PLATYLEPAS. _J. E. Gray._ Annals of Philosophy, (new series), vol. x
(1825).
CORONULA. _De Blainville._ Dict. des Sciences Nat. (1824).
COLUMELLINA. _Bivona_ (fide _Philippi_). Nuovi generi, &c. di
Molluschi, Palermo (1832).
_Compartments six, each bilobed and inwardly produced, so as to form
six midribs, which support the outwardly convex, membranous basis._
_Distribution_, throughout the tropical and warmer temperate seas.
Imbedded in turtles, sea-snakes and manatee.
This small genus, consisting of three species (though I have named
only two), is a very natural one, yet closely allied to Coronula. Most
authors have united these genera, but in doing so they destroy two very
natural little groups. Platylepas, moreover, when all its characters
are considered, has as good a claim to be generically separated from
Coronula, as has Tubicinella,--a genus universally admitted. I shall
presently recur to this subject.
_General Appearance._--The most remarkable character, and which
gives a peculiar aspect to the whole shell, is that each of the six
compartments has a medial fissure or fold; the shell thus consisting
of twelve lobes. Each fold extends from the summit to the base of
the shell, and is produced inwards in the shape of a midrib (Pl. 17,
fig. 1 _a_). The membranous basis is supported by the basal edges of
the six midribs, and is thus rendered convex. As the midrib on the
rostrum would have interfered, if fully developed, with that part of
the animal's thorax which I have called the prosoma, it is of less size
than the other midribs; and from this same cause the whole rostrum is
pushed a little on one side, generally to the left, so that the shell
is not perfectly symmetrical. The midrib of the carina is less than
the lateral midribs. The orifice is oval, and is more or less plainly
indented by six angular points,--the worn-down summits of the six
midribs. The narrow elongated opercular valves stretch from one end of
the orifice to the other. The rostrum is much broader than the carina:
the lateral compartments are but little broader than the carino-lateral
compartments. The radii are narrow, though variable in width. The
general shape of the shell varies much in the same species, being
either much depressed, or rather steeply conical; either circular, or
more commonly oval. The (so called) epidermis is usually persistent
in the lower part of the shell. The shell itself is white. The basal
diameter of the largest specimen of _P. bissexlobata_ was nearly
three-quarters of an inch.
_Structure of the Parietes, Radii, &c._--The walls of the shell in _P.
bissexlobata_ are permeated by minute pores, and have exactly the same
structure as in Coronula; the pores being completed by the union of
ledges on the outer sides of the longitudinal septa, and the latter
have little knobs formed on them at each period of growth. In _P.
decorata_ the walls are solid. Each compartment, as already stated, has
a deep nearly medial furrow, the sides of which are closely pressed
together, thus forming the midribs, and thus differing from the furrows
in Coronula, which not being pressed together, form cavities on the
under side of the shell. The inner longitudinal margin of each midrib
is thickly coated by a layer of shell, and is thus rendered much more
prominent than it would have been if formed exclusively by the inward
folding of the wall. The upper part of each midrib forms a slight,
longitudinal ridge (fig. 1 _d_) on the sheath, having become encased
by the sheath during its downward growth. Owing to the small size of
the midrib of the rostrum, there is scarcely any ridge on the sheath
of this compartment. The sheath does not descend half-way down the
walls in _P. bissexlobata_, but further down in _P. decorata_; but
perhaps the thickened inner margins of the midribs may be considered as
the downward prolongations of the sheath, and on this view the sheath
descends actually to the basal membrane. The radii are narrow: their
edges have simple septa, with the interspaces filled up solidly. The
alæ have their sutural edges smooth.
The _Basis_ is membranous, and more or less convex. This convexity
is rendered possible by the support afforded by the basal edges of
the six midribs. The so-called epidermis or membrane which covers the
shell, is prolonged in six flaps under the midribs; and hence the
proper basal membrane has six deep indentations, receiving the above
flaps. The cementing apparatus is constructed on the same simple type
as in Coronula, as has been described in the Introduction.
_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.