A Monograph on the Sub-class Cirripedia · Charles Darwin
OPERCULAR VALVES.
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_.
_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.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.