the
lines of junction being internally marked by minute apodemes. They
consist of four segments, resembling in every respect those forming the
cirri, with the important exception that there is not a vestige of the
button on the summit of the second segment; the segments are not so
thick as those of the cirri, and the terminal segment is smaller.
_Muscles and Functions of the Cirri._--For their size, the cirri and
caudal appendages have voluntary muscles of remarkable strength,
attached within their basal segments, and springing from the dorsal and
ventral surfaces of the so-called third and fourth (_l_, _m_) thoracic
segments. Other muscles, rising from within the basal segment of each
limb, run to the second segment, and from that to the third segment.
I could not distinctly make out whether any entered the terminal
segment. I have seen no other instance of muscles entering the caudal
appendages, but as in the pupa they are so furnished, we here have only
an embryonic character preserved. I may remark that the fifth and sixth
cirri, consisting of two large basal and two thinner terminal segments,
is likewise an embryonic character. From the position of the cirri, the
four hard protuberant buttons or cushions, tend to oppose each other
at a common point; and the caudal appendages fill up a gap behind,
between the cirri of the sixth pair. I at first thought, with Mr.
Hancock, that these buttons served to catch the prey; but, reflecting
on their convexity and hardness, they appear very badly adapted for
this purpose; it would, in fact, be a marvellous feat to secure, in
the dark, any moving object between four balls. On the other hand,
this very convexity, the hardness, and especially the crenated ridges,
and the powerful muscles (which from the first surprised me), are all
well explained, if we suppose the prey, being secured by the terminal
segments, to be triturated between these four balls: any part which
escaped upwards would, moreover, be retained in a sort of cage, formed
by the inwardly inflected terminal segments with their hooked spines.
This view of the very curious and unparalleled use made of a modified
portion, not of the haunch, but of an upper part of the two posterior
pairs of thoracic limbs, is in some degree confirmed by finding that
Cryptophialus, which has apparently analogous habits, requires its food
to be triturated, though in this case it is effected by very different
means, namely, by four beautifully toothed discs, with brushes of
hairs, developed within the lower end of the œsophagus.
The prey, when caught, would probably at once be carried by the
movement of the articulated thorax to the mouth (itself moveable),
and being there secured by the mouth in front, the caudal appendages
behind, the tips of the cirri above, and the broad pedicels of the
first pair on the two sides, it would be triturated by the four
crenated buttons, and would then be forced down the œsophagus by the
action of the simple jaws. I looked in vain in several specimens for
any object within the stomach. I believe, that when the specimens are
first taken, all half digested food is ejected by the mouth. Whether we
may thus account for the extremely foul condition of the rami of the
first cirri in all the many specimens examined by me, I know not; but
that these rami, which are thickly clothed with fine plumose hairs, and
are furnished with delicate muscles, act as brushes, so as to clean the
orifice of the sack, I can hardly doubt.
_Homologies._--I have as yet, to a certain extent, assumed that I have
correctly named the different parts; and a few remarks on this head may
be desirable, considering the absence of certain cirri, the singular
condition of the others, the close general resemblance of the cirri
and caudal appendages, and the fact of the latter being furnished with
muscles. The only cause for any doubt regarding the thoracic segments
is the shield of thick membrane on that segment (_k_, fig. 5), which
ought to have borne the second pair of cirri, causing two transverse
wrinkles (not distinguishable, however, on the ventral surface),
and sometimes making the segment appear as if it consisted of three
segments: if it did consist of three, as there can be no doubt about
the nature of the first pair of cirri, (not in a more rudimentary
condition than in Anelasma) or about the segment whence this first pair
arises, the two terminal oblique segments, with their appendages, would
be abdominal instead of thoracic: but this is improbable, inasmuch as
the abdomen is unusually little developed in the pupa (as presently to
be shown), and more especially from the circumstance of a monstrous
cirrus, identical in structure with the two succeeding pairs, having
been borne on a segment (_m_), which, in any case must be considered as
thoracic, for it is well known how very rarely thoracic and abdominal
limbs resemble each other. I cannot myself feel hardly any doubt on the
nature of these three pairs of appendages; for, in the first place,
the posterior appendages are articulated on and between the bases of
the adjoining pair, _exactly_ as the undoubted caudal appendages are
articulated in all other members of the family on the sixth pair of
cirri or terminal thoracic appendages. Secondly, we see in the male
of the allied genus Ibla, the very same appendages preserved as in
Alcippe, namely, the caudal, and the fifth and sixth pairs of cirri,
which latter, moreover, are generally uniramous. Thirdly and lastly,
in the likewise allied _Alepas cornuta_, we have the posterior rami of
these same fifth and sixth pairs of cirri in a rudimentary condition,
and resembling in every respect the caudal appendages. Assuming,
then, that the several appendages in Alcippe have been rightly
denominated, we have to consider the nature of their segments: in all
cirripedes, the pedicels of the cirri consist of two segments, of
which the lower one (as here) is longer than the upper one, and both
(as here) considerably thicker than the segments of the rami: in all
cirripedial pupæ, likewise, the thick pedicels of the limbs consist of
two segments, and each ramus, also (as here), of two segments: now,
with these coincidences, and bearing in mind that in Alcippe the two
upper segments do not arise from the exact middle of the summit of the
second segment, but from rather its anterior side,--bearing, also,
in mind the case just cited of _Alepas cornuta_ with the posterior
rami of these very same cirri rudimentary,--we may, I think, safely
conclude that here in Alcippe the two lower segments form the pedicel;
the two upper segments, the anterior ramus; and that the button-like
protuberance is the posterior ramus in a modified condition. As the
caudal appendages in none of the Lepadidæ, either in the mature state
or in the pupa, have two rami, we can satisfactorily understand the
absence of any trace of the button-like protuberance on the top of the
second segment.[145]
[145] I almost wish I could persuade myself that I had taken
an erroneous view of the thoracic segments, and therefore that
the three pairs of terminal appendages were all abdominal, for
then Alcippe would come into much closer relationship with
Cryptophialus; though even in that case it would form a distinct
family from it: but I cannot alter my opinion.
_Alimentary Canal._--The œsophagus runs down from the mouth, beneath
and nearly parallel to the straight row of hairs on the two sides of
the labrum: it is surrounded by the usual muscles: at the lower end it
bends down, and expanding a little, like a bell, enters the stomach.
The stomach is of considerable size and fills the main part of the
body, bulging out under the mouth, and prolonged as far as about the
middle of that segment (_l_), which ought to have borne the third pair
of cirri; here the stomach terminates in a blunt rounded point. The
tissue surrounding the stomach, and keeping it in its proper place, can
be traced to the posterior end of the thorax, but there is no rectum or
anus. I am prepared to assert positively that this is the case,[146]
for I made repeated longitudinal sections of the whole thorax in two
planes, and I subsequently cleaned the outer tissues with boiling
potash, and then, when as transparent as a sheet of glass, I examined
every part, and certainly there is no rectum (which in every case
is formed of chitine, and so is not acted on by potash) nor an anal
orifice. Singular as this fact is, it is not so improbable as it at
first appears; inasmuch as I have shown, in my former volume, that the
Lepadidæ can reject half digested food by their mouth, and secondly,
that the final stage of digestion appears to take place in the upper
part of the stomach. In the male of this very species, as we shall
immediately see, there certainly is no mouth or stomach, and apparently
no rectum or anus; so is it likewise with the males of _Scalpellum
vulgare_ and _ornatum_: in Proteolepas, there is a mouth and an
œsophagus, but no stomach, rectum, or anus. There are, I believe, no
other known instances, in the whole great class of Crustacea, of the
absence of an anus.
[146] I may venture to remark that I succeeded in every attempt,
which I made, in seeing plainly the œsophagus, and the acoustic
and olfactory orifices and sacks, which, according to all analogy,
would be of much smaller size, and far more difficult to discover,
than the rectum and anus. I may mention that, according to Mr.
Newport ('Annals of Nat. Hist.,' 1849, p. 277), the larvæ of
certain parasitic Hymenoptera have a stomach without any anus.
No crustacean, according to Milne Edwards, is destitute of this
orifice.
The stomach, in Alcippe, is much corrugated, so as to be deeply
pitted; but there are no regular cæca. The enveloping hepatic layer
is thick, brownish, pulpy, and formed of pellets of cellular matter,
not distinctly arranged in lines as is general; there is the usual
delicate muscular layer. The stomach was in every case empty, and I did
not notice the separated epithelial coat, so generally found in other
cirripedes.
_Organs of Sense._--I failed in discovering the eye, which I have no
doubt exits, as it is conspicuous in the pupa and in the male. The
olfactory pouches are seated rather laterally under the maxillæ, as in
Ibla. As in this same genus, the acoustic sack is seated remarkably
low down (fig. 5), at a very considerable distance beneath the basal
articulation of the first cirrus: the orifice is seated on a slight
prominence: the acoustic vesicle, I believe, is sub-cylindrical, with
irregular projections. I did not make out anything distinctly on the
nervous system.
_Female Generative System._--The animal we have thus far described
is exclusively female: when a longitudinal section of the thorax is
made, and the stomach removed, it can be most plainly seen that there
are no vesiculæ seminales or testes. Mr. Hancock has remarked on the
absence of the usual probosciformed penis. The male of Alcippe will
be subsequently described in detail. The female organs differ in no
respect from those of other members of the family, excepting in so
far that the layer or mass formed by the ovarian cæca (_c_) does not
lie transversely to the longitudinal axis of the whole animal, but
longitudinally under the horny disc. The ovigerous fræna are largely
developed, but serve, as previously stated, as branchiæ, and not for
their proper function of giving attachment to the ovigerous lamellæ.
The ovigerous lamella is single, and nearly corresponds, in size and
shape (as would ensue from the manner of its formation) to the under
side of the horny disc. The ova are broadly oval, and rather above
1/100th of an inch in length.
_Metamorphoses._--The larva in the first stage has been fully described
and figured by Mr. Hancock: it differs in no essential respect from
other larvæ of the family. Mr. Hancock overlooked the inferior minute
antennæ. With respect to the larvæ in the last stage, or pupa, I
obtained several specimens attached to the disc of the female, and
which were on the point of being developed into males; and another
specimen identical in all respects, but attached independently to the
shell of the mollusc, and which, therefore, I have every reason to
suppose, would have been developed into a female. In any case these
pupæ may be conveniently here described (Pl. 23, fig. 16.) They are
.025 of an inch in length; they are of the usual shape, with the
anterior end not very blunt and the postero-ventral surface somewhat
produced. The whole carapace or shell is very thin and smooth. There
are six pairs of thoracic natatory legs, situated far back towards the
posterior end of the body; each leg has the usual articulations, and
the two rami their usual long but not plumose spines; the presence of
the legs deserves notice, considering the rudimentary and modified
state of their homologues in the mature animal. The abdomen differs
considerably from the same part, as far as I have seen, in other
pupæ; it consists of only a single almost globular (fig. 17, _q_)
segment, instead of three segments; and the two caudal appendages
(_r_) are very long, and are composed each of only a single segment
(instead of two), carrying at its tip two short spines. There are two
purple eyes, 4/3000ths of an inch in diameter, which, after having
been dried and then soaked, could be seen to be compound; they are
fixed in the usual manner to two rather short apodemes, which latter
have their usual origin. But the pupa has a very unusual appearance
owing to the presence of a single dark purple eye, half the diameter
of the two larger eyes, situated behind and above the latter, and
quite disconnected with the apodemes; this is the eye of the mature
animal, which, for some reason, is here developed earlier than usual.
The prehensile antennæ are remarkable from being seated very close to
the anterior extremity: owing to this, the articulation of the second
or main segment with the basal segment, is hardly at all oblique.
The whole pupa is of exactly the same length as the pupa of _Ibla
quadrivalvis_, and so are the antennæ, (see p. 286 of my volume on
the Lepadidæ), viz., 32/6000th of an inch, but the second segment is
narrower, (being only 8/6000ths in breadth in the broadest part), and
is longer in proportion, for the disc which forms part of the total
length is only 4/6000ths in length, whereas in Ibla it was 8/6000ths;
the disc is here hoof-shaped, as in Ibla. The ultimate segment is
remarkably short and narrow, (being only 3/20000ths in width, and less
than half the size of that in Ibla); it carries (I believe) three
terminal spines, and is not notched. Altogether the antennæ more nearly
resemble those of Ibla than of any other genus in the family. From the
position of the antennæ, and from the length of the second segment, the
pupa, when cemented by the disc or third segment, to the supporting
surface, adheres, with its posterior end almost vertically upwards.
With respect to the young cirripede within the pupa, I could only
observe that its anterior end was formed into a blunt point.
_Powers of Excavation; Inorganic Deposit of Calcareous Matter;
Attachment._--Alcippe, according to Mr. Hancock, attacks only dead
shells of the Fusus and Buccinum, and always on their inner sides,
especially on the columella. The excavations, in the specimen which I
examined, were so numerous as almost to touch, and sometimes to run
into each other, the included animal being thus rendered distorted. The
orifices are directed with respect to the shell indifferently upwards
or downwards. From the shape and size of the cavity corresponding
to that of the included animal, there can be no doubt, as stated by
Mr. Hancock, that Alcippe forms its own cavity. That the action is
mechanical I think may safely be inferred from the whole outer membrane
being studded with minute, star-headed points of hard chitine, which
rise from halo-like little discs of thickened membrane, which latter
are well adapted to allow the underlying adherent muscular layer to
act on the points, and thus on the surrounding shell. Consequently
the points generally show signs of severe attrition, but they are
periodically and often replaced, at each exuviation, by new and much
sharper points. There are no points on the permanently attached layers
of the horny disc, but it particularly deserves attention, that the
renewable membrane always extends beyond the circumference of the
disc, and is there most thickly studded with the points. We have
met, in Lithotrya, with a precisely analogous fact in the extension
of the periodically moulted membrane of the peduncle, furnished with
star-headed points of chitine, and in addition with minute calcareous
beads (which, however, seem soon worn away), beyond the calcareous
discs, by which this cirripede is attached in its cavity. We need not
feel much surprise at points of chitine being hard enough to wear away
shell, when we consider what work the jaws of insects, likewise formed
of chitine, will effect.
With respect to the first commencement of the excavation, the pupa,
owing to the position of its prehensile antennæ, fixes itself with its
posterior end almost vertically upwards; and the young cirripede, after
its metamorphosis, from the greater length of the ventral integuments
formed round the eye-apodemes, must be thrown backwards into nearly
the position represented in Pl. 22, fig. 12, _b_. I have not seen a
young female at this early age, but I have traced the development of
several males, and have found that the lower end of the peduncle,
(_i. e._ what was the anterior end of the pupa), grows at quite a
remarkable rate, so as very soon to form a great bag extending beyond
the attached prehensile antennæ. Now if we suppose an analogous
structure in the female or ordinary Alcippe, and the supposition is
quite allowable, we shall almost immediately have the anterior or lower
end of the young cirripede, just in advance of its antennæ, pressing
against the surface of the shell of the mollusc; and if armed with
triturating points, as we have every reason to believe it is, it would
wear for itself a cavity. The horny disc on the ventral surface of
this protuberant anterior end of the young animal will, we may assume,
soon become cemented to the near side of the cavity just supposed to
have been excavated. And the whole animal, by further slight changes
in direction, namely, by working down more and more obliquely, will
take, as shown at (_c_), its final position. As the whole surface of
the animal, with the exception of the horny disc, is provided with
triturating points, the animal, when once imbedded, can and does
increase its cavity at both ends in length, in depth, and all round
the edges of the horny disc,--in short, in every direction excepting
directly over the horny disc. I believe, as already explained, that the
young Alcippe, (_b_, diagram), first bores obliquely into the shell;
and whatever amount of downward extension the horny disc attains before
the young cirripede assumes its proper position, with its ventral
surface upwards and parallel to the inner surface of the shell of the
mollusc, that amount determines the thickness of the plate of shell
hereafter to be left unabraded over the horny disc, as the latter
continues to extend in circumference. This plate of shell over the
horny disc is so thin, that, as mentioned at the commencement, the
colour of the ovaria is seen through; and until I reflected on the
following considerations, I was much surprised how the instinct of
the animal could so neatly guide it not to grind too deeply, and yet
to grind till only a very thin plate of shell was left over its horny
disc: these considerations are, that whatever thickness was first
given to this plate of shell, when the animal was very young and first
assumed its ultimate position, that thickness would in most cases be
always retained, owing to the flatness of the disc, and to the membrane
armed with triturating points protruding very slightly beyond and
above the horny disc, only just enough to wear away the surrounding
shell to the thickness necessary to allow of the formation of each new
zone of disc; as the disc itself is not armed, it subsequently has
no power of wearing away the plate of shell above it. Thus the horny
disc, besides giving support and attachment to the peduncle, is of this
peculiar service that it seems to guide, (somewhat like the wood-part
in a plane), the rasping powers of the lower extreme margin of the
peduncle.
I may here observe that certain radiating and often punctured lines,
mentioned and figured by Mr. Hancock, which help to render the thin
plate of shell over the peduncle conspicuous (fig. 3), are formed by
the burrows of an excessively minute annelid, the punctures being
apparently the exit orifices: I imagine that these annelids find it
difficult to commence their burrows on the smooth surface of the shell,
and that they congregate at these particular spots and thence burrow in
radiating lines, owing to their having taken advantage of the little
cliff-like edges, at the narrow and disused ends of the fissures
leading into the cavities occupied by the Alcippe, where alone they
would not be disturbed by the action of the cirri, when first they
commenced making their little burrows in the shell.
The fissure leading into the cavity is required to be broad at the
posterior end, in order that the cirri may be there freely exserted
out of the sack; and narrow in other parts, to prevent, as it would
appear, anything injurious getting in between the animal's body and
the cavity in the shell of the mollusc. As the fissure is increased in
length by attrition at the broad posterior end, which end during growth
becomes broader and broader, the lower part of the fissure has to be
narrowed, and this is effected in a very singular manner, namely, by
advantage being taken of the strong tendency, which triturated shell
with animal matter, has to set into a solid shelly mass, although
constantly agitated.[147] Mr. Hancock noticed this edging of hard
shelly matter, and naturally thought it was a secretion. Lines of
deposition (Pl. 22, fig. 4, _b_), parallel to the edges of the furrow
can often be perceived in it: its thickness and extension vary much: I
have seen it on one side alone of the orifice: it is, of course, never
found at the broad end where the process of enlargement goes on. The
peculiar worn surface with which it irregularly thins away downwards,
on the sides of the cavity, made me (together with the apparent
impossibility of such a secretion proceeding from an animal wholly
invested by a chitine membrane) suspect it to be inorganic; and this
view is certainly correct, for when a fragment is dissolved in acid,
a considerable residuum is left of bits of membrane, rubbish, and, in
one instance, even of the remnants of a foreign animal, apparently an
annelid. We have here all the circumstances favorable for inorganic
deposits of this nature, namely, finely triturated shell and chitine or
animal matter, produced by the excavation of the chamber, sea-water,
and movement.
[147] I have given some remarkable cases in my volume on 'Volcanic
Islands,' (p. 49), in which limestone, having almost the hardness
and specific gravity of marble, has been thus deposited. Almost
every coral-reef offers similar examples. The curious substance
described by Mr. Horner and Sir David Brewster, ('Philosoph.
Transact.,' 1836, p. 65), which is formed during the manufactory of
cloth, offers another example of the strong tendency which lime and
animal matter have to unite. Lately, Dr. Horsford, in 'Silliman's
North American Journal,' Jan. 1853, has discussed the chemical
theory in an analogous case on the coast of Florida; he attributes
the aggregation to the formation of a hydrate of lime through
the action of the animal matter. Mr. G. B. Sowerby, Junr., has
described a case very analogous to that of Alcippe, ('Proceedings
of Zoolog. Soc., Mollusca,' Pl. 5, fig. 4, p. 162, 1850), namely,
that of _Pholas calva_, in which a tube is formed of _inorganic_
calcareous matter, serving to narrow the entrance.
From the manner of growth of the animal, the fissure leading into the
cavity in the shell becomes much longer than the orifice leading into
the sack, and to prevent the body being unnecessarily exposed, the
upward projection of the disc, already described, is formed under the
narrow and disused end of the fissure; moreover, the two rims of the
inorganic calcareous deposit sometimes here approach so closely, as
almost or actually to touch each other; and between them, as remarked
by Mr. Hancock, there is usually a little accumulation of grains of
sand. This narrow end of the fissure is generally curled either to the
right or left hand; and I can only account for this fact by supposing
that, whilst the cirripede is young, and has not a large horny disc
attached to the cavity, it cannot keep its body straight during the
long-continued boring process.
The animal is attached by its horny disc to the thin shelly roof over
the peduncle, and likewise to the under side of the narrow end of the
fissure, but is elsewhere quite free. I carefully examined the disc
in many specimens, but could not see any cement-ducts: I believe I
saw layers of cement at the upper end of the disc, but it is not easy
to discriminate between this substance and the yellowish, somewhat
disintegrated, layers of the horny disc. The pupa certainly becomes
attached by ordinary cement, so that the attachment in early life,
at least, is normal. In some full-grown specimens, I found the lower
parts of the horny disc attached, along the edges of the layers, to the
roof of shell; and as I looked here in vain with the highest powers
for cement-ducts, or for cement, it appears to me probable that the
rough edges of these layers were united to the roof by a thin layer of
the inorganic calcareous deposit. The animal, from its very protected
situation, certainly requires to be less firmly cemented than other
cirripedes; and even in Lithotrya, which is less deeply imbedded
than Alcippe, the cementing apparatus was feebly developed. From the
length of the pupal antennæ, cemented by their terminal segments, the
position of the young cirripede (Pl. 22, fig. 12) can be changed to
a considerable extent, like a ship swinging at her moorings, but in
order to assume its final position, the animal must, I think, travel
like Lithotrya, but to a much less extent, by a short succession of
overlapping horny discs,--the old discs being partially deserted, each
new one extending beyond the last-formed one: even in the case of the
mature animal, we have seen that, under certain circumstances, it
changes, to a certain extent, its position; portions of the old disc
being deserted and attached to the roof of a deserted portion of the
cavity.
_Affinities._--In the preliminary remarks under the Family, I have
discussed this subject almost sufficiently: I will here only remark,
that the genus, though so abnormal, yet stands naturally between
Ibla and Anelasma, having clear affinities, on the one side, through
and beyond Anelasma to Alepas; and on the other side, beyond Ibla to
Scalpellum, and so to Lithotrya. Moreover, it is very distinctly
related to Cryptophialus in the succeeding Order.
MALE. Pl. 23.
On every specimen of the female Alcippe, which I carefully examined,
I found some minute parasites (or epizoons) attached to the lateral
edges of the upper part of the horny disc, and therefore lying within
the narrow end of the fissure leading into the chamber excavated in the
shell of the Buccinum. Although having had some experience in the very
anomalous forms which male cirripedes assume, yet when I first casually
inspected these parasites under a weak lens, from their transparency,
their elongated and lobed body, including an internal folded up organ,
I actually threw them away, thinking that they were probably Bryozoa.
Subsequently, a more careful inspection immediately showed the cemented
prehensile antennæ, and their cirripedial nature was demonstrated. I
soon found specimens with the perfect still adherent exuviæ of the
locomotive pupa, undistinguishable from the pupa already described as
probably belonging to the female Alcippe. But as this latter fact,
may perhaps be doubted, I must show that there is other evidence
sufficient to prove that these cirripedial parasites are the males of
the female Alcippe. Of the females, I inspected many specimens, and all
certainly were without external male organs; and in the four or five
specimens which I rigidly examined, there were no testes or vesiculæ
seminales, the latter being in all hermaphrodite and male cirripedes so
conspicuous. On the other hand, I examined at least thirty specimens
of the parasite, and they were all exclusively males, for all had
a probosciformed penis, and the greater number had their vesiculæ
seminales filled with spermatozoa, and hence were ready to perform the
act of impregnation, but undoubtedly they contained no ova. It would,
then, be very strange, if these two cirripedes of opposite sexes, thus
attached together, were not sexually related. Wonderfully different as
the parasite is from the female Alcippe, yet, in one very important
character it is related to Alcippe, and to no other member of the
Family, namely, in the sack extending down to the extreme lower point
of the peduncle; the male organs, I may add, occupying an analogous
position with the peculiar position of the female organs in Alcippe.
The lateral lobes of the peduncle in the parasite seem to represent
the sides of the broad depressed peduncle in Alcippe; and in both the
peduncle grows at its lower end--a very rare circumstance--observed
only in two genera in this Family, namely, in Anelasma, and in a
slight degree in Lithotrya. Besides these points of resemblance
between Alcippe and its parasite, which are striking, considering
their external utter dissemblance, the affinities of both point,
judging from certain small characters, in the same direction, namely,
towards Ibla and Alepas. Finally, then, I think, we may confidently
admit that this parasite or epizoon is the male of the female Alcippe:
indeed, considering the facts given in my former volume, on Ibla and
Scalpellum, I have, perhaps, here discussed the question at unnecessary
length.
The males are generally attached, as already stated, to the two
hollowed out sides of the upward prolongation of the horny disc; they
adhere by means of little patches of cement, proceeding from the
terminal segments of their antennæ, to the overlapping edges of the few
later-formed zones of the disc; hence, they lie protected, within the
narrow end and a little under the edges of the fissure leading into the
cavity in which the female is lodged. In some specimens, however, the
males are attached rather lower down on the disc, and are not confined
exclusively to its upper margin, so that they live fairly under the
roof of shell which covers the main part of the disc: but they are
never attached very low down, so as to lie far from the lower end of
the orifice leading into the sack of the female. I have two or three
times seen as many as three males on each side, but sometimes there is
only one on each side, or none on one side. A large distorted specimen
actually had twelve males, and two pupæ on the point of undergoing
their final metamorphosis, all fourteen attached on one side, and all
evidently must have been alive together! Another specimen had nearly
the same number, a few on one side, and the rest on the other side.
The male immediately, after the exuviation of the pupal carapace,
25/1000th of an inch in length, is only 23/1000th of an inch long,
but ultimately it becomes, chiefly from the growth of the lower end
of the peduncle, nearly twice this length; for the largest specimen
which I have seen, that figured, was 45/1000th of an inch long (_i.
e._ under 1/20th of an inch), and 1/100th of an inch in breadth across
the peduncle, beneath the lateral lobes. The whole external membrane
of the animal (as well as the internal membrane of the sack), is very
thin, quite structureless, and as transparent as glass; hence, even
the spermatozoa, within the vesicula seminalis, can be seen from the
outside. The whole structure of the animal is very simple. The ventral
surface can be at once recognised by the attachment of the antennæ
(fig. 19, _a_), and these organs mark the point which was the anterior
end of the male, just at the period of its metamorphosis, and before
the lower end of the peduncle had grown. These antennæ have already
been fully described; they are conspicuous from being composed of
membrane, rather thicker than that investing the body of the male,
and which external membrane can be traced entering these organs, and
appearing like cement-ducts; but within these tubular prolongations of
the outer membrane, I could obscurely see the real cement-ducts.
The part answering to the capitulum is much flattened and elongated;
it widens but little from the upper to the lower end, where it blends
with the carinal or dorsal surface (the under surface in fig. 19)
of the lobed peduncle. At the upper end there is a small orifice,
and close to this, on the ventral or rostral side, there is a thin,
apparently double projection (_i_, fig. 19) or flap of membrane, one
flap lying exactly over the other. The whole length of this capitulum
probably corresponds with that small portion of the capitulum in the
female, between the upward prolongation of the horny disc and the
lower end of the orifice; and the two broad flattened projections in
the male, probably answer to the two sharp narrow points (_a_, fig. 1,
Pl. 22) in the female. The peduncle has two lateral lobes (_h_, _g_,
fig. 19), and, whilst young, what may be called a third and medial
lobe, but this soon increases largely by growth, and forms the main
part of the peduncle. The lateral lobes are intimately connected with
the ventral surface; they tend to lie in a plane, at right angles
to the compressed capitulum, but owing to the excessive thinness
and flexibility of the whole external membrane, it is difficult to
ascertain the relative position of the different parts. Moreover, owing
to the pupa being so much flattened, these lobes are necessarily formed
folded up; and, I believe, it depends on the position, with respect
to surrounding objects, which the male ultimately holds, whether the
lobes ever assume, their apparently normal position, in a plane at
right angles to the sides of the pupa; owing, also, to the form of the
pupa, the two lobes seem generally to be actually formed of unequal
sizes, that formed in the dorsal region of the pupa being the largest.
I believe that these lobes correspond with the lateral margins of
the upper end of the peduncle of the female, which margins project
laterally beyond the sides of the capitulum. The lower lobe, or end
of the peduncle, is depressed in the same plane with the lobes; it is
of variable length; when first formed it hardly extends beyond the
basal articulation of the prehensile antennæ. Commonly it does not lie
quite in a straight line of the capitulum; and I have seen specimens
in which it stood at nearly right angles to the capitulum and to what
was the ventral surface of the pupa; this irregularity in the relative
position and sizes of the different parts of the peduncle, no doubt,
to a considerable extent, depends on the form of surface to which the
male becomes attached, just in the same way as we have seen that the
peduncle of the female becomes altered in shape during the excavation
of the chamber in which it is lodged.
I feel some difficulty on one point: in the pupa the single eye of the
future male can be clearly distinguished, and it lies some way from
the anterior end of the body; but in two males, which certainly had
just moulted, and in which none of the internal organs were as yet
developed, the eye lay close to the anterior end, directly over the
basal articulation of the antennæ. I suspect this is somehow caused by
the great change of form which supervenes, during the metamorphosis,
at this anterior end of the body; the extremely compressed body of the
pupa having to become depressed and lobed in the young male. I have
given a figure of a young male, just as it appeared (Pl. 23, fig. 18),
somewhat distorted from lying on a flat surface; _c_, being the eye.
The sack extends, in a very remarkable manner, down to the lower end
of the peduncle, the whole inside of the animal being thus freely open
to the water. In the upper part, the sack forms a mere narrow tube; it
does not appear to have been formed in the same manner as in all other
cirripedes, namely, surrounding the thorax and natatory legs of the
pupa, but in an abnormal position, along the dorsal surface, above the
sack and thorax of the pupa: a transparent line, where the new narrow
sack is in process of formation, is the first indication of the coming
metamorphosis. The sack in the capitulum of the male is not central,
but lies near the dorsal surface; the ventral interspace, between the
outside and the sack, is occupied by oblique fibres (_l_, fig. 19),
which may be striæ-less muscles, but I suspect are ligamentous fibres,
giving support to the whole projecting capitulum. These fibres enter a
little way within the lobed peduncle; they are probably homologous with
the strong muscles, which run from beneath the upper end of the horny
disc of the female to the lower end of the orifice leading into the
sack. Round the lobed peduncle,[148] there are two bands (_e_, _f_) of
thin muscular fasciæ, slightly oblique to each other, and attached at
the ends to the outer membrane; they are evidently homologous with the
external transverse muscles, which are best developed round the same
part in the female. Some of these muscles present a singular chain-like
appearance, from being strangled at intervals: they act probably in
aiding the long probosciformed penis to protrude itself out of the
sack. I could not detect any longitudinal muscles, and the lower part
of the peduncle seems destitute of muscles of any kind.
[148] I believe I saw in one specimen, most delicate transverse
muscular fibres round the lower part of the elongated capitulum.
The internal structure of the animal is very simple. Within the lower
end of the peduncle there is a dark purple eye (_c_), under the
1/1000th of an inch in diameter, a testis (_d_) and a (_b_) vesicula
seminalis. These organs falsely appear as if suspended in the middle
of the peduncle, but they are really attached, I believe within a
separate partition, to the ventral surface, occupying the same position
as the mass of ovarian cæca in the female. The eye lies on the line of
junction between the testis and the vesicula seminalis, and on their
ventral side. The testis is rounded, and consists of a mass of cells,
on an average 1/5000th of an inch in diameter. The vesicula seminalis
varies extremely in condition, being either a mere rather broad vessel,
enlarged where it joins the testis, or a bag fully as large as the
testis itself, and distended with spermatozoa, all arranged parallel to
its longer axis. There was an evident relation between the size of the
vesicula seminalis and that of the testis, the number of the cells in
the latter decreasing as the mass of the spermatozoa increased: there
was also an evident relation between the age of the male and the state
of these organs; younger and more opaque individuals, having their
testes of large size; and older specimens, with the lower end of the
peduncle arrived at its full dimensions, having the vesicula distended.
Some few old specimens had evidently discharged their spermatozoa. By
dissection I more than once distinctly traced the vesicula seminalis
entering the broad lower end of the penis. The membrane, forming
the vesicula, is ringed, and I presume is, as in other cirripedes,
contractile, so as to expel the spermatozoa. The probosciformed penis
(_m_) is of extraordinary length: it is plainly ringed, or rather
articulated, in this respect resembling that organ in Ibla and Alepas;
it tapers gradually, and terminates (as usual) with a brush of fine
bristles; it is furnished with delicate voluntary muscles, arising from
the body round its basis, and extending no doubt up to the apex, but
too fine to be traced all the way. Its broad lower end is attached in
a slight depression, on the ventral side of the sack, a little above
the point of attachment of the pupal antennæ. According to all analogy,
the spot whence the penis springs must be considered as representing
the thorax and abdomen; and the outer membrane of the penis is here, as
on this view it should be, reflexed and is continuous with that lining
the sack. Ordinarily the penis lies coiled up in complicated folds,
appearing like a large intestinal worm, and fills the lobed part of
the peduncle, which apparently serves for no other purpose than its
reception. In one case in which I dissected out the penis, I found
it in its contracted state; 41/1000th of an inch in length, equal to
that of the entire capitulum and peduncle; in a specimen, in which the
penis had been naturally exserted, the part which protruded (_m_) was
by itself rather longer than the whole animal; and as this specimen
had been placed in spirits of wine, the organ no doubt was contracted;
hence I think it probable that the probosciformed penis, when fully
stretched out, would equal twice the length of the entire animal.
There must be a nervous system; and there must likewise be a gland
(homologous with the ovaria) for secreting the cement; but I could not
distinguish parts so small. Certainly there is no mouth, or stomach, or
thorax, or limbs of any kind, or abdomen.
It is obvious that these males must be very short-lived: they perform
their masculine functions and then perish. We have seen, however,
that after the act of metamorphosis they do grow a little, and I have
reason to suspect that this is effected, as with other Cirripedes,
by moulting. The growth must be absolutely dependent on the store of
nutriment laid up within the pupa. The young male, immediately after
the exuviation of the integuments, thorax, natatory legs, abdomen,
and eyes of the pupa, consists of a pulpy cellular mass, without any
internal organs as yet formed.
Judging from the different sizes of the females which included
perfectly developed ova, I infer that they must breed more than once
during their lives; and therefore, that successive sets of males, as
in the genus Scalpellum, must become attached to them. I was not,
however, able to discover the prehensile antennæ or other remains of
the old males adherent to the females; a circumstance which I presume
is accounted for by their attachment being weak. Considering the
very small size of the male, it is not surprising that so many,--in
one case fourteen,--are required to impregnate the numerous ova of a
single female. How the males know the proper period when the ova,
lying in a sheet at the very base of the sack of the female, are ready
for impregnation, I cannot say, without it be that they perceive
the moulting of the external membrane, close to the edge of which
they are attached; for this moulting would indicate the period when
the ovigerous lamella came to the surface of the sack, and the ova
would then be soon ready for impregnation. From the position in which
the males are attached, and from the extraordinary length of the
probosciformed penis, capable of voluntary movements, I have no doubt
the males can insert the tip of this organ within the lower edge of
the orifice of the sack, and there discharge the spermatozoa, which,
by their own movements, must pass down the sides of the sack of the
female till they reach their proper destination. The position of the
males, with respect to the female's body, is almost exactly the same
as that occupied by the complemental males of _Scalpellum Peronii_
and _villosum_; the lower and narrow end of the fissure, worn in the
gasteropod shell, here affording that protection to the males, which
the edges of the opposed scuta afford to the _complemental_ males
of the above two species of Scalpellum. We cannot doubt that these
latter males aid in the impregnation of the ova of the hermaphrodites,
but they are not furnished with a very long penis, probably for the
very reason that they are _complemental_ males, and therefore not so
absolutely necessary for the impregnation of the ova as are the males
of Alcippe.
I have, in my former volume, expressed my astonishment at the extent
to which abortion had been carried in the male Ibla; but it has been
carried much further in the male Alcippe. In Ibla, the thorax is
reduced to a mere flap, and only two pairs of cirri exist in a most
useless and rudimentary state, but there is a well organised mouth,
stomach, and anus. In the males of _Scalpellum vulgare_, _ornatum_,
and _rutilum_ there is no mouth or stomach, but there is a thorax with
four pairs of minute, modified cirri, and a large abdominal lobe. Here,
in the male Alcippe, all these negatives are united, we have no mouth,
no stomach, no thorax, no cirri, no abdomen! The archetype crustacean
consists of twenty-one segments; of these the seventeen anterior
segments can be clearly made out in the archetype Cirripede: now, in
the male Alcippe, the first three segments are largely developed,
forming all that is externally visible, but the remaining fourteen
segments are absolutely aborted, but in idea may be considered as
forming the membranous depression whence the probosciformed penis
springs; for this organ normally arises at the extremity of the
seventeenth segment. To show the wonderful diversity of nature, even
in the same sub-class, I may be permitted to remark, that whilst in
Alcippe only the three anterior segments are developed, the fourteen
succeeding segments being rudimentary, in Proteolepas (hereafter to be
described) these fourteen segments are all largely developed, whilst
the three anterior segments are quite aborted, being represented only
by a thin envelope to the two threads by which this Cirripede is
attached to the supporting object.[149]
[149] It may be worth stating, that in order to procure perfect
specimens of the female and male Alcippe, pieces of the shell
inhabited by them should be dissolved in weak acids.