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.

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. 3b, and Pl. 10, fig. 1g, 1h).

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.

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.