[Illustration: Fig. 5. SCUTUM (internal view of).]
[Illustration: Fig. 6. TERGUM (external view).]
[Illustration: Fig. 7. TERGUM (internal view).]
Sessile Cirripedes, partly from being attached to surfaces having
very different characters, partly from undergoing a varying amount of
disintegration, and partly from unknown innate causes, are extremely
variable. Under the head of _Variation_, in the Family Balanidæ and
under the Genus Balanus, I have enlarged on this subject, and have
shown that there is scarcely a single _external_ character which is not
highly variable in most of the species. As whole groups of specimens
often vary in exactly the same manner, it is not easy to exaggerate the
difficulty of discriminating species and varieties. It is absolutely
necessary in most cases, for mere identification, that the valves of at
least one specimen in a group should be disarticulated and well cleaned
(for which purpose caustic potash is most useful), so that the internal
characters may be examined. Whoever attempts to make out from external
characters alone, without disarticulating the valves, the species,
(even those inhabiting one very confined region, for instance the
shores of Great Britain,) will almost certainly fall into many errors:
hence it is, and can thus only be accounted for, that I have not seen
one collection of British specimens with _all_ the species, though so
few in number, rightly discriminated; and in the large majority of
cases, either two or three species, certainly distinct, were confounded
together, or two or three varieties, as certainly not distinct, were
separated from each other.
_On the Names Given to the Different Parts of Cirripedes._
In my former volume I have stated that I found it indispensable, in
part owing to the extreme confusion of the nomenclature previously
used, to attach new names to several of the external parts of
Cirripedes. Almost all these names are applicable to the Balanidæ,
or sessile Cirripedes, and to the Verrucidæ; but a few additional
names are requisite, which, together with the old names, will, I
hope, be rendered clear by the accompanying woodcuts. In sessile
Cirripedes, the whole of that which is externally visible, may for
convenience sake be divided into the _operculum_ or _opercular valves_
(_valvæ operculares_), and the _shell_ (_testa_), though these parts
homologically present no real difference. The operculum is seated
generally some little way down within the _orifice_ of the shell; but
in very young specimens and in Verruca, the operculum is attached
to the summit of the shell, and the shell, without the operculum being
removed, can hardly be said to have any orifice; though, of course, the
opercular valves themselves have an aperture for the protrusion of the
cirri.
The shell consists of the _basis_ (called the support by some authors),
which is membranous or shelly, and flat or cup-formed, and of the
_compartments_ (_testæ valvæ_), which vary from eight to four in
number, and occasionally are all calcified together.
The compartment, at that end of the shell where the cirri are exserted
through the aperture or lips of the operculum, is called the _carina_
(fig. 1); the compartment opposite to it is the _rostrum_,--these two
lying at the ends of the longitudinal axis of the shell. Those on the
sides are the _lateral compartments_; that nearest the carina, being
the _carino-lateral_ (_testæ valva carino-lateralis_), that nearest the
rostrum, the _rostro-lateral_, and the middle one, simply the _lateral_
compartment; but these three compartments are rarely present together.
The _rostro-lateral_ compartment, which always resembles fig. 2, and
may be always known by having radii on both sides, is often absent; and
not rarely the lateral and carino-lateral compartments are confounded
together, or one is absent; in such cases the compartment that is left
is simply called the lateral one. The compartments are separated from
each other by _sutures_, which are often so fine and close as to be
distinguished with difficulty. The edge of a compartment, which can
only be seen when disarticulated from its neighbour, I have called the
sutural edge (_acies suturalis_).
Each separate _compartment_ consists of a _wall_ (_paries_), or
_parietal portion_ (_pp_ in woodcuts), which always grows downwards,
and forms the basal margin; and is furnished on the two sides either
with _alæ_ (fig. 4), or with _radii_ (fig. 2), or with an ala on one
side and a radius (fig. 3) on the other.
The _radius_[3] (adopting the name used by Bruguière, Lamarck, and
others) differs remarkably in appearance (though not in essence) from
the walls or parietal portion, owing to the direction of the lines
of growth and the state of its usually depressed surface. In the
upper part the radii overlie the alæ of the adjoining compartments:
in outline (_r_, fig. 1, 2, 3), they are wedge-formed, with their
points downwards; their summits (and this is often a useful specific
character) are either parallel to the basis or as in fig. 1 and 2,
oblique. The radii are sometimes not developed.
[3] The radii have been called by Ranzani and De Blainville
"areæ depressæ" (the parietal portions of the compartments being
the "areæ prominentes"); by Poli, "areæ interjectæ;" by Gray,
"sutures;" by Coldstream, "compartments of the second order," (the
parietal portions being those of the first order); by some authors
as "intersticia." I may here add that the scuta are the "ventral
valves" of Gray, the "anterior" of Ranzani, and the "inferior
opercular" of De Blainville: the terga are the "posterior valves"
of Gray and Ranzani, but the "superior opercular" of De Blainville:
the rostrum, on the other hand, is the "anterior valve" of Ferussac
and the "ventral" of De Blainville; the carina being the "dorsal
valve" of the latter author.
The _alæ_ (so called by Dr. Gray) are overlapped by the radii and by
part of the walls; they usually extend only about half way down the
compartment (_a_ fig. 3, 4, 1); their summits are either parallel to
the basis or oblique. The alæ of the several compartments, together
with the internal, upper, thickened surfaces of the walls, against a
shoulder of which the sutural edges of the alæ abut, have been called
(by Dr. Gray) the _sheath_ (_vagina_). The upper and greater portion of
the sheath is marked by transverse lines, caused by the exuviation of
the _opercular membrane_, as that membrane may be called, which unites
the operculum all round to the sheath, or upper internal surface of the
shell.
The _carina_ has always two _alæ_, as in fig. 4.
The _carino-lateral_ and _lateral compartments_ have always an _ala_ on
one (the rostral) side, and a _radius_ on the other (the carinal) side,
as in fig. 3.
The _rostro-lateral compartment_ (when present) has always _radii_ on
both sides, as in fig. 2.
The _rostrum_ has normally _alæ_ on both sides, as in fig. 4, but very
often from fusion with the rostro-lateral compartments on both sides,
it has _radii_ on both sides, as in fig. 2.
The walls of the shell, the basis, and the radii, are in very many
cases composed of an _outer_ and _inner lamina_, united together by
_longitudinal septa_; a set of tubes or pores being thus formed. The
points of the longitudinal septa generally project beyond the laminæ,
and are denticulated on both sides (see woodcut, further on;) the septa
are sometimes branched, several irregular rows of pores between the two
laminæ being then formed (see Pl. 7, fig. 3 _b_, and Pl. 10, fig. 1
_g_, 1 _h_).
_Operculum_, or _opercular valves_.--These consist of a pair of scuta
and a pair of terga. They are joined to the sheath of the shell by the
_opercular membrane_.
_Scutum_ (woodcut 5): this valve is generally sub-triangular, and its
three margins are the _basal_, the _tergal_, so called from being
articulated with the tergum, and the _occludent_, so called from
opening and shutting against the opposed valve. The angles are called
from the adjoining margins, as _basi-tergal_, &c.; the upper angle
being the apex. The scutum is ordinarily articulated to the tergum by
an _articular ridge_ (_crista articularis_), running up to the apex
of the valve, and by an _articular furrow_, which latter receives the
scutal margin of the tergum. The articular ridge, instead of projecting
straight up from the valve, when laid flat on its external surface,
often bends over to the tergal side, and is then said to be _reflexed_.
On the internal surface of the valve, there is almost always an
_adductor pit_ or _cavity_ (_fossa adductoris_), for the attachment of
the adductor scutorum muscle: this pit is often bounded on its tergal
and basal sides, by a ridge, called the _adductor ridge_ (_crista
adductoris_), which, in its upper part, is often confluent with the
articular ridge. Beneath the adductor ridge, in the basi-tergal corner
of the valve, there is often a _lateral-depressor pit_ (_fossa musculi
lateralis depressoris_), for the attachment of the so-called muscle;
and this pit is sometimes furnished with crests.
_Tergum_, (woodcut 6 and 7):--this valve, also, has three margins,
the _scutal_, _basal_, and _carinal_; its upper end, or _apex_, is
sometimes _beaked_; on the basal margin a _spur_ (_calcar_) depends;
the outer surface of the valve is depressed or longitudinally
_furrowed_ (_sulcus longitudinalis_) in the line of the spur. The
part called the spur is often so broad, that the name becomes not very
appropriate. The angles are denominated, from the adjoining margins,
as _basi-carinal_, or _basi-scutal_ angle, &c. On the under side, in
the upper part, there is an _articular ridge_, and on its scutal side,
an _articular furrow_, receiving the articular ridge of the scutum. In
the basi-carinal corner of the valve there are often crests for the
attachment of the tergal depressor muscle.
_Sack_, _Body_, _Cirri_, _Mouth_.--A slit-like orifice between the
opercular valves leads into the _sack_, in which the body is lodged.
The body consists of the six (perhaps the seven) posterior thoracic
segments of the archetype Crustacean; the first of these six segments
(or first two, if there be seven segments) is developed on its dorsal
aspect into a part, which I have called the _prosoma_[4] (see fig. 1,
_c_, Pl. 25). There is no abdomen. The thoracic segments support six
pairs of _cirri_. Each cirrus consists of a two-jointed _pedicel_,
carrying two multiarticulated _rami_. Rarely there are articulated
_caudal appendages_ (_appendices caudales_) on each side of the anus.
The prominent mouth consists of a _labrum_, _palpi_, _mandibles_,
_maxillæ_, and _outer maxillæ_, the latter resembling a lower lip:
these organs may be conveniently spoken of, after Milne Edwards, as
_gnathites_. Within the sack, attached to its carino-lateral end, a
folded membrane forms the _branchiæ_. The sheets of ova lying within
the sack are called the _ovigerous lamellæ_.
[4] A discussion on the homologies of the different parts is given
under the head of the Metamorphoses of the Balanidæ.
I have often found it convenient to designate the membrane investing
the body, lining the sack, &c., by its proper chemical name of
_chitine_, instead of by horny, or other such equivalents; but when
covering parts of the shell, for brevity's sake I have often spoken of
it as an _epidermis_, but I do not believe that such is its nature.
When this membrane sends into the body of the animal rigid projections
or crests, for the attachment of muscles or any other purpose, I call
them, after Audouin, _apodemes_. For the underlying true skin, I use
the term _corium_.
_Relative position of parts._--The centre of the generally flat
basis, which is cemented to the supporting surface, is properly the
_anterior_ end, and the tips of the terga, often hidden within the
shell, are properly the _posterior_ end of the external covering; but
I have found it more convenient to speak of the _upper_ and _basal_
surfaces and aspects, which hardly admit of any mistake. A line drawn
from the centre of the basis, along the middle of the rostrum to the
tips of the scuta, shows the strictly _medio-ventral_ surface of the
shell; and another line drawn from the centre of the basis, along the
carina, to the tips of the terga, shows the strictly _medio-dorsal_
line; but from the crooked course of these lines, I have found it far
more convenient to speak of the _rostral_ and _carinal_ end or aspect
of the different parts of the shell; this is the more necessary with
respect to the internal parts of the animal, owing to their remarkable
changes of position during the metamorphosis, whence it comes that the
dorsal surface of the thorax faces partly dorsally, partly anteriorly
or downwards, and partly even ventrally; and the ventral surface of the
whole posterior part of the thorax faces upwards or posteriorly; but
when we refer these parts to the _rostral_, _carinal_, _basal_, and
_upper_ ends of the shell, there can be no mistake. There has moreover
been great confusion in these relative terms, as applied by different
authors.
When a sessile Cirripede is held in the position in which they have
generally been figured, namely with the basis downwards and the
scuta towards the beholder, then the _right_ and _left_ sides of the
Cirripede correspond with those of the holder.
I have followed the example of Botanists, and added the interjection
(!) to synonyms, when I have seen an authentic specimen bearing the
name in question.
Every locality, under each species, is given from specimens ticketed
in a manner and under circumstances appearing to me worthy of
confidence,--the specific determination being in each case made by
myself.