CORONULA. _Lamarck._ Annales du Museum, tom. 1 (1802).
DIADEMA. _Schumacher._ Essai d'un Nouveau Syst., &c., 1817.
CETOPIRUS (SED NON CORONULA). _Ranzani._ Memoire di Storia Naturale
(1820).
POLYLEPAS. _J. E. Gray, (Klein)._ Annals of Philosophy, (new series),
vol. 10, 1825.
_Compartments six, of equal sizes: walls thin, deeply folded, with
the folds forming cavities, open only on the under side of the shell:
opercular valves much smaller than the orifice of the shell._
_Distribution_, mundane, imbedded in Cetacea.
This genus and the three following, namely, Platylepas, Tubicinella,
and Xenobalanus, have very considerable claims to be separated as a
sub-family, as has been proposed by Drs. Leach and Gray. Although in
structure and habits they are certainly closely related together; yet
only few characters can be predicated of all four in common,--some
characters failing in one genus and some in another. All four, however,
differ from the foregoing genera in the opercular valves not being
articulated together, being simply united by tough horny membrane;
but Chelonobia makes some approach in this respect. All four have the
branchiæ composed of a double fold; but this can hardly be considered
a character of much weight, as may be inferred from the remarks made
on this subject at p. 153 of the Introduction, where the differences
of these four genera from the other genera of the family are discussed
at some length. Altogether I have been led to conclude, though with
much hesitation, that these genera had better not be separated as the
sub-family of Coronulinæ.
Our present genus, Coronula, is closely related to Platylepas, and
likewise to Xenobalanus, though this latter genus is so very different
in external aspect that it might easily be mistaken for a pedunculated
Cirripede. Lamarck and some other authors have placed the species of
Chelonobia under Coronula, but this has arisen, as explained under that
genus, from a misapprehension of their structure; the folded very thin
walls in Coronula having been compared with the radiating septa of
the very thick walls in Chelonobia. I may further add, that Coronula
has been divided into two genera by Ranzani, on palpably insufficient
grounds.
_General Appearance._--The three recent species of this genus have a
very handsome and striking appearance. The shell is highly symmetrical,
owing to the six compartments being of the same size and having
exactly the same outline. The general shape is either depressed or, as
expressed by the name, like a crown. The walls are longitudinally and
slightly ribbed, owing, as we shall presently see, to their wonderfully
folded structure; and the surface is marked by very fine longitudinal
striæ, crossed by finely beaded lines of growth. Hence the walls
offer a strong contrast in appearance with the six, smooth radii,
of equal breadth. The symmetrically hexagonal, or rounded-hexagonal
orifice of the shell is closed by a thick, nearly horizontal membrane,
supporting, towards the rostral end, the small opercular valves, with
a slit, having protuberant lips, in the middle, for the protrusion of
the cirri. The opercular membrane is attached all round, but a short
distance beneath the summit of the shell. In regard to size, I have
seen a specimen of _C. diadema_ two inches in height and two and a half
in diameter.
_Structure of the Shell._--The structure at first appears singularly
complicated, and quite unlike that of any other Cirripede; but the
whole results simply from the folding of the very thin walls, which
in all essential respects are constructed like those in Balanus.
In a young specimen of _C. balænaris_, having the orifice of the
shell 2/10ths of an inch in diameter, I found the upper part of each
compartment only slightly sinuous, not more so than is common in many
varieties of Balanus, but more symmetrical, for each compartment had
three slight furrows, making, for the whole shell, eighteen furrows.
The ensuing changes during the growth of the shell will be best
understood by looking at the diagrams _a_, _b_, _c_, in fig. 10, Pl.
15, which are supposed to represent the basal margins of the walls
of a single compartment: (_a_) shows the simply sinuous wall of the
young shell. As the shell grows, the furrows rapidly grow deeper and
deeper (_b_), and wider and wider; at the same time, the folds or
ridges between the furrows gradually become drawn out at their ends
into transverse loops (_c_), the extremities of which ultimately
become closely pressed together--the furrows being thus converted into
cavities, extending from the top to the base of the compartments. This
structure in the mature shell will be best understood by looking at the
transverse or horizontal section (Pl. 16, fig. 7) of the rostral end of
the shell of _C. diadema_, in which species the folding of the walls
is simpler than in _C. balænaris_ or _C. reginæ_: the walls (_e_) are
represented by a double line connected by little cross lines,--(_h_)
being one of the transverse loops at the outer end of one of the folds,
and (_f′_) one of the cavities between the folded walls, open at the
bottom of the shell, and occupied by the epidermis of the Whale. The
walls of the compartments, as here represented, (A being the rostrum,
C C the lateral compartments, D D the alæ of the carino-lateral
compartments,) are separated from each other by the broad radii; but
if the section had been taken low down near the basis, the end of the
folded wall of one compartment would have been separated from that
of the adjoining compartment only by the close suture: this will be
understood by a glance at the entire shell, given in fig. 3, Pl. 15.
The rostrum of the same species, viewed from the inside, is shown at
fig. 1, Pl. 16; here it may be observed, that the basal margin (_e e′
e′′_) of the folded wall is extremely oblique, the outer portion having
extended downwards much more than the inner portion: this obliquity
is more clearly shown in the lateral view (fig. 2) of a lateral
compartment, for this figure will equally well serve for a lateral view
of the rostrum (fig. 1), if the ala (_a′_) be supposed to be removed.
The section of the rostrum in fig. 7, (A), will now be intelligible in
relation to the view of the rostrum given in fig. 1, if it be borne in
mind that the section has been taken high up, near the letters _a_,
_b_, _c′′_ in fig. 1.
As above stated, the folds or ridges in the young shell become more
and more transversely drawn out at their ends into the transverse
loops, till the latter join and touch each other. In _C. balænaris_
the lines of junction are simple, though very close; in the other
species, the ends of the transverse loops, where touching, are finely
and elegantly toothed, and thus locked together. These teeth appear
single when the shell is viewed either externally or internally, but
when the walls along the lines of junction are forced apart, they are
seen really to consist of transverse rows of minute teeth. These teeth
are less distinct, forming only sinuous ridges in _C. barbara_ (fig.
6, Pl. 15); in all cases the teeth are formed by the modification
of the very minute beads, which ornament the lines of growth on the
external surface of the shell. In the rostrum of _C. diadema_, as seen
internally (fig. 1, Pl. 16), the serrated and closely-fitting lines of
junction (_f_) between the ends of the transverse loops of the folded
walls are plainly shown. It is the more or less rounded surfaces of the
transverse loops which give to the external surface of the parietes its
longitudinally ribbed structure: the ribs are plainest in _C. diadema_,
fig. 3, Pl. 15. The shell, in fact, as seen externally, consists of but
an extremely small portion of the external surface of the whole length
of wall, being exclusively formed of the transversely looped ends of
the radiating folds, together with the radii. Owing to the ends of the
transverse loops being so closely pressed together, the furrows are
practically converted (as already remarked) into cavities, open only on
the under side of the shell, and extending from the oblique bases of
the compartments up to their apices; and these are invariably filled
by the black epidermis of the Whale. Owing to this circumstance, the
skin of the Whale has been mistaken by some authors for parts of the
Cirripede! In _C. diadema_, in which the summit of the shell is often a
little disintegrated, the whale's skin is often there exposed, forming
three black spots at the top of each compartment. It should always be
remembered that these flattened and deep cavities are furrows, which
homologically ought to be open in longitudinal lines along the external
surface of the shell, from the top to the bottom.
As the shell increases in diameter, each of the original eighteen
transverse loops, forming the exterior surface of the shell, increases
in breadth; and they would have had to increase extremely, had not
some of the transverse loops become, during growth, divided into two
or three new transverse loops, in a manner strictly analogous with the
first formation of the eighteen folds in the young shell. In Pl. 15,
fig. 7, 8, 9, we see how one of the circumferential transverse loops,
by the formation of a medial furrow, or rather bay, becomes developed
into two transverse loops; and it is rather important to observe
that three new loops might equally well have been contemporaneously
formed. By the repeated formation of new circumferential loops and
the consequent formation of new folds, the wall of the shell, when
old, especially in _C. balænaris_, becomes folded in a wonderfully
complicated manner, as may be seen in Pl. 15, fig. 5, which is an
exact tracing of the extreme basal edge of the wall of a shell of
_C. balænaris_; to perceive the full amount of complication, it is
advisable to trace the wall of any one of the compartments, from one
suture (_s_) to another. In this figure the sutures are purposely
drawn a little open. It may be seen that the new transverse loops, and
consequently the new folds of the walls, have been, in this species,
mostly formed in symmetrical order, on both sides of the six sutures;
this results from the transverse loops on the sutures almost always
giving rise contemporaneously to three new transverse loops. In _C.
diadema_ the transverse loops on the sutures usually divide into only
two new loops, one on the rostral side and one still remaining at the
suture; hence the folding of the walls in this species is much less
symmetrical. The number, however, of the transverse loops and the exact
pattern of the folding is variable in all four species of the genus.
I may further add, to show the complication of the folds, that in a
shell of _C. balænaris_, having a basal diameter of two inches, and
which had the walls as little folded as ever they are, yet I found,
by careful measurement, that the entire basal edge of the wall, if
stretched straight, would have extended for a length of fifty-two
inches! Therefore, if the wall had not been folded, but had been simply
circular, as in ordinary cirripedes, the basal diameter of the specimen
would have been between sixteen and seventeen inches!
The central membranous basis is flat, but the bottom of the folded
walls of the shell is concave, which is caused by the outer ends of the
folded walls having grown at a greater rate than the inner ends. The
concavity is deep in _C. diadema_; in _C. balænaris_ it is much less
so, and here the inner hood-like ends of the folded walls are rather
abruptly, but in a variable degree, produced downwards, generally even
slightly beneath the level of the circumference of the shell; this
fact is of interest in relation to the peculiar, depending, spur-like
processes in the genus Platylepas. A lateral view of a compartment in
both these species, is given in Pl. 16, figs. 2 and 3; and by supposing
in each case a compartment to stand opposite, at a distance which may
be judged of from fig. 5, the vertical sectional outline of the whole
shell will be understood: in fig. 3 of _C. balænaris_, however, the
inner hood-like ends of the folded walls are not produced so much
downwards as is usual.
In the same manner, as the outside of the shell consists of the
transversely expanded ends of the folded walls, pressed closely
together, so the cavity in which the animal's body is lodged, is formed
by the inner and less closely joined ends of the folds, lined by the
thick sheath (_a_, fig. 1 and 7, Pl. 16), which latter extends down
very near to the basal membrane. The cavity for the body, is small
compared to the whole shell; in _C. diadema_ it is deeply cup-formed,
with a small, flat, membranous bottom or basis; in _C. balænaris_ it
is wider and shallower, with a broader bottom, and with the upper
edges of the walls more inflected. In both species, the thick membrane
connecting the opercular valves to the shell, is attached all round
near the summit of the sheath. The uppermost portion of the sheath is
not marked by concentric lines, as in most of the Balanidæ, owing to
the opercular membrane not being, as we shall presently see, regularly
moulted. A portion of a single wall, when closely examined, is found to
be formed of an outer and inner lamina, united by longitudinal septa,
and is thus permeated by minute, square, longitudinal pores,--exactly
as in the normal structure of Balanus. The walls are extremely thin,
and are striated longitudinally, owing to the slight projection, on
both the inner and outer surfaces, of the longitudinal septa; they are
thicker in the part forming the external transverse loops, being here,
in _C. balænaris_, as much as 15/1000ths of an inch in thickness; but
when forming one side of the spoke-like folds, the thickness is only
6/1000ths of an inch. The inner lamina is thicker, contrary to what is
usual, than the outer lamina; the sharp tips of the longitudinal septa
project a little beyond either lamina, giving to the basal edge of the
wall a serrated outline. It is singular that the thin outer lamina
is first formed as a rim or ledge on each side of the longitudinal
septa; these ledges being not closely united for some little space up
the wall, as is represented in the enlarged drawing of a bit of the
basal wall of _C. diadema_, Pl. 16, fig. 6. The open clefts thus left
are, of course, covered by the so-called epidermis, for otherwise
the included threads of corium would have been exposed. Each fresh
period of growth, in the case of _C. diadema_ (fig. 6) and _reginæ_,
and to a certain extent in _C. balænaris_, is marked by little knobs
on the longitudinal, slightly prominent, septa, and this prefigures
an analogous strongly marked structure in Tubicinella. A fine thread
of corium runs up each pore to the summit of the compartment; for
these pores are not, as in Balanus, cut off by transverse calcareous
septa, or have their upper ends solidly filled up with shelly matter.
As, however, the summit of the shell in Coronula is sometimes
disintegrated, the threads of corium within the pores would have
been exposed, had not each thread formed for itself, as I suppose, a
transverse membranous septum near the summit of the shell; at least
this is the case with the larger pores of the radii. The walls, where
closely pressed together in the spoke-like folds, are disunited at
the extreme base, but above this they are firmly calcified together.
A ribbon of corium runs along the basal edge of each spoke, and sends
threads of corium up the parietal pores on each side, and its upper
edge serves to deposit calcareous matter (homologous with the layers
of the sheath) and thus to unite the two walls firmly together. In
_C. diadema_, the walls of the terminal transverse loops are simply
calcified together like the spoke-like portions; but in _C. balænaris_
the opposite sides of the loops are united by septa (see the transverse
section in Pl. 15, fig. 2 _a_), making from five to eleven longitudinal
tubes within each transverse loop; these tubes being larger than the
parietal pores. When a piece of the shell is dissolved in acid, no
tubuli can be discovered, which may be accounted for by the thinness of
the walls; nor are there any spines on any of the external membranes.
The number of the pores, in the parietes of a moderately-sized specimen
of _C. balænaris_, I calculated was at 3400, each occupied by a thread
of corium springing from the eighteen branched ribbons, diverging from
the corium, surrounding the base of the sack. To this number must be
added between 300 and 400 larger threads of corium running up the
tubes in the transverse loops; and no less than about 2300 fringes and
threads occupying the pores in the six radii: thus we see that the
dermal system in Coronula is wonderfully complicated.
_Radii._--The radii are very wide in _C. diadema_. In all the species
their summits are square or parallel to the basis. Their internal
structure is remarkable: as the walls in this genus are extremely thin,
so are the _proper_ radii, for in fact they consist in this and all
cases, as we know, of one margin of the wall modified by its lateral
growth against the opposed compartment. But as the radii in Coronula
are not plicated, like the walls, the shell would have been excessively
weak along the six lines of suture, had not the radii been strengthened
by numerous sinuous plates, springing from the inner lamina of the
proper radius, and running downwards, attached to the folded wall
of the compartment to which the radius belongs, and with their free
edges pressed against the folded wall of the opposed compartment.
These plates give out short transverse denticuli, making altogether a
beautiful structure, as is best seen in Pl. 16, fig. 3, but also in
fig. 2 and 4, and _d_ in fig. 1. In the section, fig. 7, the proper
radius (_d_) is seen to be continuous with the wall (_e_), and to be
very thin, in fact forming but a small portion of the compound radius:
it is formed of an outer and inner approximate lamina, separated by
septa, which are nearly horizontal, and which consequently cannot be
shown in the transverse section fig. 7. The outer lamina of the radius
is imperfect, or does not reach quite to the suture, leaving the septa
a little exposed (imperfectly shown in Pl. 16, fig. 3), exactly as is
the case with the outer lamina of the parietes, at the basal margin
of the shell. In all common Balanidæ, a ribbon of corium runs up each
of the six sutures, and sends in fine threads between the septa of
the radii, but here a thread of corium runs up a minute, cylindrical
pore, situated on the line of junction between the radius and the
wall whence it arises; and from this longitudinal thread the finer
threads spring which pass between the horizontal septa of the proper
radius: this cylindrical pore is rather large in _C. balænaris_, but
excessively small in _C. diadema_ (see a black dot (_d′_) in section,
fig. 7), and is solidly filled up in the upper part of the shell. The
plates (fig. 3) which run down from the inner lamina of the proper
radius, and form the greater part of its thickness, are occupied by
fringes of corium, extending up from a ribbon of corium, running along
each suture, like those which run along the bases of the spoke-like
folded walls. This difference in the origin of the ribbons of corium,
occupying the interspaces between the plates and the pores in the
proper radii, shows the essential difference between the latter and the
thick inner portions of the compound radii. In _C. balænaris_ (fig. 3),
the compound radius extends from the outside of the shell to the sheath
and to near the basal edges of the folded walls: in _C. diadema_ (fig.
2), it does not extend so far inwards and downwards; and in _C. reginæ_
(fig. 4), even still less so. It ensues from this circumstance that
when, in these two latter species, a transverse section is made across
the middle of the shell, a large chamber (_v_, fig. 7), occupied by the
ovaria, is found on each line of suture (_s s_), separating the radii
(_d_) and alæ (_a_).
The _Alæ_ in _C. diadema_ (section, Pl. 16, fig. 7, _a′_) and _C.
reginæ_, are of remarkable thickness, nearly equal to that of the
radii, and in _C. balænaris_, of considerable thickness; this is
evidently to give strength to the shell, which is weakest along the
lines of suture, notwithstanding that the radii have been specially
thickened. The edge of each ala presents a miniature resemblance of
the edge of the radius, namely, a central ridge sending off on both
sides sinuous plates, themselves denticulated. In _C. balænaris_, the
ala rests almost entirely on the inner surface of the compound radius;
but in the other two species, in which, as already stated, the radii
and alæ are separated by chambers, the ala rests on a plate (_c′′_ in
fig. 1, 4, and 7), which extends from the top of the radius to the
bottom of the sheath, narrowing downwards (_c′′_, fig. 1), and is a
specially developed portion of the sheath for the radius to rest on.
In _C. diadema_ and _reginæ_, the sides of the folded walls, at the
ends of the chambers (_v_, in section, fig. 7), are strengthened by the
deposition of layers of shell in connexion with the sheath. The sheath
extends close down to the basal membrane in _C. balænaris_, and does
not project freely: in the other two species it depends freely, but
does not run quite so low down. I have only further to remark, that the
sutures, though very strongly united, are not calcified together; for
they easily separate after the action of caustic potash.
The _Basis_ (Pl. 28, fig. 1 _a_-1 _c_) is membranous and is formed of
concentric slips: each slip has eighteen angles, corresponding with
the open ends (see Pl. 16, fig. 5) of the eighteen folds of wall. A
ray of membrane runs under each of these folds, being prolonged from
the basal membrane; but these rays can hardly properly be called parts
of the basal membrane, for they split at each period of growth along
the middle, and the two halves are drawn from under the basal edges
of the walls, and thus come to invest their outer surfaces. The basal
membrane, and the whole cementing apparatus, which is much simpler than
in Balanus, has been fully described (p. 135) in the Introduction.
_Opercular Valves._--In the sessile cirripedes hitherto examined, the
four opercular valves are inclined towards each other, and nearly fill
up the orifice of the shell, being united to the walls by a more or
less narrow circular border of membrane; this membrane being attached
rather low down to the sheath. In Coronula the opercular membrane is
stretched like the skin of a drum, almost horizontally across the top
of the shell; it is, however, generally attached to the sheath rather
lower down at the carinal than at the rostral end: hence the animal's
body, as remarked by Burmeister, is attached almost horizontally;
but this we shall see, under Xenobalanus and Tubicinella, cannot be
considered a character of much importance. The valves are quite small
compared with the opercular membrane, and certainly are of little
functional importance. The scuta in _C. balænaris_ and _diadema_ (Pl.
15, fig. 3 _b_) are sub-triangular; but the under or growing surface
is elongated and arched. These valves stand almost at the rostral
end of the orifice, instead of on each side of it. In _C. balænaris_
the terga are small, with the under surface oval: in _C. diadema_
they are either quite aborted, or are represented by a barely visible
plate of shell, parallel and close to the tergal margin of the scutum.
The aperture leading into the sack is formed by a nearly medial slit
of considerable length, furnished with irregularly protuberant,
inwardly inclined lips. These lips are formed by the development of
an inner fold or crest of membrane, which can be just detected in
most Cirripedes: the lips include a double fold of corium, and are
covered by a delicate tunic, continuous with that lining the sack, and
homologically continuous with the opercular membrane. The opercular
membrane is very thick, tough, and yellowish; it is, in parts, finely
plicated in lines radiating from the apices of the valves: these plicæ
consist of membrane in an altered condition, being harder, more horny,
and of a browner yellow; the plicæ are large at the rostral end of
the scuta, and projecting beyond these valves, they afford attachment
to the rostral depressor muscles. Rims of similarly modified membrane
(Pl. 15, fig. 2 _b_) connect the scuta and terga together. The rims and
plicæ are occasionally moulted together with the opercular membrane.
In almost all hitherto described Balanidæ, a new opercular membrane is
formed at each period of exuviation, and as soon as formed, the old one
is generally moulted, together with the other membranes of the body:
the case is very different in Coronula, in which it is evident, from
the lines of growth on the valves and sheath, that a new membrane is
formed only at rather long intervals, and that it is formed in some
degree extensible, so as to allow of some growth in the shell. Two or
even three of these membranes are retained at the same time, one over
the other; and thus, by their joint thickness, they afford protection
to the included animal's body, and compensate for the smallness of the
opercular valves. In a large specimen of _C. balænaris_, two inches in
diameter, there had been formed, since its existence as a very young
shell, not more than eight opercular membranes, whereas the other
membranes must have been moulted within this same period at least
thirty times. In a young specimen of this species, having the orifice
of the shell only two tenths of an inch in diameter, I found the
opercular membrane, as usual, double. This membrane is not furnished
with spines, nor is it penetrated by tubuli as in most other genera.
The tissue left after the opercular valves have been dissolved in acid,
presents no tubuli, or any other recognisable character, and is not
divided (as is usual) into layers.
_Muscles of the Sack._--These muscles differ considerably from
those of other sessile cirripedes hitherto described. There are the
usual five (or properly six, the two tergal muscles being here, as
elsewhere, confluent) bundles of fasciæ; but they hardly can be called
bundles, they are so much spread out and thin. In _C. balænaris_,
each rostral muscle consists, in different specimens, of from three to
five principal fasciæ; these, at their upper ends, are attached to the
cartilaginous plicæ at the rostral extremities of the scuta; at their
lower ends, they do not reach (as in all the previous genera) to the
basal membrane, but after converging, they diverge again into a little
fan of fibres, which are firmly attached to the corium low down on the
sides of the sack. The lateral depressores of the scuta consist each
of about three fasciæ, and they terminate downwards like the rostral
muscles. The tergal depressores are spread out into a thin sheet;
upwards they reach to the basal edges of the lips of the sack-aperture,
and downwards they curve a little towards the rostral and opposite end
of the shell, and extend nearly to the basal membrane. These tergal
muscles include two fasciæ, larger than the others, which extend rather
further, both upwards and downwards, than the other fasciæ. But the
most novel character in these several muscles is that in their lower
portions they do not exhibit transverse striæ, thus showing a tendency
to become involuntary as in pedunculated cirripedes. This circumstance,
and their feebleness, is easily accounted for by the thick unyielding
nature of the opercular membrane, and the feebly developed character
of the opercular valves. In _C. diadema_, the tergal muscles are much
spread out, having, as in _C. balænaris_, a larger fascia on each side;
but the lateral depressores of the scuta form a well defined nearly
cylindrical bundle; the rostral pair are extremely weak and spread out:
I could perceive only feeble transverse striæ on some of these muscles,
and on others there was not the least trace of striæ. I may add that
the adductor scutorum muscle is well developed, as are the eight pairs
of muscles which unite the animal's body to the under surfaces of
the scuta. The action of the adductor scutorum serves to close the
sack-aperture, but towards the carinal end necessarily with very small
force; protection in this part can only be afforded by the protuberant,
valvular lips, and by the dorsal surfaces of the inwardly curled cirri,
with their tufts of bristles.
_Mouth._--The mouth (Pl. 26, figs. 3, 4) is much elongated
transversely, and does not differ essentially from that of most other
Balanidæ. The labrum is notched and not in the least bullate, though
in _C. diadema_ there is a slight prominence outside close beneath
the notch. The palpi are of large size, with their bristly apices
touching each other. The mandibles are very strong, with from four to
five main teeth, which are remarkable by presenting only rudiments
of being laterally double; but between the second and third, and
between the third and fourth main teeth, there is a small intermediate
tooth,--these I have not met with in any sessile cirripede hitherto
described. The maxillæ are small, with the two upper spines remarkably
strong. The outer maxillæ are on their inner faces bilobed. Between
these organs there is a minute projection, or mentum, flattened in the
longitudinal axis of the body; I have not noticed this in any previous
Cirripede.
The _Cirri_ are short and extremely much flattened: the three anterior
pairs have their rami unequal in length by two or three segments; the
posterior edges of their pedicels are fringed by tufts of extremely
fine hairs. The pedicel of the first cirrus is very broad; its rami are
short, with the segments very broad. The rami of the second and third
cirri are short, with the segments protuberant in front and thickly
clothed with spines; the terminal segments have some short, thick,
claw-like spines. The three posterior pairs have protuberant segments,
each supporting three or four pairs of short, strong, main spines, with
a small intermediate tuft: the dorsal tuft is small.
The prosoma is of large size. The _stomach_ is large, without cæca, but
with some internal longitudinal plaits (in _C. balænaris_ at least),
showing a tendency to the formation of cæca. In the stomach of _C.
balænaris_ I found a considerable quantity of a conferva, too much, I
think, to have got in accidentally.
_Generative system._--The vesiculæ seminales are large; and at their
broad blunt ends, in _C. balænaris_, four separate vasa deferentia
enter, of which fact I have seen no other instance. The ovarian tubes
do not extend up the sides of the sack, but lie at the bottom, over
the basal membrane; in _C. balænaris_ they send six short ray-like
prolongations into the six sutures; in _C. diadema_ they send similar
prolongations into the sutures, and fill up, as I believe, the six
chambers (Pl. 16, fig. 7, _v_) lying between the radii and alæ: I have
examined only one specimen of _C. diadema_ in spirits, and this had
the ovarian tubes in an early state of development, when they can with
difficulty be distinguished from the pulpy corium; the orange-coloured
masses, however, which filled the six cavities, resembled the layer
which certainly consisted of undeveloped ovarian tubes and cæca, lying
over the basal membrane. The ova are wonderfully numerous; their length
is 3/400ths of an inch. The larvæ have been noticed in the anatomical
introduction.
_Branchiæ._--These are immensely developed, covering almost four fifths
of the area of each side of the sack. Each consists of two nearly
equal folds, attached vertically to the carinal end of the sack, and
transversely across the upper end extending to the animal's body. The
outline of the free part is rounded. Both folds of both branchiæ are
deeply plicated on both sides; hence the superficies of the whole is
very great. We shall find that this structure is common to the three
following allied genera, but with these exceptions, I have observed
double branchiæ only in one species of one other genus, namely, in
_Chthamalus dentatus_.
_Attachment._--The shells adhere with remarkable strength to the
whale's skin. Having, until recently, examined only separated
specimens, and observing portions of the whale's skin adhering to
the outside, and solidly filling up the cavities on the under side,
I did not doubt that the shell had the power of forming, by its own
action, a deep cavity in the skin of the whale. Inspection of the basal
outline of the walls of the shell (Pl. 16, fig. 5) of _C. balænaris_,
will show how singularly unfitted its structure is for any burrowing
process; and I was led to speculate on the possibility of the pupa
being able to bury itself deeply in the skin, but rejected this
view as opposed to what is known of the habits and structure of the
pupæ of other Cirripedes. Having now examined several specimens of
_C. diadema_ adhering, in a group, to a large piece of skin, in Mr.
Stutchbury's collection, it has become evident that the attachment
is as much owing to the upward growth of the whale's skin, as to the
downward growth of the Coronula. In Pl. 15, fig. 4, a vertical section
is given of the whale's skin, taken through the place whence a shell
of _C. diadema_ has been removed; consequently we here see nothing
but the whale's skin: the upper black layer is the dark horn-coloured
epidermis, forming the general surface of the whale's body, and
resting on an orange-coloured fibrous layer, which is lightly shaded
in the drawing. The two horns in the section are two of the eighteen
projections, formed entirely of the dark epidermic layer, which fill up
the eighteen flattened cavities produced by the folding of the walls.
Outside the horns we see the section of a circular furrow, in which the
circumferential margin of the shell was lodged; and between the horns,
there is the central hollow, within which, when lined with shell, the
cirripede's body was included. The circular furrow is formed in main
part by the epidermic layer being thinner there than on either side,
and partly by the orange-coloured, underlying fibrous layer curving a
little downwards, from having apparently yielded to the pressure of the
circumferential margin of the shell. With respect to the cause of the
thinness of the epidermis under the circular furrow, I do not know how
much to attribute to mere mechanical compression or stretching, and how
much to the pressure of the shell, having checked[124] its formation.
In the case of very young and small shells, it is hardly possible that
their pressure can have in any way influenced the formation of new
epidermic layers under the thick old layers; and we must believe, at
least in these cases, that the whole effect is mechanical, the sharp
basal edges of the shell having indented the epidermis; but this is
not more surprising than that the radicle of a plant should penetrate
hard ground. Whether the indented epidermis in the circular furrow
becomes ruptured, I am not sure; ragged layers may commonly be observed
outside the shell, but it is very possible that these may be the ends
of layers of epidermis which have been preserved by the covering of
the shell, whereas, on the surrounding parts of the whale's body,
these same layers have been removed by disintegration. To return to
the section,--the outline of the boss of orange-coloured fibrous
tissue, under the central hollow, clearly shows that it must have
been formed by its own upward growth, for it stands above the general
surrounding level of the corresponding layer. This same conclusion is
still more obvious with respect to the eighteen flattened prominent
horns, formed of the dark epidermis; the manner in which the epidermis
has been forced, moulded, and packed into the eighteen flattened and
curved cavities of the shell, so as to adhere to them with considerable
tenacity, is extremely curious. The prominence of these horns is so
great that it appears to me quite impossible to account for them,
excepting by a special formation of epidermis beneath each cavity.
The basal membrane of the Coronula, which lies at the bottom of the
central hollow, adheres by its own cementing apparatus; and when the
larva first attaches itself, this adhesion must be very important, as
it allows the basal edges of the shell, during their slow downward
growth, to press firmly on the whale's skin, and so slowly indent it
with the circular furrow. The final cause, probably, of the cavities
on the under side of the shell in this genus, formed by the singularly
convoluted parietes, is to allow of the upward growth into them of the
epidermis of the whale, thus securing a firm attachment and allowing
the shell to exert a strong downward pressure, and thus effect its
partial imbedment, and protection from the enormous force of the waves
to which it must be exposed.
[124] Formerly I was inclined to believe, as stated in my former
volume on the Lepadidæ, that the cement injured the true skin of
the supporting animal, but this, at least in such cases as the
present, I do not now at all believe.
With respect to _C. balænaris_, I have seen only specimens, preserved
on shrunk and twisted whale's skin, with the underlying fibrous
layer not preserved; but the cavities in the shell were filled by
horns of epidermis, exactly as in _C. diadema_. There is, however,
this difference in the attachment of the two species, that in _C.
balænaris_, owing to its depressed form, the circumference of the shell
indents the whale's skin, not vertically downwards, as in _C. diadema_,
but very obliquely outwards; and, consequently, buries itself much more
completely, but less deeply, under a folded and apparently ruptured
flap of the epidermis. In young specimens, of the size of a shilling,
the entire shell, with the exception of the operculum, is thus covered
up and protected whilst young and tender.
_Geographical Distribution._--The genus is found wherever whales
occur, that is, from the Arctic to the Equatorial regions, in both
hemispheres. It is asserted that sometimes as many as a couple of
hundred specimens will adhere to a single whale. _Coronula barbara_,
a form closely allied to _C. diadema_, existed during the Red Crag
period; and Bronn has described some fossil specimens from Italy.
_Affinities._--In the wonderfully convoluted shell,--in the parietal
tubes not being either filled up by calcareous layers or being crossed
by calcareous septa,--in the outer lamina of the shell between the
longitudinal septa near the basis being imperfect, Coronula differs
from all the foregoing genera; in the two latter respects it agrees
with the three following allied genera, viz., Platylepas, Tubicinella,
and Xenobalanus. The equal size of all six compartments of the shell,
has been observed in very few genera besides Coronula. In a new
opercular membrane not being formed at each exuviation, and in two or
three of these membranes being persistent, and in their being attached
high up the sheath, this genus agrees with the three following genera
alone. In the valves tending to be rudimentary, and in the protuberant
lips of the sack-aperture, we have a close alliance with Xenobalanus.
The muscles of the sack being spread out, and tending to lose their
transversely striated character, are great peculiarities in Coronula,
Tubicinella, and Xenobalanus. The simplicity of the cement-ducts is
a remarkable character, observed in two of the following genera, but
not in Tubicinella. The double branchiæ is a peculiarity common to all
four genera. Neither the mouth nor cirri offer any new characters of
much importance in Coronula or in the three allied genera: the lower
teeth of the mandibles not being laterally double, but having small
intermediate teeth, is the newest feature in the mouth.