[136] According to Bock, in the 'Naturforscher' of 1778, this
term was used by Rumph for a Chelonobia, but as it was before the
adoption of the binomial nomenclature, according to the Rules, it
may be passed over, and does not interfere with the priority of
Schumacher.
_Distribution_, Northern Europe, Mediterranean, Red Sea, Madeira,
West Indies, Tierra del Fuego, Chile, Peru.
The shell in this genus is extremely unsymmetrical, not two of the six
pieces of which it is composed quite resembling each other. At first
it appeared hopelessly difficult to identify, in a homological sense,
these six valves, with those of ordinary cirripedes, but the difficulty
soon quite vanished. The operculum consists of two moveable valves on
one side, namely, a scutum and tergum, but without any moveable valves
on the opposed side: the scutum, though remarkable from being much
smaller than the tergum, can be easily recognised by giving attachment
to the animal's body and to the adductor scutorum muscle. The four
other pieces are articulated together, and form the shell surrounding
the sack, in which the animal's body is enclosed: of these, the two
against which the moveable scutum and tergum shut, are smaller, differ
greatly in shape, and are articulated together in a different manner
from the remaining two pieces; from these facts alone there would be
a strong presumption that they were of a different nature. The fixed
valve, against which the scutum shuts, is either furnished with a
remarkably prominent plate (_a_ in fig. 1 _c_, _s′_; compare this with
_s′_ in the reversed shell in fig. 1 _e_), or is hollowed out, as in
_V. nexa_, for the attachment of the adductor scutorum muscle. Thus
it is rendered probable that this fixed valve is a modified scutum;
but a surface of attachment for one end of the adductor muscle might,
perhaps, have been developed on any other valve, or a scutum might
have become fused with a lateral valve of the shell; the shell on
this latter view being rendered in idea more symmetrical. But when
a very young specimen is carefully examined, it is found that the
moveable and fixed scutum, the moveable tergum and its opposed valve
or fixed tergum, at the first period of calcification, resemble each
other quite closely; but that, as each zone of shell is added, the
differences become rapidly greater and greater: hence, it may be
considered as directly proved, that the two fixed valves (S′ and T′ in
all the figures in Pl. 21), which are opposed to the moveable valves
of the operculum (S and T), consist of an extraordinarily modified
scutum and tergum. It has been shown (p. 129), that at the period of
the metamorphosis, the two scuta, the two terga, and the carina of
the Lepadidæ, commence their growth, under the form of the so-called
"primordial valves," and so differ from all the other valves when such
occur: now, in two species of Verruca, I have found closely analogous
primordial valves on the apices of both the moveable and fixed scutum
and tergum (thus affording strong additional evidence that their nature
has been rightly interpreted), and on one of the two remaining valves,
namely, that at the posterior or carinal end of the shell. Hence, we
may safely infer, that this latter valve, which, though very much more
developed on one than on the other side, is so far medial as to curl
round and cover the line of opening between the moveable and fixed
tergum, is really a carina. The sixth valve differs only very slightly
in shape from the carina, and is directly opposed to it; therefore,
in accordance with all analogy, it must be the rostrum. Consequently,
the shell in Verruca consists of a moveable scutum and tergum, a fixed
scutum and tergum, a carina and rostrum, and, as we shall immediately
see, a membranous basis--the basis being, as in all sessile cirripedes,
the homologue of the peduncle in the Lepadidæ.
The moveable scutum and tergum stand at about right angles with the
fixed pair; and as these latter form a part of the wall of the shell,
which is always steep on this side, the moveable pair, which close the
orifice, are nearly horizontal or parallel to the basis and surface of
attachment. Hence, the animal's body, which is attached between the two
scuta, but nearest to the moveable scutum, also, lies nearly parallel
to the surface of attachment; and I was consequently at first led to
suspect that the basal membrane was one side of the shell in a modified
condition; but the presence of the prehensile antennæ of the pupa in
nearly the middle of this membrane, and the sheet of cement-tissue on
its under side, demonstrate that this membrane, though lying on one
side of the animal, is the true basis. To make all the parts in Verruca
hold the same position as in other cirripedes, relatively to the
surface of attachment, we must develope the carina and rostrum equally
on both sides of the true longitudinal axis of the shell, and insert
the newly-developed portion between the basis and the fixed scutum
and tergum, reducing the latter in size, and tilting a little up the
moveable scutum and tergum; and by this means the animal's body would
be turned, so that its dorso-ventral longitudinal plane would stand at
right angles to the basal membrane.
Extraordinarily great as is the difference between the right and
left sides of the whole shell, yet in all the species it seems to be
entirely a matter of chance whether it be the right scutum and tergum
with the right side of the rostrum and carina, or the left scutum and
tergum with the left side of the rostrum and carina, which become
abnormally developed. Nor does there seem to be any relation between
the side of the operculum to be attached, whether right or left, and
the nature of the surface of attachment; for I have seen many specimens
adhering to perfectly level surfaces, and to quite cylindrical branches
of Laminariæ; and in these cases, however the larva might attach
itself, there could be nothing to favour the development of one side
more than the other. Although the attached scuta and terga are larger
than the moveable pair, yet, owing to the small development of the
carina and rostrum on the attached side, the upper or unattached side
must be considered as the most developed. In this respect, and in
the circumstance of either right or left side being modified, we are
reminded of the structure of _Pœcilasma Kæmpferi_ (described in my
former volume on the Lepadidæ), in which the valves on the side of the
capitulum, nearest to the crab's body, to which the specimens were
attached, were somewhat less developed than those on the opposite side.
I may add, that in ordinary Crustaceans, as I am informed by Professor
Bell, the unequal development of the thoracic limbs seems quite
capriciously to affect either the left or right side of the body.
_General Appearance of the Shell._--The shell is in most cases much
depressed and irregularly circular; the side formed by the fixed
scutum and tergum is always steeper than the other side: the colour
is white or pale brownish, and in _V. nexa_ pale red. The surface is
naked. The size is small, rarely exceeding a quarter of an inch in
diameter, and the whole shell often appears like a mere scale on the
surface of attachment. The most remarkable feature in the external
aspect is due to the suture between the rostrum and carina, which is
formed by oblique, interlocking plates or folds; as all these plates
continue to be added to at their extremities during growth, the upper
plates become longer than the lower ones; and the plates on both
sides of the suture together form a triangular area, with the broad
end uppermost, somewhat like the radius of a sessile cirripede: they
act, also, like a radius, for their growth serves to separate these
two valves, and so adds to the diameter of the shell. The suture
between the rostrum and fixed scutum and that between the carina and
fixed tergum are nearly of the same nature, but the former is more
conspicuous than the latter; neither are so conspicuous as that between
the carina and rostrum: accordingly as the right or left scutum and
tergum are moveable, so the suture, second in plainness, (see Pl. 21,
fig. 1 _a_, and 1 _d_,) is placed to the left or right hand. The fourth
suture, between the fixed scutum and tergum, as viewed externally,
is straight, and so very obscure that it has been overlooked by some
authors, and the shell described as consisting of only three nearly
equal pieces, for the fixed scutum and tergum together are about equal
in size to the carina or rostrum. The orifice approaches more nearly
to an unequal-sided triangle, with the apex broadly truncated, than to
any other figure. The operculum fits with remarkable closeness, and
is surrounded by a slight rim, formed by the edges of the four other
valves.
_Moveable Scutum and Tergum._--The scutum (S in 1 _b_ and 5) is narrow
and very small, barely equalling half the size of the tergum, and
therefore proportionally much smaller than in any other cirripede; in a
very young shell, however, (of _V. Strömia_) less than a pin's head in
size, the scutum equalled the tergum in size. The valve is remarkably
thick; it is generally depressed down the middle; but in _V. nexa_
this part is longitudinally ribbed. The occludent margin is curved.
On the tergal margin there are two articular ridges (with a deepish
furrow between them), of which the upper one (′ in S, in fig. 1 _b_,
and 5) extends from the apex about half-way down the valve; and the
other, or lower articular ridge (′′ in S), generally runs down nearly
to the basal margin: an angle, running from the apex to the basi-tergal
corner of the valve, appears like a third articular ridge, but cannot
properly be considered such. The above two articular ridges interfold
with analogous ones on the scutal margin of the tergum, and so lock the
valves together. On the under side (fig. 1 _f_), the surface is bounded
along the occludent margin by a slight rim: there is generally a very
slight depression for the adductor muscle; but in _V. Spengleri_ there
is a straight, short, sharp (Pl. 21, fig. 2), prominent adductor ridge.
The _moveable tergum_ is broad and rhomboidal. Externally a prominent
axial ridge (′′′ in T, in fig. 1 _b_, &c.), which widens downwards,
runs from the apex of the valve to the basal point, and there
projecting slightly, causes the scutum to be indented; this indentation
on the scutum appears like a third articular ridge, lying beneath
(′′), S, in fig. 1 _b_, &c. Above the lower and axial ridge, on the
scutal margin of the tergum, there is a middle articular ridge, which
locks in, between the lower (′′) and upper articular ridges (′) of the
scutum (S). Again above the middle ridge there is an upper and third
articular ridge (′), which is either quite distinct, as in fig. 5, T,
or more commonly is formed by the occludent margin of the valve, as in
T, fig. 1 _b_. The broad extremity of this upper articular ridge is
often produced into a slight projection, or shoulder, and this always
underlies the scutum, of which the under and upper surface is indented
or furrowed (see fig. 1 _f_), in order to receive this shoulder. The
upper articular ridge of the scutum (′, S, 1 _b_) locks in between the
upper articular ridge or occludent margin (′, T), and the middle ridge
(′′, T) of the tergum.
Hence, altogether, there are three articular ridges on the scutal
margin of the tergum, the occludent margin being generally counted
as one; whereas, on the tergal margin of the scutum, there are only
two ridges, though, as before noticed, an outer indentation, which is
developed as a ridge in _V. nexa_ (fig. 5, S), might almost be counted
as a third articular ridge.
I may here just remark, that the furrow between the two ridges on the
tergal margin of the scutum, resembles the articular furrow in the
scutum of the Balanidæ; but it may be doubted whether the resemblance
be more than superficial, as this furrow, in the case of Balanidæ,
receives the edge itself of the tergum, whereas here it receives only
a ridge, proceeding from the apex of the tergum, to a nearly middle
point on its scutal margin. Finally, I may add, that the tergum in
this genus, in general shape, in growth (presently to be referred to),
in the manner in which the upper scutal shoulder is overlapped by
the scutum, and in the presence of the axial ridge, presents a very
striking resemblance to certain old fossil species of Pollicipes, and
to a limited extent to the living species of Lithotrya.
The scutum and tergum being interlocked, move together; they can be
firmly shut by the contraction of the long adductor scutorum muscle.
Their opening appears partly due to the elasticity of the membranous
hinge (representing the opercular membrane), by which they are attached
transversely, just beneath the summit of the carina and rostrum. No
doubt the protrusion of the cirri effectively aids the act of opening.
These valves are not capable (nor, of course, the other valves) of any
other movement; for there are no muscles for such movements.
_Fixed Scutum and Tergum._--The fixed scutum is larger than the fixed
tergum, and therefore has the same proportions as the homologous valves
in ordinary cirripedia, but reversed proportions compared with the
moveable scutum and tergum. The shape of neither valve can hardly be
described. The _fixed Scutum_ (S′ in all the figs.), externally, seems
at first to consist of two portions, namely, a curved occludent rim
(_a_), closely resembling the opposed occludent margin (_a_) of the
moveable scutum (S), having in fact undergone very little modification,
(as may be best seen in Pl. 21, fig. 1 _b_); and secondly, of a much
modified portion (marked ′′), which resembles in outline and state of
surface the rest of the walls of the shell, and may be called the
parietal portion. The rostrum (A) curls round the end of the occludent
portion, under an edge (_b_), evidently answering to the basal margin
(_b_) of the moveable scutum, and is simply united to this portion
by membrane, but beyond this part, it is articulated to the parietal
portion (′′) of the fixed scutum, by oblique interlocking ridges, like
those forming the suture between the rostrum and carina. In _V. nexa_,
however, (fig. 5) the rostrum (A) does not curl round any part of the
interlocking fixed scutum (S′), but articulates with it by a straight
suture. Internally, the fixed scutum (S′ in figs. 1 _e_, 1 _c_, of
reversed shells) has a surprisingly large, thin adductor plate (_m_ in
fig. 1 _c_, 1 _b_), with a rounded outline, projecting nearly parallel
to the basis or surface of attachment; the adductor scutorum muscle
is attached to its upper surface, and consequently the animal's body
lies between this plate and the moveable scutum. In the fixed scutum of
_V. nexa_, however, there is a deep pit, instead of a plate, for this
muscle.
The _fixed Tergum_, likewise, consists of two portions--a middle
and lower, or parietal portion, and a rim or upper portion; the rim
consists of two unequal arms, answering to the two upper margins (not
merely the edges) of the rhomboidal moveable tergum; the longer rim
(_x_ in T′, see fig. 5) answers to the carinal margin (_x_) of the
moveable tergum, and may be called the carinal rim; and the shorter
rim (_o_ and ′ in fig. 5) answers to the occludent margin (_o_ and
′) of the moveable valve, and may be called the occludent rim. The
carina curls round the end of the carinal rim, under an edge, _z_ (much
foreshortened in T′ in fig. 1 _b_, and best seen in fig. 5), answering
to at least a large part of the basal margin (_z_) of the moveable
tergum, and interlocks, by a serrated suture, with the edge of the
parietal portion of the valve. Internally (fig. 1 _e_, less plain in 1
_c_) there is a transverse ledge, notched in the middle, and sometimes
deeply hollow beneath, running across the valve in about the line of
the adductor plate of the fixed scutum: this ledge, in fact, marks
and is partly caused by, the line of separation between the central
or parietal, much modified, and the scarcely modified, upper or rim
portion of the valve. The use of this ledge is apparently to give
attachment, as does the under side of the adductor plate of the fixed
scutum, to ligamentous fibres, presently to be mentioned, by which the
shell is attached to the basal membrane: the carina and rostrum being
so much more gently inclined, do not stand in need of a ledge for their
attachment.
By comparing the moveable scutum and tergum with the corresponding
fixed valves, in all the species, the modification of the latter may
be clearly made out to have been effected as follows; and the case
appears to me a striking and interesting one. The moveable scutum
and tergum lie in the same plane, and are articulated by the means
of three ridges on the tergum (including the occludent margin), and
by two on the scutum. The fixed scutum and tergum have to be curved,
and to be greatly increased in size; and this is brought about, as we
shall see, by the large development of a certain small portion of each
valve. Comparing first the moveable tergum (T) with the fixed tergum
(T′), the umbo of growth matches the umbo of the four margins of the
moveable valve, the carinal (_x_), basal (_z_), and occludent (_o_ in
fig. 5), margins can be identified with certainty in the fixed valve,
from their close similarity in shape, their absolute apposition, or
correspondence in position. There remains only the scutal or articular
margin, with its three articular ridges; of these, the uppermost (′),
inasmuch as in most of the species it is hardly distinct from the
occludent margin, can, as we have just seen, be clearly identified, and
is overlapped, as it normally should be, by the upper tergal corner
of the fixed scutum: the second or middle articular ridge, though
not so distinct as in the moveable valves, can be plainly recognised
(′′), T′, in fig. 1 _b_, and 5; and it serves its normal function of
articulating the two valves together. But when we look in the fixed
valve for the third or axial ridge (′′′), we find in its exact place,
namely, extending from the umbo to the extreme opposite end of the
valve, between the second articular ridge (′′) and the basal margin
(_z_, see fig. 5), only that portion of the valve which I have called
the parietal portion; consequently, I do not doubt that this really is
the axial ridge largely expanded. So again in comparing the moveable
scutum (S) with the fixed scutum (S′); two of the three margins of the
former, namely, the occludent (_a_, see fig. 1 _b_) and basal (_b_),
can be identified without a doubt in the fixed valve: the third and
tergal margin remains; this should have two articular ridges; of these
the upper one, still serving its normal function, can be detected in
all the species (′ in fig. 1 _b_), and can be seen pretty plainly (′
fig. 5) in _V. nexa_: but of the lower and other articular ridge there
is no sign,--excepting indeed the whole parietal portion of the valve,
which, from holding an exactly homologous position with the lower
articular ridge of the moveable valve, I cannot doubt in this ridge
expanded and curiously metamorphosed. Hence, in both fixed scutum and
tergum, it is the outermost or lowest of the articular ridges which
has been modified and expanded, so as to rest on and be fixed to the
surface of attachment. It would appear as if it had resulted from one
ridge in each of these valves having been thus used up by expansion
(so to express myself), that the suture between the fixed scutum and
tergum is more simple than any other suture in the whole shell; and it
is owing probably to this straightness, and consequent _tendency_ to
weakness, that the valves do not grow along this line, and so do not
become separated from each other during growth, as on the three other
lines of suture. As it actually is, owing to this suture never being
separated, it is even stronger than the others; its edges on the inside
(fig. 1 _c_), I may add, are a little inflected or prominent.
_Rostrum_ and _Carina_: these valves differ from each other, only in
the former (A) being rather the largest, and in being more plainly
articulated with the fixed scutum, than is the carina (B) with the
fixed tergum. Their umbones stand in their normal places, at the
two ends of the orifice leading into the sack, that is, facing the
dorso-ventral longitudinal plane of the animal; but they are very
unequally developed on the two sides, and hence they rise very
obliquely from the surface of attachment. Their summits are nearly
square, which is caused by the continued growth on both sides of the
oblique plates or ridges, by which they are articulated with the
adjoining valves. These plates strikingly resemble, as already stated,
the radii in certain species of Chthamalus. Without these articulating
plates, the outline of the rostrum and carina would have been
triangular, with the apex upwards. In _V. nexa_, in which the walls
of the shell are almost perpendicular, the rostrum (A, fig. 5) is very
peculiar and patelliformed, with the umbo sub-central: this results
from the development of a border at the upper end of the valve. In this
same species, the basal edges of the rostrum, carina, fixed scutum and
tergum, are rectangularly inflected, so as to form a ledge round the
basis, as in the case of some few species of Chthamalus,--the ledge
appearing like part of the real basis. During the growth of the shell,
the upper internal ends of the carina and rostrum are either rendered
solid, or a ledge is formed on the inside across their summits, hollow
beneath, like the sheath of the Balanidæ, to which solid or hollow
ledge the basal margins of the moveable scutum and tergum are attached
by a rim of membrane, forming a hinge.
_Direction of Growth: Minute Structure of Valves._--The shell grows
downwards all round its basal margin. As far as the diametric growth
of its upper part is concerned, there may be said to be only three
valves, for the fixed scutum and tergum never become, as already
stated, separated; on the three other lines of suture, the valves are
added to on both sides; and thus the whole upper part of the shell,
and the orifice, increases in diameter. The moveable scutum and tergum
grow along their basal margins, and along the margins by which they
are articulated together; but the scutum in this latter respect, less
than the tergum. The summits of the moveable scutum and tergum, during
continued growth, become either worn away, or they project freely; in
this latter case, an internal ledge is added round the upper end of the
fixed scutum and tergum, so as to keep the orifice accurately closed.
In _V. nexa_ the rostrum, with its sub-central umbo, is anomalous, as
already stated, owing to a broad upper internal border growing in a
direction almost directly opposed to the basal growth of the moveable
opercular valves.
In young specimens, on the apices of both scuta and both terga, and
on the carina, but not on the rostrum, _primordial valves_ may be
distinguished, resembling the valves, so called, which first appear (p.
129, Introduction) after the metamorphosis in, the Lepadidæ. In the
Verrucidæ, however, they are calcareous; and the minute transverse
cylinders, of which they appear to be composed, stand further apart,
causing the surface of the primordial valve to be marked with little
separate circles, instead of by hexagons.
The shelly matter of which the valves are composed is translucent: it
is remarkably destitute of any investing membrane. The under surface
is marked with rows of minute approximate pores, parallel to the lines
of growth, into which the corium enters: after a portion of shell has
been dissolved in acid, these threads of corium are seen to change,
a short distance within, into cylinders of yellow chitine, running
obliquely through the substance of the valve. These cylinders are about
1/2000th of an inch in diameter, but in parts they are spindle-shaped
and twice as thick: they vary in length, about 1/100th of an inch
being the average length: these cylinders at their upper ends suddenly
contract into a point, more or less long, or are produced into a very
fine tortuous tubulus of chitine, imbedded in the shell: I have seen
in no other Cirripedes tubuli of this structure. There are other
ordinary tubuli, such as occur in the valves of most Cirripedes, about
1/6000th of an inch in diameter, and which sometimes alternate with
the above-described thicker cylinders. There are no external spines.
From the number and length of the tubuli of both kinds, the tissue left
after the action of acid is singularly complicated.
_Basis._--The basal membrane is thin, and is divided, but not very
plainly, into concentric slips, marking the successive increments of
growth. In the middle of it, in two young specimens, I found with great
difficulty the pupal prehensile antennæ: they were of small size,
measuring from the extreme edge of the main or second segment to the
end of the disc, only 27/6000ths of an inch: the disc appeared narrow
(as in Pollicipes and Scalpellum), with a single spine at the proximate
end: the ultimate segment, placed as usual at about right angles to
the disc, bore two groups of shorter and longer spines, but I could
not count how many. The antennæ were enveloped in a mass of cement of
a yellow colour, resembling in all its characters the cement of other
Cirripedes. In only one case, I believe I saw bifurcating cement-ducts,
of extreme tenuity, viz. 1/15,000th of an inch in diameter. The sheet
of cement on the whole under side of the basal membrane, not rarely
shows a very irregular reticulated structure. For convenience sake,
it will be best to defer the discussion on the very anomalous, though
slight, powers of excavation which this genus possesses, and which I
must attribute to the effects of some substance secreted probably by
the cement-organs. I will here only mention, that the specimens which
have excavated a depression, are less firmly attached than those, which
have not acted on their support; and that, in the former case, the
basal membrane, for a considerable space in the middle, becomes quite
detached.
_Animal's Body._--The body is much flattened and, owing to the
little development of one side of the shell, lies parallel to the
surface of attachment. The prosoma is but little protuberant. The
articulations of the thorax are unusually straight and transverse.
The _Mouth_ is also much flattened: it is placed rather distantly
from the adductor scutorum muscle, owing to the production of the
lower margin of the labrum. The _Labrum_ is not notched, or even
hollowed out in the middle, or (excepting in _V. nexa_) bullate;
its crest is surmounted by about eight (more numerous in _V. nexa_)
little teeth, or by some fine bristles. The _Palpi_ are of moderate
size, with their tips nearly meeting; they are slightly curved, and
have bristles only on their outer sides and extremities: they are
apparently capable of being lifted up and down by a muscle attached to
them, just outside the rounded swelling on each side of the labrum to
which they are articulated: in _V. nexa_, however, the palpi are very
small and narrow, and their tips do not nearly meet. In this genus,
therefore, we find the swollen state of the labrum and the size of
the palpi--characters generally invariable and of high classificatory
importance--variable. The mandibles have three upper main teeth,
with two or three minute lower teeth, or, in _V. nexa_, with the
lower part pectinated with small spines: in _V. Strömia_, I have seen
traces of the second tooth being laterally double--a character of
some importance. The _Maxillæ_ have a notch under the upper pair of
large spines, with the lower part bearing, as usual, a double row of
bristles, and forming a large step-formed projection: these organs are
furnished with the usual apodeme and muscles. The _Outer Maxillæ_ are
prominent, and deeply lobed on their inner surfaces, the two lobes
being clothed with bristles.
_Cirri._--The first pair are attached, as usual, on each side of the
mouth, and stand some way apart from the five posterior pairs. The
second and third pairs differ considerably in structure from the three
posterior pairs, which are much elongated. The _first pair_ (excepting
in _V. nexa_) is short, with the two rami slightly unequal in length,
and with the segments thickly clothed, as usual, with spines. The
_second pair_ is remarkable from the posterior ramus being more than
twice as long, and containing thrice as many segments, as the anterior
ramus, which is barely as long as the shorter ramus of the first pair:
the segments in the anterior ramus of the second pair (only five in
number in a full-sized specimen) are broader and more protuberant in
front, and more thickly clothed with spines (the terminal spines being
doubly pectinated), than are the segments on the posterior ramus; on
the latter, the uppermost segments have their bristles arranged in
front in simple pairs, with the dorsal spines long, the lower segments
being more thickly clothed with bristles, owing to the development of
lateral rows. The _third pair_ resembles in every respect the second
pair, except in being a little longer, and in the bristles on the
posterior ramus being less crowded, more resembling the arrangement of
those on the posterior cirri. In _V. nexa_, however, there is not so
great an inequality in length or dissimilarity in structure in the two
rami of the second cirrus, and only a very slight difference of any
kind in the two rami of the third pair. _Fourth_, _fifth_, and _sixth
pairs_ have numerous elongated segments, bearing four or three pairs of
long slender spines in front, with a single minute bristle between each
pair, and with two or three slender spines in the dorsal tuft.
There is a considerable amount of variation in the proportional
length, and in the number of the segments, of the several cirri in
_V. Strömia_; in some specimens the two rami of the fourth pair were
unequal in length; in some, nearly all the cirri on the lower or
attached side were shorter than those on the upper side.
_Caudal Appendages._--These are of most unusual length, sometimes
even exceeding those of _Ibla quadrivalvis_, which surpasses, in this
respect, all other cirripedes. They arise on each side and over the
anus. They consist of numerous (sometimes as many as twenty-four),
unequal, cylindrical, thin segments, bearing, at their upper ends,
a circle of long and very slender spines. They sometimes equal two
thirds or even four fifths of the length of the sixth cirrus; but
their length, and the number of their segments, (sometimes imperfectly
divided), varies much in different specimens of the same species,
and sometimes even on opposite sides of the same individual. In some
very young shells, as big as a pin's head, the caudal appendages were
proportionally extremely short, and consisted of only two or three
segments. No muscles enter these organs; and when the animal is taken
out of its sack, they project straight out behind, instead of being
curled in, like the cirri.
_Anatomical Structure._--The animal's body is attached to the two
scuta by the adductor scutorum, and by the other usual muscles running
towards the mouth, and surrounding the prosoma. The whole external
covering or shell has no other muscles; Verruca thus differing from the
Balanidæ and Lepadidæ; but the shell is attached all round, near its
circumference, to the basal membrane, by a band of very short fibres,
appearing like muscles, but really ligamentous, as determined for me
by Professor Quekett. _Branchiæ_ are entirely absent. The _alimentary
canal_ presents all the usual characters, but in the prosoma is rather
abruptly bent back on itself. The orifices of the two _olfactory
pouches_ are not at all prominent; they are placed directly under the
outer maxillæ, (homologically in their middle segment), just above
a small, medial, tongue-like apodeme. The orifices of the _acoustic
sacks_ appeared to be in their usual position beneath the basal
articulations of the first pair of cirri. The _vesiculæ seminales_
occupy their usual position in the prosoma; they are not much
convoluted; they unite before entering the penis. The probosciformed
penis is imperfectly ringed; it is thick and short, and tapers much
more abruptly than is usual; it supports a few very thin hairs. The
_ovarian cæca_ are spread over the basal membrane, at the bottom of
the sack; hence they in fact lie almost on one side of the animal: they
consist of two main trunks, proceeding out of the animal's body at the
rostral end of the sack, which then branch and inosculate. In specimens
of _V. Strömia_ collected by Mr. Peach for me, in Cornwall, during the
first week of April, there were included two ovigerous lamellæ, placed
transversely across the rostral and the carinal end of the sack: the
lamellæ were .11 of an inch in length; they appeared loose and not
attached, as in the Lepadidæ, to any ovigerous fræna. The ova, in their
earliest age, have one end much pointed, and are 8/1000ths of an inch
in length; they become blunter and increase a little in size before
the larvæ burst forth. The larvæ, both during their earliest stage and
after the first moult, have been excellently figured and described[137]
by Mr. C. Spence Bate: they present no particular characters distinct
from the larvæ of other Cirripedes. I will only further add, that
the structure of the prehensile antennæ still adherent to the basal
membrane, indicates that the larva in its last stage,--that is the
locomotive pupa,--has a normal character.
[137] 'Annals and Mag. of Nat. Hist.,' 1851, Pl. 7, fig. 8-10.
_Affinities._--These have been sufficiently discussed under the family;
I need here only remark that all the species, with the exception of _V.
nexa_, are intimately allied together.
_Range--Habits--Geological History._--The genus Verruca ranges, being
represented by four species, from Iceland to Cape Horn. The species
that is found in Tierra del Fuego extends up the west coast to Peru.
Our northern form, _V. Strömia_, (if I may trust a specimen in the
British Museum, apparently ticketed in an authentic manner), occurs
also in the Red Sea; and this is the only locality in the eastern
hemisphere whence I have seen this genus. The species seem generally
to live in rather deep water: I procured _V. lævigata_ from nineteen
fathoms, on the east coast of Patagonia: _V. Strömia_ is found,
according to information given me by Professor Forbes, on the British
shores, between five and fifty fathoms, and on the steep shore off
Mull, in ninety fathoms; but Mr. Thompson assures me that he once
saw it adhering to tidal rocks and likewise to some floating bark.
Generally the species are attached to living organic bodies, especially
shells of Mollusca and of Cirripedes, to Gorgoniæ, and Laminariæ;
less frequently to rocks. We shall immediately see that it has slight
powers of excavation. This genus is geologically older than any true
sessile cirripede or member of the Balanidæ: _V. Strömia_ is found in
the Glacial Deposits and in the Red and Coralline Crag of England:
another species (in a state not to be identified) occurs in the ancient
Tertiary formations of Patagonia; and another in the Chalk of England
and Belgium. The fact of this Family ascending to a Secondary epoch
accords, in an interesting manner, with its affinities; inasmuch as
though in appearance a sessile cirripede, it is almost equally related
to the Lepadidæ and Balanidæ, and is more nearly related to the
Lepadidæ than to the Balaninæ, or typical members of the Balanidæ: of
the latter, none have hitherto been found in any Secondary deposit,
whereas the Lepadidæ culminated during the Cretacean period.
_Powers of Excavation._
My attention was called to this subject by Mr. Hancock, whose excellent
researches on the boring of Mollusca are well known. _Verruca Strömia_,
when attached to shells destitute of an epidermis, excavates, as he
informed me, a slight depression, deepest in the middle; but when the
epidermis is present no effect whatever is produced. We shall presently
see that the central depression is in some degree distinct from that
of the circumference. I have since found Mr. Hancock's observations
strictly applicable to _V. lævigata_, _V. Spengleri_, and to an ancient
tertiary species from Patagonia. From having found that the following
cirripedes, viz., Lithotrya, Alcippe, and Cryptophialus, all form
their deep excavations by mechanical means, and from having read the
above-mentioned memoirs by Mr. Hancock on the boring of mollusca, I
was strongly impressed with the idea that the action in Verruca would
likewise prove mechanical: but from the following facts I have come to
the conclusion that the excavation must be due to a solvent, probably
poured out from the cement-ducts, which debouch on the under side of
the basal membrane.
In the first place, an epidermis, as just stated, perfectly preserves
the shells of the various species of mollusca and certain cirripedes,
to which I have seen Verruca attached: this is well shown by comparing
the effect produced on the same shell in parts covered by the epidermis
and in parts whence it has been abraded; or where the shell of the
Verruca had fixed itself, whilst very young, within a crack in the
epidermis, and had subsequently, by its growth, turned up the edges,
and had then acted on the underlying shell; whereas the specimens
attached to the sound epidermis had not produced the smallest effect.
Again, I have seen an epidermis-covered mussel-shell encrusted by a
hard nullipora, on which _V. lævigata_ was attached; and here the
calcareous nullipora, under the middle of the basal membrane, was
entirely corroded away, whilst the underlying epidermis and the shell
beneath it, were not in the least affected. The protection afforded
by the epidermis is still more strikingly shown by contrasting shells
with very sharp prominent ridges, when thus invested and when naked,
to which Verrucæ have been attached: I have given a figure (Pl. 21,
fig. 6) of a piece of an invested Venus, from the surface of which a
_V. Spengleri_ had been just removed; on the other hand, I have seen a
Peruvian Discina in which even sharper ridges, covered by epidermis,
were left absolutely untouched, although projecting deeply into the
shell of an attached _V. lævigata_. I have seen several specimens of
this latter Verruca (which has the power of corroding naked shell as
deeply as its congeners), attached to the membrane-covered variety of
_Balanus lævis_, the shell of which was thus perfectly preserved: now
this membrane is little more than the 1/2000th of an inch in thickness;
it is not hard, and so brittle that it generally separates with the
Verruca, leaving the underlying shell of the _B. lævis_ with its
lines of growth glossy and perfect: it appears to me impossible that
a membrane so thin and brittle could resist an action, if mechanical,
which has worn away from twenty to forty times as great a thickness of
hard shell; but the thinnest film of any matter on which acid does not
act, as of grease in certain forms of printing, will perfectly preserve
the underlying substance, and as I have ascertained by putting on a
drop of acid, is the case with this membrane. I have removed several
scores of shells of _V. Strömia_ from the stems of Laminariæ, and when
the latter were washed and slightly dried, generally not the least
effect could be seen, except that the spots where the shell had adhered
were glossy from the still adherent basal membrane: yet the stems of
Laminariæ are far from hard. In some cases, however, the attachment of
the Verruca seemed to have produced a very slight depression on the
Laminaria, but this, I think, may be safely attributed to the growth
of the surrounding surface; for I have seen exactly the same effect
produced by the attachment of the discs of the antennæ of a Lepas,
and these discs, with their long spines, could not possibly produce
any excavation; nor is Lepas or its pupa in any case a burrowing
animal. Again, I have seen a few specimens of Verruca attached to
Gorgoniæ, and they had not acted in the least on the horny axis. I
have examined numerous specimens of _V. Strömia_ attached to three
pieces of slate-rock, and to one piece of red sandstone, all from
different localities, and no effect whatever had been produced; yet
the slate-rock, especially in one instance, was soft. Mr. Bate, to
whom I am indebted for some of these specimens, also informs me that
he could discover no impressions on the slate-rocks, whence specimens
of the Verruca had been removed. On the other hand, I have had two
specimens of limestone, with attached Verrucæ, one coarse and very
impure, and the other hard and marble-like; and in both cases there was
a distinct central slight cavity, including loose gritty matter. The
loose particles evidently resulted from the unequal action either of a
solvent or of some mechanical power on the rock, for it is improbable
in the highest degree that the shells should have fixed themselves
exactly over small collections of loose particles, even if such could
possibly have remained on projecting surfaces of sea-washed rocks.
The above facts seem to indicate pretty plainly that the excavation
of the support does not depend on its hardness, but on its containing
calcareous matter, liable to be acted on by some solvent: but as this
view, considering what we know of Lithotrya and of the two other
burrowing genera of cirripedes to be hereafter described, appears
improbable, I will add a few additional observations. I most carefully
examined the shell and basal membrane of Verruca, and likewise the
tissues left after the dissolution of the shell in acid, and could
detect no structure at all fitted for boring; and what appears more
important, there was no apparent difference in the state of the
specimens which had and had not excavated a hollow; and this, I think,
would certainly have been the case (as in Lithotrya) if the action had
been mechanical. It is not easy to ascertain, owing to the small effect
at any time produced, at how early an age Verruca begins to act on
its support; but I found two sets of specimens only 1/20th of an inch
in basal diameter, which had certainly commenced. The ribbed shell,
(Pl. 21, fig. 6), especially the middle rib, shows, in a somewhat
exaggerated degree, the _typical_ form of the excavation; it may be
here seen that the excavation is of the same depth for some little
distance from the circumference towards the centre, but that in the
middle it suddenly becomes deeper. I have seen several specimens with a
central hollow, without any, or with scarcely any, marginal depression,
and likewise the reversed case. These several facts show that the
central excavation cannot be due to an equable action, prolonged
during the whole growth of the shell, having thus affected the middle
more than the circumferential parts, for in this case the excavations
would have sloped into each other. In specimens which have not at all
acted on their support, the whole basal membrane is firmly attached,
as in all ordinary cirripedes, to the supporting surface; but in those
which have acted, the middle portion of the basal membrane is quite
unattached, and the circumferential portion is, I think, less firmly
attached than is usual; but between these two portions, there is a
circular zone strongly cemented to the supporting surface, and which
alone keeps the shell in its place. Now, on the mechanical theory,
to account for the circumferential hollow, the basal edges of the
shell together with the circumference of the basal membrane must be
subjected to movement, but the shell is united to the basal membrane
by corium and by transparent structureless chitine (both of which may
be left out of question) and by a circle of short fibres, which adhere
at their lower ends to the firmly cemented circular zone, and by their
upper ends to the shell; and these fibres have been very carefully
examined by Professor Quekett, and pronounced to be not muscular,
but exclusively ligamentous, and therefore incapable of moving the
edge of the shell. The basal membrane over the central hollow is, as
stated, quite loose: its lower surface, formed by a reticulated layer
of horny cement-tissue, shows no signs of abrasion, and the membrane
is so brittle and tender, that in specimens which have been once dried
and then well soaked, it almost invariably cracks when the shell is
removed, owing to its mere adhesion to the delicate inner tunics of
the sack; yet on the mechanical theory, the wearing of the central
hollow must have been caused by the action of this middle portion of
the basal membrane,[138] which, it may be repeated, is destitute of
muscles. From the presence of the prehensile pupal antennæ, enveloped
in cement, nearly in the centre of the basal membrane, it is certain
that this spot was originally attached to the supporting surface, and
has since been detached from it; as, moreover, the central hollow goes
on increasing in diameter with the growth of the shell, it is certain
that the inner edge of the firmly attached circular zone of basal
membrane must likewise continually go on becoming detached: it may,
then, be asked by what force can the basal membrane, seeing that it
is united to its own shell above only by fibres of ligament near the
circumference, be continually torn away from the underlying support,
to which it is strongly cemented? On the other hand, on the theory
of a solvent slowly poured out from the cement-ducts, its separation
from its support is simply explained. It might be supposed that the
calcareous matter, when dissolved, would not be able to escape from the
central hollow, owing to the basal membrane being so firmly cemented
all round it; but the attachment is by a reticulated layer of cement;
and I infer that it must be permeated by open passages, from the fact
of the hollow being often filled, in dried specimens, by a bubble of
air, instead of the basal membrane being pressed closely down into the
hollow, as would have been the case had the hollow been hermetically
sealed up. I have seen a few instances in which the bottom of the
central hollow was occupied, (as was remarked to me by Mr. Hancock),
by a little chalky and gritty matter; and in the case of one of the
specimens of calcareous rock, before alluded to, by coarse grains and
oxide of iron; this seems quite compatible with a solvent acting more
readily on certain parts of the rock or shell than on other and less
soluble parts or particles.
[138] Mr. Hancock suggests to me that the basal membrane, on the
mechanical theory, need not itself move; the motion of epithelial
scales, were they transferred into cutting agents, might be
supposed to be sufficient. But of such scales, though I used very
high powers, I could see no trace; and their presence on the under
side of the layer of cement seems hardly possible. Moreover,
according to Von Siebold ('Anatomie Comparée,' tom. 1, p. 412),
ciliary action has not been observed in any Crustacean, or indeed
any Articulate animal. This same statement is likewise made in
Annals and Mag. of Nat. Hist. 1854, p. 136, by Dr. T. Williams.
The greatest depth of the central hollow, in any specimen seen by me,
even measuring from the top of a rib in the case of a ribbed shell,
to the deepest point, was only 1/50th of an inch; but considering how
much depressed the shell of Verruca is, I have no doubt that this small
gain of space is of service to the animal: we must suppose the loose
middle portion of the basal membrane is stretched slightly, or splits
and is repaired, so as to fit the hollow. With respect to the even
much slighter circumferential excavation, it barely equals in depth
the thickness of the extreme edges of the walls; it must, I presume,
give strength to the shell when _laterally_ pushed; but it certainly
appeared to me that the individuals which had excavated a depression
for themselves, could be pried vertically up much more easily than
those which had not acted on their support. Finally, we must suppose
that the hypothetical solvent is poured out of the cement-ducts at
the extreme circumference of the basal membrane, which is almost
loose and destitute of cement, so as to slightly corrode outwards and
downwards the calcareous support; the action here then stops, and this
rim of basal membrane becomes, after a new rim has been formed and
as the shell grows outwards, firmly cemented down to the now slightly
excavated surface of attachment; but during all the time the solvent
goes on acting in the middle, and continues, during the whole growth
of the shell, to encroach on and dissolve the supporting surface from
under the inner edges of the previously cemented down, circular zone of
basal membrane. I have discussed this subject at considerable length,
as it appears to me an interesting one. In this case we have the action
of ciliæ[139] and of respiratory currents, to which in the case of
Mollusca so much has been attributed, entirely eliminated. It is, also,
an interesting fact, that within the same Order we should have some
Cirripedes boring by simply mechanical means, and others by a chemical
solvent.[140]
[139] See the previous note to p. 516.
[140] The solvent may be carbonic acid gas, as suggested by Mr. C.
S. Bate in the case of Mollusca ('Report of British Association,'
1849, p. 73), but here, under the basal membrane, we cannot have
the respiratory currents, or the ciliary action (see note, supra),
as likewise suggested by Mr. Bate.