XENOBALANUS GLOBICIPITIS. _Steenstrup._ Videnskabelige Meddelelser
fra den Naturhist. Forening i Kjöbenhavn, for Aaret, 1851. Tab.
3, fig. 11-15.
_General Appearance._--The shell is in an almost rudimentary
condition, and appears like a small white irregular star, imbedded up
to its top in the skin of the porpoise. Out of this thin, star-shaped
shell, a cylindrical, flexible, peduncle-formed body springs, which
forms the main part of the animal; it is narrow where coming out of
the central cavity of the star, but soon acquires its full diameter;
at the upper end it has a reflexed hood, and hence is broader,
and this has the appearance of forming a capitulum, like that of
a pedunculated Cirripede. This pseudo-capitulum is formed by a
membranous reflexed collar or hood, which is very narrow at the lower
end of the orifice, close under the mouth, and becomes wider and
wider towards the upper and carinal or posterior end of the orifice;
hence the lower reflexed edge of the hood is only slightly oblique or
even nearly transverse. The orifice leading into the sack is large,
and nearly in the same straight line with the peduncle; it is a
little hollowed out in the middle at the upper end, and on each side
of this medial hollow, there is a small rounded projection or horn,
not perforated, but hollow, as may be seen by turning up the hood and
looking at its under side. These two little horns curiously bring to
mind the ear-like appendages in _Conchoderma aurita_ (_Otion_), but
these latter are perforated, open into the sack, and point outwards.
The peduncle-formed body answers, as we shall presently see, to the
main part of the shell in Tubicinella, and the hood, as it would
appear, to the lips of the sack-aperture, which project between its
scuta and terga; of these valves there is not here a trace. The
whole surface is smooth, and is formed by rather thin membrane, of
an orange colour; but from the colour of the underlying corium, the
whole appears of a dark chocolate red, the reflexed hood being rather
lighter coloured. It is singular how closely the colour resembles
that of some dark varieties of the above-mentioned _Conchoderma
aurita_, and likewise of _Anelasma squalicola_, both pedunculated
Cirripedes, having oceanic habits, and destitute, to a remarkable
degree, like Xenobalanus, of shelly valves.
The largest specimen which I have seen was very nearly two inches in
length: in this specimen the star-shaped shell measured, from extreme
point to point, nearly a quarter of an inch in diameter, but the
internal cavity only about one eighth of an inch. This latter measure
gives also the diameter of the peduncle, where coming out of the
shell; the diameter just beneath the hood, was in this same specimen
rather more than a quarter of an inch, and therefore greater than the
diameter of the points of the shell. The depth of the shell from the
upper rim to the basal membrane, in one specimen which I measured,
was only one twentieth of an inch, and this specimen had its
pseudo-peduncle one inch and three quarters in length, consequently
thirty-five times as long as the shell was deep.
_Structure of Shell._--The almost rudimentary shell (fig. 4
_b_) consists of a small, thin, six-rayed disc, formed of six
compartments, each of which, instead of being outwardly convex, as
in ordinary Cirripedes, is deeply bowed inwards. The narrow sutures
(_s s_) separating the six compartments, run along the middle of
the six rays, each ray being composed of the bowed ends of the
walls of the adjoining compartments. The rays are a little curved
towards the carinal end of the shell. It is remarkable that the
rostrum is smaller and less deeply folded inwards than the other
compartments, and the lateral compartments are a little smaller
than the carino-lateral compartments, which is exactly the reverse
of what is usually the case. Only about four zones of growth have
been preserved in any specimen, and consequently the shell is very
nearly of the same diameter at the top and bottom; for the upper end
of the shell is rapidly removed, as in Tubicinella, by the scaling
off of the upper rims of the sheath, and by the disintegration of
the walls. The zones of growth are commonly not piled exactly over
each other, but rather obliquely, as represented in fig. 4 _b_. Each
zone projects, forming a prominent, sharp, toothed ridge round the
shell. In Coronula (Pl. 16, fig. 6) and its allies, the outer lamina
of the wall is formed by the union, a little above the basal margin,
of ledges running along the sides of the longitudinal septa. In
Xenobalanus (Pl. 17, fig. 4 _c_) similar ledges are less perfectly
joined, and apertures seem always to be left in transverse rows
under the transverse toothed ridges, which latter are best seen
in fig. 4 _b_. The apertures, of course, are covered by membrane.
The transverse ridges are surmounted by knobs arising from the
longitudinal septa; and the knobs themselves are capped by other
little heads, which are not represented in the drawing. Owing to
these projections, and to the prominence of the transverse ridges
and of the longitudinal septa, the external membrane is attached so
firmly to the shell that even with the aid of caustic potash it can
hardly be separated.
The internal cavity of the shell is small: it approaches a hexagon
in shape, with the rostral side very short, and the lateral sides
curved inwards. It is lined by a rather thick sheath, which descends
very near to the basal membrane; the sheath is divided into very
distinct, successive convex zones of growth. The external membrane
of the pseudo-peduncle is attached with remarkable strength to
these rib-like zones of the sheath. The alæ (_a_ in fig. 4 _b_) are
represented by mere angular shoulders, received into very slight
notches, and placed at the inner ends or entrances of the double
walls, or rays as I have called them. With respect to the radii, they
also are in an extremely rudimentary condition; but a thin layer of
shell, apparently continuous and homologous with the sheath, extends
from the sheath along both sides of each ray, and on the rostral side
(whence the radius ought to arise), about half-way from the end of
the ray, gives rise to a projection or ridge (_d_, in fig. 4 _b_)
which runs from the top to the base of the shallow shell. From this
longitudinal ridge, septa, parallel to the basis, extend to nearly
the extremity of the ray or double wall. These represent the radii;
but they never grow, so as to increase the diameter of the shell.
These radii evidently correspond to the additional or pseudo-radii
in Coronula, which in that genus lie between the parallel, as here,
and folded parietes. Of the true radius, having the same thickness
as the paries, I here saw only traces in an internal, very slight,
longitudinal ridge running up the shell, close to the outer extremity
of each ray or double wall.
The membranous _Basis_ is united all round to a rather wide flap of
membrane which is inflected from the outer surface of the shell. The
cement-glands appear to be mere enlargements of the cement-trunks,
which latter extend in two nearly straight and parallel lines. From
each gland two cement-ducts proceed, one of which runs parallel to
the cement-trunk.
The pseudo-peduncle, forming the main part of the animal, has, as far
as external appearance is concerned, been sufficiently described.
The part forming the hood apparently answers to the protuberant lips
of the operculum, and the lower part to the sub-cylindrical shell of
Tubicinella; both shell and peduncle in the two genera being wider
at top than at bottom. If in imagination we chip away (an action
always in progress) the whole upper part of the shell of Tubicinella,
leaving only two or three zones of growth at the base, we shall
convert it into a Xenobalanus, with every internal part and organ
occupying the same relative position: for it should be borne in mind
that the shell of Tubicinella is lined close down to the basis by
the opercular membrane, and this is strictly comparable with the
outer membrane of the pseudo-peduncle of Xenobalanus. The body, as
in Tubicinella, is attached in a vertical position, with the longer
axis of the thorax and of the much elongated prosoma extending in
the direction of the longitudinal axis of the pseudo-peduncle. The
point of attachment of the body to the lower part of the hood is
much elongated, but presents the usual muscles running to near the
base of the labrum and embracing the prosoma. The adductor scutorum
muscle is well developed, and is placed close beneath the mouth,
where the collar or hood commences folding over: hence, no doubt, the
lower end of the large and long orifice into the sack can be closed
by the adductor; but the upper end probably can only be blocked up
by the outer surfaces of the curled cirri. The scuta, I may add,
if such had existed, would certainly have covered the point where
the adductor muscle is attached to the exterior membrane. The sack
extends down to within the almost rudimentary shell: the tunic lining
it is unusually strong; indeed, in the reflexed hood-like portion,
it is as strong as the properly external membrane. The latter seems
to be moulted in large strips, and not in a single piece, like the
opercular membrane in Balanus: just above the shell, fragments of
three or four of these outer coats are retained by their very firm
attachment to the sheath. The two layers of corium, lining the outer
membrane of the pseudo-peduncle and the inner tunic of the sack,
instead of being, as usual, united almost continuously together,
stand some little way apart, and are connected by longitudinal septa;
hence, in a transverse section, especially of the lower part, the
sack is surrounded by an irregular ring of square tubes of corium.
The muscles surrounding the sack and imbedded in the inner fold of
corium, are very thinly spread out; they branch, and even sometimes
cross each other; they are more numerous at the carinal and rostral
ends, but certainly cannot be said to form six (or five) bundles, as
in all other sessile cirripedes. Some of the fasciæ extend down to
the very basis, and some up to the summit, to near the two little
horn-like projections. I could not perceive any transverse striæ on
these muscles. Altogether, they are very weak, and cannot have much
power in moving the whole peduncle-like body.
_Branchiæ._--These are largely developed: they are attached to two
approximate, longitudinal, fleshy crests, which extend more than
half-way down the sack, along the carinal margin. Each branchia is
double, the two folds being united where attached in a transverse
line across the sack, on a level with the attachment of the body.
The inner fold is much smaller than the outer; not extending half so
far down the sack, and not extending so far transversely; it is also
hardly at all plicated. The larger and plicated fold extends down
considerably below the lower end of the prosoma, and altogether fully
equals one third of the entire length of the animal, measured from
the shell to the summit of the orifice. Both folds are formed of very
delicate membrane.
_Mouth._--Labrum unusually prominent, as measured from its basal
margin to the crest, which is but slightly notched, hairy, and
without teeth. Palpi broad, heart-shaped, clothed on their inner
sides by a thick brush of spines, which here, as on the other
gnathites and cirri, are almost all doubly serrated. On the outer
margin of the palpi there are a few longer spines. Mandibles villose,
with five teeth, of which the fifth is very small and of irregular
shape: the inferior angle is broad and pectinated. There are no
intermediate teeth between the second, third, and fourth teeth, as
in the three foregoing genera. Had I not known that the lower main
mandibular teeth were always laterally double in the Balaninæ, and
had I not observed how obscure this structure was in Coronula and
Tubicinella, I should have overlooked the merest vestiges of double
teeth in the present genus; indeed, in some specimens the teeth
seemed to be absolutely single. The maxillæ are villose: their edge
exhibits a trace of being notched under the two great upper spines.
The outer maxillæ are bilobed, but not very plainly: between these
organs there is no little prominent mentum, as in the three previous
genera.
_Cirri._--The cirri are short, particularly the three anterior pairs.
The segments in all are singular, from being so much compressed, so
short, and of such great breadth; they are protuberant in front.
In the second and third cirri, the broad _lateral_ faces of the
segments, with the exception of the posterior face of the posterior
ramus, are almost bare of spines. In the three posterior pairs of
cirri (Pl. 29, fig. 6), the segments are protuberant in front, and
support three pairs of short thick spines, with an intermediate tuft;
the dorsal tufts are unusually small: their pedicels are remarkable
from the upper segment, and the upper part of the lower segment,
being produced into a rounded protuberance, dotted with spines: I
have met with a similar structure only in _Scalpellum vulgare_. In
the third pair (fig. 5) there is only a trace of this structure; and
in the second pair the anterior margin of the pedicel is straight,
and clothed with three tufts of bristles. The pedicel of the first
cirrus is very broad, and clasps the mouth.
The cirri and mouth are dark chocolate red, like the outside of the
animal and the upper part of the sack. The thorax is redder and
paler. The four posterior articulations of the thorax are straight
and transverse; the next segment, or that corresponding with the
second pair of cirri, is slightly inflected, in the usual way,
towards the prosoma. The prosoma is pale coloured, extraordinarily
elongated, and bluntly pointed; it extends down (see the dotted
outline in Pl. 17, fig. 4 _a_) about one third of the length of the
whole animal. The orifice leading into the acoustic sack forms a
freely depending little point beneath the basal articulation of the
first cirrus. The stomach in the uppermost part is deeply and closely
plaited longitudinally, but has no cæca; it runs down (externally
coated, as usual, by the testes) to the lower point of the prosoma,
and is then doubled back on itself, so that it is very long.
_Generative System._--The probosciformed penis is short and thick,
and covered with very minute tufts of bristles: there is no
knife-edged projection at its dorsal base. The vesiculæ seminales are
much convoluted and of great length. The ovarian tubes form a small
sheet within the rudimentary shell, in the normal position, over
the basal membrane; and likewise higher up between the two folds of
corium surrounding the pseudo-peduncle; they do not, however, appear
to occur round the lower part of the peduncle: they extend highest
on the rostral and carinal sides, and lowest on the two lateral
faces. The ova are wonderfully numerous; they are 15/2000ths of an
inch in length: they form, instead of two thin lamellæ, two almost
cylindrical packets, which are held together by most feeble membrane.
Each packet, in the upper part lies between the two folds of the
branchiæ; and in the lower part, is embraced only by the larger outer
fold. The two packets of eggs sometimes cohere together at their
lower ends.
_Imbedment._--The shell is imbedded up to its summit, but the shell
is very shallow. The imbedment seems due either to the compression
of the epidermis of the porpoise, or to its formation beneath the
shell having been checked; the outline of the true skin under the
dark-coloured epidermis is not in the least affected. The epidermis
fills up the bay-like spaces formed by the inwardly folded walls, and
firmly adheres to them.
_Affinities._--This genus presents very varied affinities to
Tubicinella, Coronula, and Platylepas. To the latter it is more
especially allied in the compartments, being singly folded inwards,
though the sides of the folds are not here closely pressed together,
as in Platylepas: in both these genera the fold is less deep in the
rostrum, of which fact, in Platylepas, the final cause is evident,
but here there seems no cause, excepting the simple one of affinity.
Xenobalanus is further allied to Platylepas, in the lesser size of
the inner fold of each branchia, compared with the outer, and in
the structure of the cement-glands, and to a certain extent in that
of the sheath. To Coronula the special alliance is shown by the
remarkable character of the pseudo-radii lying between the parallel
and adjoining walls, and in the general character of the cirri: in
Coronula we have the terga sometimes quite aborted, and the scuta
of small size, thus exhibiting a tendency to the entire absence of
opercular valves, so remarkable in Xenobalanus. To Tubicinella, the
alliance is still more plainly shown in the external shape of the
whole animal, wider at top than at bottom,--in the opercular membrane
descending almost to the very base--in the relative positions of the
different parts and organs--in the upper end of the shell continually
scaling off--in the prominence of the transverse external ridges--in
the sheath being divided into successive zones of growth, and being
prolonged nearly to the basal membrane--and in the edges of the alæ
being smooth. Internally, the resemblance is also plainly shown,
in the strength of the internal tunic of the sack--in the branchiæ
springing from two approximate fleshy crests--in the freely depending
acoustic orifices--in the form of the thorax and prosoma--and
consequently of the alimentary canal.
Seeing the state of the almost rudimentary shell, it is not difficult
to imagine its total disappearance. Thinking of this, it occurred
to me to doubt for a few minutes, whether Anelasma, described in my
volume on the Lepadidæ, which presents many points of resemblance with
Xenobalanus,--viz., in colour, in the strength of the internal membrane
of the sack, in the size of the orifice with its thin membranous
margin, in the entire absence of opercular valves, and in epizoic
habits,--might not in truth be a sessile cirripede, in an extremely
altered condition; for the cirri of Anelasma are in so rudimentary a
state, and the mouth in so modified a condition, that the internal
characters by which pedunculated cirripedes can be distinguished from
the Balaninæ, though not from the Chthamalinæ, are almost lost. But
if Anelasma had belonged to either of the two sub-families of the
Balanidæ, the sack would have penetrated almost to the bottom of the
peduncle; there would have been only a single layer of longitudinal
muscles round the peduncle; and there would not have existed ovigerous
fræna. I believe that Anelasma and Xenobalanus are only analogically
connected, being no more related together by true affinity, than any
other two genera in the Lepadidæ and Balanidæ.
_Sub-Family_--CHTHAMALINÆ.
_Shell with the rostrum having alæ, but without radii: rostro-lateral
compartments without alæ on either side: parietes not porose._
_Mouth with the labrum bullate; palpi hardly touching each other: third
pair of cirri with the segments resembling those of the fourth pair._
The shell in this sub-family consists of four, six, or eight
compartments, with the addition, in Catophragmus, of several whorls
of supplemental compartments or scales, like those forming the lower
part of the capitulum in Pollicipes. The rostrum has alæ and no
radii; in shape and size it resembles the carina. The rostro-lateral
compartments are destitute of alæ; in all cases they overlap the
adjoining compartments, and have radii (when such are developed) on
both sides. In the genus, Pachylasma, however, the shell must be
looked to very young, in order to detect this normal structure, for
soon the true rostrum and rostro-lateral compartments become blended
together, making a compound rostrum, destitute, as in the Balaninæ,
of alæ, but furnished with radii. The parietes are never porose, nor
furnished with regular symmetrical ribs (representing the longitudinal
parietal septa) on their inner surfaces: sometimes, however, on the
basal internal edges, there are some irregular depending points. In
conformity with the simplicity of the parietes, the Radii (which are
seldom much developed) have likewise a simple structure; and often are
merely formed by the simple lateral growth of the parietes. The scuta
and terga are articulated together more deeply than is usual in the
Balaninæ; and the terga never have a long spur.
Looking to the animal's body. The Labrum is always swollen and
bullate, and though hollowed out, is never notched in the middle. The
Palpi are rather small, and have not their tips nearly touching each
other. The mandibles generally have their lower main teeth laterally
single, though sometimes they are double: the inferior angle is always
pectinated. The maxillæ are always notched under the upper pair
of great spines. Of the _Cirri_, the third pair much more closely
resembles, in external structure and powers of movement, the fourth
than the second pair, though sometimes a few of the basal segments,
especially on the anterior ramus, are thickly covered with bristles, as
on the segments of the second pair. In three species belonging to two
genera, there are caudal appendages, resembling those generally present
amongst the Lepadidæ, but never found in the Balaninæ. The Branchiæ
are generally smaller than in the Balaninæ; and are sometimes quite
rudimentary.
In the introductory description (p. 152 and 176) of the Family
Balanidæ, I have already discussed the relations of the Chthamalinæ to
the Balaninæ, and of the several genera to each other, so that I need
not here re-enter on the subject.