Portion of edge of basis of [Balanus tintinnabulum], a, a, outer lamina; b, b, inner or upper lamina; c, c, c, porose or cancellated radiating septa.
Regarding the very remarkable means by which the basis of sessile Cirripedes is cemented to the surface of attachment, it will be convenient to defer for a little the description, on account of its necessary length.
Structure of the Opercular Valves (Scuta and Terga).
These are situated on each side of the slit or orifice leading into the sack; from their shape, their powers of movement, their separation by flexible membrane from the shell, to which they serve as a lid, they appear at first as if they constituted an element very distinct from the shell itself, but this is not the case. They are, together with the opercular membrane, as essentially as the whole of what is externally visible, a part of the modified carapace, of which they occupy the upper or posterior extremity: from tracing the metamorphoses, or even by comparison of a [Balanus] with Pollicipes, there can be no doubt of the truth of this conclusion. The opercular valves are four in number,—a pair of scuta and a pair of terga; but the latter in [Coronula diadema] and [reginæ], are either aborted or represented by a mere rudiment; and in [Xenobalanus] both scuta and terga are quite absent. In several cases, more especially in the genus [Pyrgoma] (Pl. [13], fig. [1 b]), the scutum and tergum on each side are calcified together, so that sometimes not even a trace of the line of junction can be discovered. In most cases the scutum is firmly united, being articulated in a manner presently to be described, to the tergum; but in [Coronula], [Tubicinella] (Pl. [17], fig. [3 c]), and [Platylepas], the ends of these valves are simply approximated.
Scuta.—These valves are important, inasmuch as the animal’s body is attached to them; in Pl. [25], fig. [1], the broken line, surrounding a, b, shows where the body has been cut, in removing the scutum on the near side, the other scutum, S, being left articulated to the tergum, T. In shape the scuta are generally sub-triangular; but in some species of [Pyrgoma] and in [Chelonobia], &c. they are much elongated. The lines of growth are usually prominent; and along the occludent margins the alternate, or sometimes every third or fourth line, is developed into a knob, which produces a serrated edge, serving to lock the two opposed valves together; there is, however, no trace of this structure in [Coronula] and [Tubicinella]. In some species of [Pyrgoma], a ledge of considerable breadth (Pl. [13], fig. [3 e], &c.) is developed along the occludent margins of the two scuta, as well as of the two terga, giving them an anomalous structure. The Terga differ considerably in outline in the different genera and species: their shape approaches more nearly to a triangle than to any other regular form; but there is generally a projection or spur on the basal margin, on the side towards the scutum. In some species of [Pyrgoma], the tergum is of so irregular a shape as to defy description. In most cases, a longitudinal depression or furrow runs down the valve, from the apex to the extremity of the spur; and it not rarely happens that the sides of this furrow become folded inwards and almost closed. The spur probably answers to the basal point of the usually sub-rhomboidal tergum in Pollicipes and Scalpellum.[22] The tips of the terga in some species of [Balanus], &c., are specially modified into sharp points or beaks (Pl. [2], fig. [3 b], [3 d]), bowed a little inwards, and projecting considerably above the tips of the scuta; this is effected by the medial, uppermost part of the valve being internally thickened and hardened, and then, by the disintegration of the two margins and the external surface, the internal modified portion becomes exposed. The whole valve, also, at least in such cases as in [Balanus psittacus], appears to be forced slowly upwards in the articular furrow of the scutum. I am assured, by a competent observer, that the beaks of the terga in [B. porcatus] can give an object placed within the orifice of the shell a sharp tap.
[22] In comparing the Tergum of one of the [Balanidæ] with that of a typical member of the Lepadidæ, for instance, that of [Balanus] with that of Pollicipes, apex corresponds with apex: the extremity of the spur in [Balanus] corresponds with the basal point of the whole valve in Pollicipes: the scutal margin, (which in [Balanus] homologically extends down to the extremity of the spur), corresponds with the scutal margin of Pollicipes: the carinal margin in [Balanus] corresponds with the upper carinal margin in Pollicipes: the basal margin of [Balanus] on the carinal side of the spur, corresponds with the lower carinal margin in Pollicipes: lastly, (and this is the chief difference), in [Balanus] there is no appreciable occludent margin, the apex of the valve being brought close to the upper angle of the scutal margin; in [Chthamalus], however, there is yet left some remnant of an occludent margin,—which margin in Pollicipes is conspicuous.
The scutum and tergum, with the few exceptions above stated, are articulated together at a large or open angle. The articulation (see Pl. [11], fig. [5 b], [c], [d], and fig. [6 b], [c]) is effected by the margin of the tergum being a little inflected, and lodged in a furrow in the margin of the scutum. This furrow in the scutum has its further border generally prominent and often reflexed or curved over; I have called it the articular ridge; it, also, is lodged in a furrow in the upper part of the tergum, which again is bordered by a ridge, viz., the articular tergal ridge. So that in both scutum and tergum there is an articular furrow, bordered in each case, on one side by the margin of the valve, and on the other side by the so-called articular ridge. In [Chelonobia] (Pl. [14], fig. [1 b]) the articular ridge of the scutum is horny. When, as often happens, the scuta and terga have been much worn, the manner of their articulation (Pl. [18], fig. [1 a]) is pretty well shown even from the outside; in this case their external appearance is very different from what it is in those individuals (fig. [1 c]) of the same species, which have not suffered disintegration. This articulation of the scuta and terga is prefigured amongst the Lepadidæ, in Pollicipes mitella, and in Lithotrya.
The scuta are brought together by a short, strong, straight, adductor muscle (Pl. [25], fig. [1, a]); its attachment leaves (with very few exceptions, as in [Tubicinella]) a rounded impression, or even pit, on the under side of the valve in its upper part. This pit is frequently bounded, on its lower side, by a sharp ridge, which, though not in actual connexion with the adductor muscle, I have, for convenience sake, called the adductor ridge; it serves apparently to give support to the animal’s body; in some few cases (as in [B. psittacus], Pl. [2], fig. [3 c]) it is confluent at its upper end with the articular ridge, and converts the whole basi-tergal corner of the valve into a deep cavity. In some of the species of [Pyrgoma] (Pl. [12], fig. [5 c], [7 b]), and in some varieties of [Creusia], this adductor ridge is enormously developed, so as to depend far beneath the true basal margin, or that to which the opercular membrane is attached. At the basi-tergal corner of the valve, there is generally a small pit or impression, and sometimes distinct crests, for the attachment of the lateral depressor muscle. At the rostral end there is, also, a small cavity formed by the overfolding of the occludent margin (rarely furnished with crests) for the attachment of the rostral depressor muscle. In the Terga, at the basi-carinal corner, there are usually crests, though sometimes feebly developed, for the attachment of the tergal depressor muscle. But in [Chelonobia], [Coronula], [Tubicinella], [Platylepas], and in some other cases, there are no crests. The crests, when well developed, are furnished with rectangular sub-crests or denticuli on both sides; in fact they resemble, and are probably homologous with, the denticulated ribs or septa in the parietes, radii, and basis. Altogether the scuta and terga are attached, as far as muscles are concerned, to the shell and sack, by three longitudinal pairs.