Chapter XXV: Introduction (21)

_Scuta and Terga._--These are narrow and long, stretching the whole length of the orifice of the shell. The two valves touch each other, but are not properly articulated together: their upper surfaces scale off in layers: they resemble each other very closely in shape (fig. 1 _c_), being simply oblong. The membrane covering the valves supports some very minute spines, 2/1000th of an inch in length. When a portion of valve is dissolved in acid it is seen to be penetrated by very numerous tubuli. The opercular membrane is rather thick: in _P. decorata_ I found it occasionally double; in this species it supports some very minute spines; it is attached about one third down the sheath, of which the upper part is not marked (as in Balanus) by regular concentric rings. The aperture between the valves, leading into the sack, has protuberant lips, as in Coronula and Tubicinella. The normal five bundles of muscles surrounding the sack, differently from in the two genera just mentioned, do not thin out and spread out downwards, but retaining their full thickness, reach the basal membrane, to which they are attached.

_Mouth_: the parts present no difference from those in Coronula; between the outer maxillæ we have the same small flattened mentum-like projection; and in the mandibles, the same small intermediate teeth between the second and third, and between the third and fourth main teeth, as in that genus. The main teeth here are more plainly double laterally. There are teeth on each side of the central notch of the labrum. The cirri differ little from those of Coronula, but the segments are not nearly so much flattened and broad, and the second and third pairs are not so short. The segments of the sixth pair support four pairs of main spines, with a few minute intermediate spines, but with no intermediate tuft.

_Branchiæ_: these consist on each side (at least in _P. decorata_) of a double fold, much less plicated than in Coronula; the fold nearest to the animal's body is rounder, and not quite so large as the outer fold.

_Range and Habits._--The _P. bissexlobata_ ranges from the Mediterranean and west coast of Africa to the West Indies, to the north-east coast of Australia, and, judging from the name given by Chenu, to California: the second species, _P. decorata_, inhabits the Pacific Ocean; and the third imperfectly known species, was deeply buried in the skin of a sea-snake, off Borneo. The _P. bissexlobata_ is attached both to turtles and to the skin of the manatee: some specimens in the British Museum, attached to the latter, from the west coast of Africa, were entirely buried in the skin, with the exception of the operculum. The specimens of _P. decorata_, which I obtained at the Galapagos Archipelago, were buried about half their depth in the softer parts of the skin of the green turtle. I do not doubt that the imbedment is effected in the same manner as in the case of Chelonobia.

_Affinities._--This genus, in general structure and habits, approaches closely to Coronula; but it differs from all the species of that genus in the following respects, which appears to me fully to justify its generic separation:--The outline is often oval, instead of circular; the compartments are not of equal sizes and shapes; each compartment has here only one inward furrow or fold, and this has its sides pressed closely together, and is encased, but not obliterated, by the sheath; the rostrum is not quite medial; and the radii are narrow and nearly solid. The opercular valves, again, are here more developed, and their microscopical structure is different. The opercular membrane is less constantly double; and, what is much more important, the muscles of the sack are not here spread out, and show no tendency to become rudimentary and lose their transverse striæ. The cirri differ only in not being so broad and flattened. The branchiæ are not quite so much developed. Finally, I may add, that in several of the above respects in which Platylepas differs from Coronula, it approaches ordinary sessile cirripedes.

1. PLATYLEPAS BISSEXLOBATA. Pl. 17, fig. 1 _a_-1 _d_.

CORONULA BISSEXLOBATA. _De Blainville._ Dict. des Sciences
Naturelles, tom. 32, (1824), Tab. 117, fig. 1.

PLATYLEPAS PULCHRA. _J. E. Gray_ (!). Annals of Philosophy, (new
series), vol. 10, (1825).

COLUMELLINA BISSEXLOBATA. _Bivona_ (fide _Philippi_). Nuovi generi di
Mollusch. (1832), Tab. 3, fig. 1.[126]

CORONULA CALIFORNIENSIS. _Chenu_ (!). Illust. Conch., Tab. 1, fig. 4.

[126] This memoir was published in the 'Effemeridi Scientifiche e
Litt. per la Sicilia,' according to the 'Bibliographia Zoologiæ et
Geologiæ,' by Agassiz and Strickland.

_Shell with the transverse lines of growth conspicuous: parietes permeated by pores; sheath descending barely half-way down the parietes._

_Hab._--Mediterranean, attached to turtles. River Gambia, attached
to manatee. Honduras, attached to manatee. Moreton Bay, lat. 27°
S., Australia, apparently attached to the dugong of that coast.
California(?) Mus. Brit., Stutchbury, and Cuming.

_General Appearance._--Shell generally much depressed, and broadly
oval or circular; sometimes steeply conical. Orifice oval, generally
not large. Surface rather plainly marked by closely approximate
lines of growth, which with the lobed outline gives to the whole an
elegantly sculptured appearance: occasionally the longitudinal ridges
formed by the parietal septa are distinct. Basal diameter of largest
specimen three quarters of an inch.

_Structure of Shell._--I have nothing material to add to the generic
description. The midribs are not so prominent as in the following
species, and hence the basal membrane is less convex. The origin
of the midrib as a fold is very plain. The sheath descends barely
half-way down the walls, and is a little hollow on its under margin,
on each side of the midrib. Beneath the sheath the parietes are
finely ribbed (fig. 1 _d_), but to a variable degree. I may here
remark, that in the specimens taken from manatee, on the coast of
Africa and at Honduras, the internal ribs extended further up and
were plainer, and the opercular valves seemed to be a little narrower
than in the other specimens, so that I at first suspected that they
were specifically distinct, but I could make out no other than these
small and variable points of difference.

_Scuta_: oblong (fig. 1 _c_), about twice as long as broad, with the
rostral end rounded, rather narrower than the other end, and curled a
little inwards. _Terga_, of nearly the same shape and nearly as long
as the scuta: the carinal end is rather more pointed than the scutal
end of the valve, and when viewed internally, the growing surface
of this end is seen to be bluntly pointed. In both valves the upper
layers of shell usually scale off.

_Mouth and Cirri._ The only differences, compared with the following
species, that I perceived, were that the hairs at the end of the
palpi were here rather longer, and the fourth tooth in the mandibles
rather less distinct. The rami of the first cirrus are very unequal
in length.

2. PLATYLEPAS DECORATA. Pl. 17, fig. 2 _a_-2 _b_.

_Shell with fine longitudinal ridges, ornamented in the lower part by minute beads; parietes not porose: basal membrane equalling in convexity the shell._

_Hab._--Pacific Ocean; Galapagos Archipelago; Lord Hood's Island, Low
Archipelago; Mus. Cuming, Darwin.

_General Appearance and Structure._--Shell oval, with the orifice
large. The walls are thick, and of less height from top to base than
in the last species. The surface (and this is the chief external
character) is marked by fine longitudinal ridges, each of which,
when closely examined, is found to be double (fig. 2 _b_), and near
the basis, where not abraded, to be ornamented with minute beads on
each side: these beads are largest on those ridges which border the
sutures. The parietes are not permeated by pores; on their inner
surfaces there are a few rather prominent but short ridges, instead
of the many finer ridges, as in _P. bissexlobata_. The sheath is of
great thickness; in young specimens layers scale off its summit, all
round the shell, as in Tubicinella; it is only slightly hollowed out
at its lower margin; it descends more than half-way down the walls.
The midribs, formed by the folded walls, are more prominent than in
the last species, and descend lower down; hence the basal membrane is
more convex than in the last species, for it projects downwards as
much as the shell projects upwards. In medium-sized specimens, the
midribs project inwards to a distance exceeding half the breadth of a
compartment. A less proportional length of the midrib, in an inward
direction, is formed by the inward folding of the wall, and a greater
length by the thickening of its inner longitudinal margin, than in
the last species. The basal edge of the inner end of the midrib is
smooth.

_Dimensions._--The average size of the numerous specimens which
I obtained at the Galapagos Islands was about .2 of an inch in
diameter, and I found none larger; but Mr. Cuming's specimens from
Lord Hood's Island are half an inch in diameter.

_Opercular Valves._--These closely resemble those of the former
species, with the exception that the carinal end of the growing or
under surface of the Tergum is much squarer.

_Cirri._--The only difference which I could perceive was, that the
rami of the first pair were not quite so unequal in length.

_Species Dubiæ._

3. PLATYLEPAS ----?

_Hab._--Imbedded in the skin of a sea-snake, taken off Borneo.

I am indebted to Dr. Gray for a single specimen of this supposed species, but as it is very young and imperfect, wanting the opercular valves and cirri, I do not choose to name it. The shell presents all the usual characters of the genus; the rostrum, I may remark, being pushed to the left side. The parietes are permeated by pores of considerable size, which shows that the species is distinct from _P. decorata_. On the inner basal surfaces of the walls, there are two or three very distinct ridges on each side of the midrib; and this fact, together with the size of the parietal pores, makes me suspect that it is not an immature specimen of _P. bissexlobata_.

10. _Genus_--TUBICINELLA. Pl. 17, fig. 3 _a_-3 _c_.

TUBICINELLA. _Lamarck._ Annales du Museum, tom. 1 (1802), Tab. 30,
fig. 1.

CORONULA. _De Blainville._ Dict. des Sciences Naturelles, (1824).

_Compartments six, of equal sizes; shell sub-cylindrical, wider at the top than at the basis, belted by several large transverse ridges._

_Hab._--Southern Pacific Ocean, Western South America, New South
Wales, Cape of Good Hope; imbedded in whales, and often associated
with _Coronula balænaris_.

This genus is closely allied to Coronula, and perhaps De Blainville was right in uniting them. But Coronula, as it now stands, is so natural a genus, that it seems a pity that a form so entirely different in general aspect, as Tubicinella, should be forced into it. The main difference between these genera consists in the walls being here not folded, in the simpler radii, and in the general shape of the shell and of the included animal's body; but there are many other minor points of difference. The most novel character in Tubicinella consists in the shell being lined almost close down to the basis by the opercular membrane. In the opercular membrane thus forming a long tube, and in the general shape of the animal's body, we shall presently see that Tubicinella is closely related to Xenobalanus. In numerous other respects Tubicinella is almost equally allied to the latter genus, to Platylepas, and to Coronula. Finally, several points of structure indicate that Tubicinella may be considered as a Coronula, with the shell much simplified in structure.

1. TUBICINELLA TRACHEALIS. Pl. 17, fig. 3 _a_-3 _c_.

LEPAS TRACHEALIS. _Shaw._ Nat. Miscell. (1789-1813), vol. 17 (1806?)
tab. 726.

---- TRACHEÆFORMIS. _Wood._ General Conch. (1815), tab. 4, fig. 1-3.

TUBICINELLA TRACHEALIS. _J. E. Gray._ Annals of Philosophy, (new
series), vol. 10, (1825.)

---- MAJOR ET MINUS. _Lamarck._[127] Annales du Mus. Nat., tom. 1
(1802), Tab. 30, fig. 1-2.

---- BALÆNARUM. _Lamarck._ Animaux sans Vertèbres, (1818).

---- ---- _Chenu._ Illust. Conch. (Plate).

---- ---- _Sowerby._ Genera of Recent and Fossil Shells (Plate).

---- LAMARCKII. _Leach._ Encyclop. Brit. Suppl., vol. 3 (1824), Pl.
57.

CORONULA TUBICINELLA. _De Blainville._ Dict. des Sciences Nat., tom.
32, Pl. 117, fig. 5 (1824).

[127] It may be observed that I have here broken through the great
law of priority; for it appears to me too grossly incorrect to
retain the specific name either of _major_ or _minus_ in a genus
including a single species. Lamarck himself seems to have been
of this opinion, by giving, in 1818, the new specific name of
_balænarum_; but Shaw's name of _trachealis_ has the clear right to
priority, if _major_ or _minus_ be rejected.

_General Appearance._--Shell elongated, sub-cylindrical, with the
upper end rather wider than the lower, and therefore widening in
a direction the reverse of that usual with sessile cirripedes.
The shell is often a little bent to one or the other side: it is
surrounded by from two or three to about ten very prominent, strong,
blunt ridges or belts, placed at rather irregular distances from
each other. The surface is finely striated longitudinally. The six
compartments are of nearly equal sizes and shapes. In full-grown
specimens the parietes are not wider at the base, and often they are
even a little narrower, than at the summit of the shell: in young
specimens the parietes do widen a little downwards. The radii are
narrow; but in young specimens they are proportionably much broader
(Pl. 17, fig. 3 _b_) than in old specimens. The whole compartment,
including the radius and wall, is always a little wider at the summit
than at the base, in accordance with the shape of the whole shell.
The operculum consists of four nearly equal-sized, similar valves,
projecting above the upper end of the shell, which is always broken
and jagged: the valves are united to the sheath by a very thick,
much folded membrane. The aperture leading into the sack is bordered
by very prominent lips, projecting above the opercular valves; the
latter have their upper layers always scaled off. The shell is
imbedded in the whale's skin up to the level of its operculum. The
largest specimen which I have seen was barely one inch (.95) in
diameter at the summit, and 1.5 in length; the longest specimen which
I have seen scarcely attained a length of one inch and three quarters.

_Structure of the Shell._--The parietes are thin, and if the sheath
(which extends to near the basis) be removed, they are rendered
extremely thin. They are formed by an outer and inner lamina, united
by fine longitudinal septa, projecting at the basis beyond the
laminæ. The pores thus produced (which in a transverse section are
oblong in outline), run up to the summit of the shell, and are not
filled up by shelly matter; but I presume that the included tubular
threads of corium are protected, at the broken upper end of the
shell, by transverse membranous septa. The outer lamina of shell,
as in Coronula, is formed, though obscurely, by the union of ledges
projecting from the longitudinal septa. The circular prominent belts,
surrounding the shell, are formed by the longitudinal septa, at
certain, irregular and rather distant periods, growing outwards; the
wall at each belt being increased to nearly twice its thickness in
other parts. At each belt the threads of corium within the parietal
pores lend off minute branches to supply the thickened wall. These
belts, which continuously surround the shell, correspond (as is best
seen in young specimens), with the little knobs or beads, which, in
Coronula (Pl. 16, fig. 4), rise separately, and not quite regularly,
on the longitudinal parietal septa, and which, I believe, are formed
at every successive period of growth; here they are much larger,
stand in straight transverse rows, become confluent, and are formed
only at occasional intervals. The whole external surface of the shell
is covered by membrane, stronger and more persistent than is usual
with most cirripedes.

Internally the sheath extends almost to the basis of the much
elongated shell, and terminates in a slight shoulder: it is divided
as in common Balanidæ, and differently from in Coronula, into zones
of growth, but these are very broad; at the upper end of the shell,
which, as will hereafter be explained, is always breaking away,
the sheath readily yields along the oblique planes, which separate
the zones of growth and dip outwards: a similar but less strongly
marked structure occurs in Platylepas, and in no other genus. We
shall presently see that the sheath presents a much more anomalous
character, in being lined down to its basal edge by the innermost and
last-formed layers of the opercular membrane.

_Radii._--The radii are narrow. The belts which surround the shell
are prolonged, with slightly diminished prominence, across the radii,
their formation being simply due to the radii being here thicker than
in other parts. It can be seen more plainly in Tubicinella, than in
other Balanidæ, that the membrane externally investing the shell,
splits along the radii during the diametric growth of the shell,
and is continually repaired and added to along these lines by new
longitudinal slips of membrane. The radius consists (as usual) of an
inner and outer lamina, which latter does not extend quite to the
line of suture--a slightly gaping fissure being thus left. The two
laminæ are connected by septa, which are not denticulated, but near
the outer lamina bi- or tri-furcate, and the ends of the branches
thus formed spread out, forming a sort of outer scalloped lamina, in
advance of the true outer lamina. The fine threads of corium running
between these septa, do not spring, as in all common cirripedes, from
a fillet of corium occupying the actual suture, but from two nearly
circular threads of corium occupying two tubes, which run along the
line of junction between the radius and the compartment whence it
springs. In Coronula alone we have a nearly similar structure; for
the fine threads of corium occupying the _proper_ radius, spring from
a _single_ very minute tube (Pl. 16, fig. 7, _d′_), occupying the
same position with the _two_ tubes in Tubicinella. I may further add,
that the structure of the proper radius in Coronula is precisely the
same as here just described, but being on so very minute a scale, I
did not there describe it so carefully as I have here done.

_Alæ_: these are only remarkable from their extreme thinness; for
they are not thicker than the inner lamina of the radius. Their
sutural edges are quite smooth. Forming part of the sheath, they
extend down close to the basis of the shell; where, instead of, as in
general, ending abruptly in a rectangular shoulder, they slope off
into their own compartment.

_Basis._--The basal membrane is complicated, owing to the shell, when
full-grown, barely, or not at all, increasing in diameter, and, in
consequence, membrane after membrane, each with its own cement-ducts
attached to it, are thrown down one nearly over the other. In the
Introduction (p. 143--Pl. 28, fig. 3), I have fully described the
cementing apparatus, which is very curious from one of the ducts
always having a loop with two spurs projecting from it. The basal
membrane does not equal in diameter the base of the shell, for the
membrane externally covering the walls is inflected inwards all round
for a considerable width, and is then united to the basal membrane:
in Coronula, the basal membrane extends only under the internal
cavity of the shell, and not under the folded walls, and therefore
presents a somewhat analogous structure.

_Opercular Valves._--The scuta and terga are nearly of the same size
and shape: they are mitre-formed, and higher than broad. They do not
fill up the orifice of the shell. The scutum is a little larger than
the tergum, and rather less symmetrical, the rostral corner of the
valve being a little produced. There is no hollow or crest for the
adductor muscle, which is small. In the tergum there is no trace of
a spur. The two valves are not articulated together, but standing
close to each other are united, as well as the scutum to the scutum,
and tergum to the tergum, by thick, brown, tough, yet soft membrane,
in layers continuous with, but differing in appearance from, the
surrounding opercular membrane. The layers of shell, forming the
valves, are thick, and only the three or four lower layers are
usually preserved, the upper ones having symmetrically scaled off,
leaving snow-white surfaces. Owing to the thickness of the successive
shelly layers, and to the circumstance of each new layer being but
very little larger than the last, the scaling off of the old upper
layers is a quite necessary process; for otherwise the orifice into
the sack would have been encumbered and almost closed by four long,
slightly tapering points, prolonged upwards from the basal layers
that form the four existing valves. The same scaling off process
takes place in Platylepas, and amongst pedunculated cirripedes in
Lithotrya. Microscopical examination does not exhibit any fine
spines on the membrane investing the valves, or any tubuli in the
shelly layers after their dissolution in acid: in this respect the
valves resemble those of Coronula. The summits of the valves project
freely for about a third of their own height, above the level of
the membrane by which they are surrounded. The orifice leading into
the sack is bordered by very protuberant lips, standing up even
considerably above the upper freely projecting portions of the valves.

The _Opercular Membrane_, connecting the valves and the top of the
shell, is thick and tough, and deeply folded in concentric wrinkles.
As in Coronula, it consists of two or three separate membranes (each
composed of many laminæ) one over the other, united to successive
shelly layers of the opercular valves. As the upper shelly layer
scales off, the membrane attached to it is likewise thrown off. The
innermost laminæ of the last-formed opercular membrane extend down,
closely attached to the sheath, to its basal edge, and therefore
nearly to the basis of the shell; the outer and older laminæ, all
closely attached one within the other and to the sheath, extend to
a less and less distance downwards; consequently, the animal's body
is enclosed in a tube, thinning out downwards, formed by the laminæ
of the successive opercular membranes, surrounded outside by the
shell: only in the following genus, Xenobalanus, shall we meet with a
nearly analogous structure. As the shell of Tubicinella increases in
diameter, from the growth of the radii, the opercular membrane lining
the sheath is necessarily split along the six lines of suture, in the
same manner as is the membrane externally investing the shell; in a
like manner, also, it is repaired and added to by new longitudinal
slips of membrane. Of this structure, in the opercular membrane, I
have seen no other instance; for in most genera the old opercular
membrane is moulted, and a new and larger one formed at each period
of growth; in Coronula, in which the opercular membrane is likewise
for a time persistent, it does not run far down the inside of the
shell, and each new membrane is formed large and extensible, so as
to allow, without splitting, of some increase in the diameter of
the shell. The opercular membrane at the summit of the shell, in
Tubicinella, is folded in concentric lines, and so deeply, that
the basal edges of the opercular valves are generally hidden: this
folding arises partly from each last deposited and innermost membrane
being originally formed slightly folded, but chiefly from the rapid
downward growth of the shell, and the consequent downward movement of
the whole animal's body, together with the opercular valves to which
the body is attached, and this necessarily tends to wrinkle and fold
the opercular membrane. Owing to the opercular membrane extending far
down inside the shell, and being firmly attached to the sheath, as
the upper part of the shell breaks away and disintegrates (which we
shall presently see is constantly taking place), small particles of
shell are left adherent to the circumferential and folded parts of
the opercular membrane; and this at first much perplexed me.

_Muscles of the Sack_: these extend down almost to the base of the
shell, but in the lower part they spread out and become thin and very
irregular, not even corresponding on the opposite sides of the body.
The fasciæ in the upper part show very distinct transverse striæ, but
lower down these become either obscure or entirely deficient. In all
these characters the muscles of Tubicinella and Coronula resemble
each other. The rostral depressor muscles of the scuta consist each
of four small bundles of fasciæ; the lateral depressores run not
quite straight down, but in a curved course towards the carinal end
of the sack: the tergal depressores are proportionally smaller than
in ordinary sessile cirripedes, but they project and form two crests
(with some fasciæ between them), which support the Branchiæ. The
membrane lining the sack, I may here mention, is unusually strong.

_Branchiæ._[128]--These are enormously developed; the two together
covering two thirds of the area of the sack. Each consists of two
folds, both deeply plicated. They are attached longitudinally to
the two crests, including and formed by the muscles running from
the terga to near the basis of the shell. The branchiæ are likewise
attached transversely to the sack, under the basal margins of the
terga.

[128] These have been described by Professor Owen in the second
volume of the 'Descriptive Catalogue of the Museum of the Royal
College of Surgeons.'

_Mouth._--The labrum is very finely hirsute, without teeth; the palpi
have a short row of moderately long spines along their exterior
basal margins. The mandibles have four rather narrow, sharp teeth,
which (excepting the first) have double points: between the second
and third, and again between the third and fourth teeth, there is a
single small intermediate tooth: the inferior angle is irregularly
pectinated. The maxillæ are small; there is a small notch beneath the
two upper great spines, and a second notch near the inferior angle.
Between the outer maxillæ, there is a square-topped mentum. Hence we
see that the mouth in all its few peculiarities, resembles that of
Coronula.

_Cirri._--These are short, with short and broad segments protuberant
in front. The pedicel of the first cirrus is very broad, and
exteriorly clothed with fine hairs: its rami are slightly unequal
in length. The second and third cirri are very short. The three
posterior pairs are remarkable from the pairs of main spines being
placed so close one under the other, and in an oblique direction,
that at first they appear to form a single crowded transverse row:
the dorsal tufts are rather large.

_Body._--The body is remarkable from its nearly vertical position,
and from the much elongated pyramidal form of the prosoma, extending
down nearly to the bottom of the sack. The membrane investing the
prosoma, presents a few circular folds, falsely appearing like
articulations. The œsophagus enters the stomach rather obliquely.
With respect to the generative system, I have only to remark, that
the vesiculæ seminales are of great length, and convoluted to a
remarkable degree. The ovarian cæca, form a thick layer at the
bottom of the sack; they do not appear to extend up the shell round
the sack. The only other point, which I shall here mention, is that
beneath the basal articulation of the first cirrus, there is a
longitudinal swelling, ending in a freely projecting point, .06 of
an inch in length; at first, I thought, that we here had a rudiment
of a filamentary appendage like those found in several Lepadidæ;
but closer examination showed an orifice at the apex, leading into
the acoustic meatus, in which the singular, wrinkled, heart-shaped
acoustic vesicle, mentioned in the Introduction, hangs suspended.
Alongside the freely depending point, with an orifice at its end,
there is a smaller upward projecting point, without any orifice, but
hollow within and lined by corium; I believe it opens internally into
the acoustic meatus.

_Attachment and general Growth of Shell._--All the specimens which
I have seen have been attached in groups. They are buried up to
the level of the operculum in the whale's skin; and their summits,
I suspect, lie even beneath the general surface of the body of
the whale. It is certain that the shells grow much at their basal
ends. As in the case of Coronula, the flat membranous basis does
not actually penetrate the skin; but the general pressure of the
whole group of shells seems to push inwards the skin of the whale,
and directly beneath each shell the formation of new epidermis
is apparently checked. Between the shells, however, though close
together, the epidermis continues to be formed, and is pushed upwards
between them, in the same manner as it is forced into the flattened
cavities on the under side of the shell of Coronula.

The manner in which a full-grown shell assumes its ordinary shape,
at first appears very perplexing:[129] it has to change from a
cylinder, at first probably not much above the 1/50th of an inch in
diameter, to a cylinder nearly one inch in diameter: and this is
not effected by the growth of the radii, for the radii never reach
the basis, and the basis of course has to increase in diameter
like the rest of the cylindrical shell. The radii serve only to
keep the summit of the shell wider than the basis, which is the
natural shape of this species; and in large-sized specimens, this
purpose is sometimes aided by the parietes during their downward
growth decreasing slightly in width. In ordinary nearly full-sized
specimens, the parietes are of the same width at the top and bottom,
but in some large-sized specimens, as just stated, they even become
narrower towards the bottom; as they grow only at the bottom, one
does not at first see how they can ever increase in width, or how
the older shells can have acquired their present diameter. But an
examination of young specimens, from .1 to .3 of an inch in diameter,
at once serves to show how the shell attains its full size and shape:
for here the parietes are all found to increase downwards sensibly
in width, though at a much slower ratio than in other sessile
cirripedes; in larger, but not full-grown specimens, a similar
increase can by care be detected: hence by long-continued growth at
the base of the shell, with the removal of the upper part, a young
Tubicinella of small diameter will be converted into an old one of
large diameter, retaining during all the time its sub-cylindrical
form, with its summit rather broader than its base. With respect
to the removal of the upper part of the shell, this seems almost
constantly going on, for the summit of every specimen invariably had
a freshly broken aspect. The peculiar structure of the sheath, which
is the strongest part of the shell, namely, its division into oblique
layers, separable by a slight force, doubtless is subservient to the
repeated breakage of the summit. In some species of Tetraclita and
of Balanus, gradual disintegration of the upper part of the shell is
a necessary element in the growth of the animal, in order that the
orifice may increase in size, and here we have mechanical breakage
equally necessary.

[129] I am indebted to Dr. J. E. Gray for calling my attention to
this subject, and explaining to me several points.

Some curious results follow from the peculiar growth of Tubicinella
just described. At Plate 17, fig. 3 _b_, we have a careful drawing of
a lateral compartment, together with its radius, (which latter does
not here concern us), taken from a shell .2 of an inch in diameter.
The two protracted dotted lines show the form which this compartment
would have assumed, if it had continued growing downwards at the
same rate of increase in width as hitherto. But the increase in
width always seems to become less and less as the shell grows older;
hence the dotted lines, representing the wall after long-continued
growth, ought to have been drawn diverging or widening still more
slowly than they do. The lateral compartment in fig. 3 _a_ is the
exact size of the compartment of a large specimen nearly one inch in
diameter; in this specimen the parietes, far from increasing in width
downwards, had commenced, as is represented, decreasing. Compartments
of all intermediate sizes between those figured at 3 _a_ and 3 _b_
can easily be shown in different specimens. From these facts we may
safely infer, that if the whole growth of the compartment 3 _a_ had
been preserved, instead of its upper end having been continually
chipped away, it would have had even a more tapering form than that
represented by the whole and dotted lines in the two figures, and
would have exceeded six inches in length! this of course being also
the length of the whole shell. The young Tubicinella, of which 3
_b_ is a compartment, was imbedded in the whale's skin nearly up to
the level of its operculum; if it had lived, it would no doubt have
grown to the length just specified, viz., above six inches, but as
all the growth is at the lower end, the bottom of the shell, it might
be thought, would necessarily have become buried in the whale's skin
to this same depth; and the summit of the shell, on this same view,
would have been buried to a depth by as much less as the height or
length of the old shell itself, namely, by about one inch and a half
less than the six inches. As far as I can judge from an examination
of several large groups of full-grown specimens, preserved in their
imbedded condition, the summits of the shells seem always to lie a
little beneath the surrounding level of the whale's skin, but not
nearly to the extent here just inferred. Nor can I believe that the
epidermis of the whale had ceased being formed under these specimens,
whilst it had gone on being formed all round them, to the thickness
of between four and five inches, and that it had subsequently
disintegrated to this same thickness,--which processes would account
for the summit of the shell being still on nearly a level with the
surface of the whale. The view which seems to me most probable, is,
that the rapid downward growth of the shell, besides indenting the
whale's skin, at the same time slowly pushes the whole shell out
of the skin, and thus continually exposes the summit to the wear
and breakage which seems to be necessary for its existence. On this
view, the very peculiar form of Tubicinella, which is retained during
life, namely, the slightly greater width at top than at bottom,
is beautifully explained, viz., for the sake of facilitating the
protrusion of the shell; for the ordinary conical shape of sessile
cirripedes, with the apex upwards, would have rendered the pushing
out of an imbedded shell almost impossible; on the other hand, we can
see that the likewise very peculiar, concentric, prominent belts may
be necessary to prevent too easy protrusion.

_Fossil Species._--I do not believe that this genus has hitherto
been found fossil. The _Tubicinella maxima_ of Ch. Morren, said to
have been found (see Bronn, Index Palæontologicus) in the chalk of
Belgium, I have good reason to believe does not really belong to this
genus.

11. _Genus_--XENOBALANUS. Pl. 17, fig. 4 _a_-4 _c_.

XENOBALANUS. _Steenstrup._ Videnskabelige Meddelelser. Aaret, 1851.

SIPHONICELLA (sine descript.) _Darwin._ Monograph on the Lepadidæ, p.
156 (1852).

_Shell almost rudimentary, star-formed, composed of six compartments, with a long peduncle-formed body rising from the middle: opercular valves none._

_Hab._--North Atlantic Ocean, attached to Porpoises; Mus. R. T. Lowe,
Steenstrup.

This Cirripede, in appearance the most anomalous of its family, has affinities distinctly pronounced. Four years ago the Rev. R. T. Lowe sent me some specimens, which he had obtained from a porpoise between Madeira and England; and I named them in MS. Siphonicella, from their relationship to Tubicinella,--a fact which I mention only because Sir C. Lyell has alluded to this genus under the above name (without any description), in his anniversary address to the Geological Society, as have I, in my volume (p. 156) on the Lepadidæ. Since that time Professor Steenstrup has described and named the genus, fully recognising its place and affinities, and has most kindly sent me a magnificent group of specimens.

This genus singularly resembles, in general appearance, some of the pedunculated Cirripedes, so much so that in the specimens sent me by Mr. Lowe, in which the almost rudimentary shell was, from disintegration and its deep imbedment, not plainly visible, I did not in the least doubt that I was examining a new genus of Lepadidæ. I may mention, as a proof how truly all the parts and organs are correlated in Cirripedes, that I was at first in despair when I found a species to all appearance pedunculated, with its labrum not bullate, its palpi of large size, its third pair of cirri totally unlike the fourth and succeeding pairs, and with only a single layer of muscles round the peduncle; but when, in addition, I found that there were branchiæ, and that these were double, I felt convinced that I was dissecting a disguised sessile cirripede, and that its true place was near Coronula: soon, I found the imbedded and almost rudimentary shell, of which a mere fragment would equally well have declared the true position and relationship of the whole animal. Though Xenobalanus, in external aspect, is so completely masked, yet in its habits, namely, in living attached on Cetaceans, as in its essential structure, it displays its real affinities. In the course of the following description, it will be seen that in the shell, the affinity is almost equally close to Coronula and Platylepas, but that, considering the whole animal, the affinity is somewhat closer to Tubicinella. Xenobalanus may indeed be described as a Tubicinella without opercular valves,--with the opercular membrane thickened down to the basis,--and with the shell, excepting the few last-formed basal zones of growth, almost wholly removed by the breakage of its upper end; this remnant of a shell, however, presenting some strong points of resemblance to Coronula.

1. XENOBALANUS GLOBICIPITIS. Pl. 17, fig. 4 _a_-4 _c_.

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.