When a Cirripede is alive, the action of the cirri is really beautiful: from the position of the thoracic segments, the posterior cirri (three pairs in the [Balaninæ] and four pairs in the [Chthamalinæ]) form a sort of semicircle facing the mouth: the anterior cirri stand further apart, and are opposed in pairs to each other, with the first pair pointing beyond the mouth. Together the cirri form a hollow cone, not circular but elongated, with the mouth situated at the lower anterior end. The posterior cirri are protruded, by the movement of the whole thorax, curled up, close along the carinal end of the orifice; as they are protruded, they diverge, both by the movement of their pedicels, and, as I believe, by the separation of the thoracic segments. As the two rami of each separate cirrus are uncurled, they also diverge a little; as do the double rows of spines on the segments in each ramus, by their elasticity. By the movement of the thorax, the cirri are then swept towards the rostrum; and, lastly, they are brought perpendicularly down towards the mouth with a rapid movement, which would be beautifully adapted to catch any object floating or swimming in the water; hence I have called the action of the cirri, captorial. When the shell of a [Balanus] is broken open, the second and third pairs of cirri are repeatedly clasped over the mouth with a convulsive movement, in a manner indicating, I think, that their chief function is to seize and carry to the mouth any object entangled by the sweeping movement of the three posterior pairs. The first pair is also well adapted to aid in this seizing action; but I suspect that the long anterior ramus likewise acts as an organ of touch, warning the animal of danger. The mouth being itself moveable as a whole,—the outer maxillæ being capable of a backward and forward sweeping action, and being furnished with orifices apparently olfactory,—the inner maxillæ having more diversified movements,—the toothed mandibles overhanging the œsophagus,—and the œsophagus itself possessing a powerful swallowing movement, are all admirably adapted to secure any prey, when once entangled by the cirri.
Mouth.
The mouth, in the sub-family [Chthamalinæ], cannot be distinguished from that of the Lepadidæ, which has been pretty fully described in my former volume. In the [Balaninæ], however, the labrum differs considerably in not being swollen; that is, in its outer and inner fold of membrane being close together, and in having a central notch: the palpi are also larger, and the lower teeth on the mandibles, are laterally (Pl. [26], fig. [5]) double, as will be more particularly stated under these two sub-families. I have given a drawing (Pl. [26], fig. [1]) of the mouth, seen from above, of [Balanus perforatus], with the right-hand palpus (d′) and outer maxilla (a′) cut off, in order that the labrum (e), mandibles (c), and inner maxilla (b) might be better shown; the cut-off bases (x, x) of the first cirrus on each side are also shown. In fig. [2] we have the deep supra-œsophageal cavity in [Bal. improvisus] torn open and laid flat, with the inner surfaces of the labrum (c) and outer maxillæ (a) exhibited, the palpi, mandibles, and inner maxillæ having been removed. Figs. [3] and [4] will presently be referred to; they are parts of the mouth, with the muscles, &c. removed, of [Coronula]. The mouth differs extremely little in the different genera and species of the [Balanidæ], much less than amongst the Lepadidæ. In the [Balaninæ], the crest of the labrum is sometimes hairy, instead of having, as is usual, from two to six teeth on each side of the central notch: in [Balanus improvisus] (Pl. [26], fig. [2]) and [eburneus], and in [Chelonobia], the crest on each side of the central notch (e′) is furnished with a row of finely graduated teeth. A sub-triangular portion of the inner fold of membrane of the labrum, which overhangs the œsophagus, is always thickened and yellowish; it is also often punctured in patterns (Pl. [26], fig. [2], f), which, I believe, give attachment to little muscles that serve to open the upper end of the œsophagus. Opposite to this thickened, sub-triangular portion of membrane, the thin membrane forming the supra-œsophageal cavity (or the cavity surrounded by the gnathites) is strengthened by a pair of curved ribs (h, fig. [2]) of thickened yellowish membrane, running down from the inner bases (a′′) of the bilobed outer maxillæ to the opening of the œsophagus (g): a broad branch from each of these ribs supports the sides of the orifice of the œsophagus; and this branch almost joins on to a slightly thickened rim or bar (f′), which branches off from the upper part of the sub-triangular (f) inner fold of the labrum. This structure, in [Bal. improvisus], is represented in Pl. [26], fig. [2], as well as it could be, considering that the deep supra-œsophageal cavity has to be torn open; and then laid flat.
The Palpi (Pl. [26], fig. [6]) differ little, except in size, in the different genera, being squarish, more or less elongated, or even approaching to club-shaped: in most of the [Balaninæ] they are larger even than the mandibles, of which they normally form a part. Their upper margins, especially towards their free extremities, are always thickly clothed with spines; and there is generally a single row, either short (r) or long, of spines of greater length, which arise from a little above, and stand almost in a parallel line to, the basal margin. On the internal surface, there is sometimes a row (t) of very short little spines, which overhang the crest of the labrum. The Mandibles (Pl. [26], fig. [5]) have from three to five teeth; the lower point or angle is generally pectinated. In [Coronula] and its close allies, there are some small teeth intermediate between the four or five main teeth; and in these genera, though members of the sub-family [Balaninæ], the lower teeth exhibit only rudiments of being laterally double.[33] The Maxillæ sometimes have a notch under the upper large pair of spines, and in [Octomeris brunnea] there is a double notch: in many species of [Balanus], the inferior corner stands up like a step (Pl. [26], fig. [7], a): in many other genera and species, the whole edge is straight. In all, or almost all cases, the row of spines on the middle portion is double. The Outer Maxillæ are always bilobed on their inner faces, and are clothed with bristles. On all the gnathites, the bristles are often doubly serrated.
[33] M. Martin St. Ange describes, in his ‘Mémoire sur l’Organisation des Cirripèdes,’ pp. 15 and 32, “une petite langue” in the mouth of Lepas; but I may venture to assert that such does not exist; it is merely the point of union between the outer maxillæ. M. St. Ange, in his comparison of the mouth of Lepas with that of Phyllosoma, compares the mandible of the latter with the palpus of Lepas; the first maxilla of Phyllosoma with the mandible of Lepas; and so on with the other gnathites.
Muscles and functions of the Gnathites, and their confluence.—The outer maxillæ appear at first like a deeply-lobed lower lip, for they reach over almost to the labrum (Pl. [26], fig. [1]), and thus partially cover the other organs; they are separately capable of a strong and rapid, to and fro movement, by which no doubt they sweep any prey, entangled by the cirri, towards the other gnathites. Each outer maxilla is furnished with a pair of muscles, apparently a flexor and extensor; there is also a little muscle between the two maxillæ, I presume for the purpose of bringing them together. The outer and inner maxillæ generally stand close together, and in several genera a little way apart, from the mandibles; but there is no trace of any labrum or true lower lip, bounding the mandibles and orifice of the œsophagus. The outer and inner maxillæ and mandibles are not opposed in pairs to each other, but against the thickened inner fold of the labrum; almost in the same manner as the posterior pairs of cirri are not opposed one to the other, but to the mouth.
I have described pretty accurately the muscles of the mandibles in my former volume, and there given a drawing (Pl. 10, fig. 1) of them. There are four muscles: first, the depressor muscle, which is the largest, and is attached, at its upper end, to ligamentous apodemes under the free toothed portion of the jaw; and at its lower end, spreading considerably out, is attached to a concavity close above the basal margin of the labrum; to understand the action of these muscles, it should be borne in mind that the mandible almost faces the labrum. In some genera, as in [Coronula],[34] the swelling near the basal margin of the labrum (Pl. [26], fig. [3], k), caused by the internal concavity for the above muscle, is conspicuous. The depressor muscle is opposed by a small elevator, attached to the mandible close by the depressor; thence it runs upwards, and is united at its upper end to the base of the palpus, at the point where the latter adheres to the labrum: I have ventured to call this muscle the elevator, from being apparently so well fitted for this purpose; but I feel some little doubt, from having observed an apparent slight movement in the palpi of living Balani; and this is the only muscle entering those organs. The free part of the mandible is articulated on a square, thickened piece of membrane, forming part of the side of the mouth (Pl. [26], figs. [3], [4], c1; and Pl. 10, fig. 1, a, b, in my volume on the Lepadidæ); to this square piece of membrane, two short muscles are attached, one above the other, and which ought, in the Plate in my former volume, to have been represented crossing the depressor muscle at nearly right angles; at their further ends they are attached to about the middle of the labrum, where, at least in [Coronula] (Pl. [26], fig. [3], i), a slight concavity can be detected. The action of these two muscles must be to draw the whole mandible against the labrum; and the depressor muscle might, at the same time, draw the toothed edge downwards, and thus force any prey into the œsophagus.
[34] This is figured by Burmeister in his ‘Beiträge zur Naturgeschichte der Rankenfüsser,’ Tab. 2, fig. 6.
The inner maxillæ are likewise furnished with four muscles, very nearly as figured in my former volume (Pl. 10, fig. 10); namely, two muscles, one inside and the other outside the curious apodeme, which in the [Balanidæ] (Pl. [26], fig. [7], b′) is as invariably present as in the Lepadidæ: these two muscles are attached at their lower ends to the outer membrane of the mouth, close to its basal articulation: the outer one of these two muscles would, I presume, act as an elevator, and the inner one as a depressor; the free part of the organ working on the top of the apodeme, like an axe, on a hinge, on the top of a pole. But there is also a larger depressor muscle, in an analogous position with that (i. e. the first-mentioned muscle) of the mandibles; and a fourth muscle, crossing the latter depressor at nearly right angles, and attached (as far as I could make out) on the side of the orifice of the œsophagus, close under the mandibles: the action of this latter muscle would be to draw the whole organ towards the labrum.
I must not conclude my description of the mouth, without drawing particular attention to its peculiar compounded nature. It is prominent, and is capable, as a whole, of movement; it is separated from the body by a fold or articulation, which can be traced all round. It is, as we have seen, composed of a broad labrum and three pairs of gnathites, which latter have only their terminal segments free; and these surround a conical hollow, at the bottom of which lies the opening of the œsophagus. The prominence of the whole mouth appears to result from the lateral fusion of the two basal segments of the three pairs of gnathites. I have examined the mouth of ordinary Crustaceans, and can see no trace of a structure like this. That there has been some union of the parts is indisputable; for the palpi, which in ordinary Crustaceans are quite free, are here firmly united to the upper and outer corners of the labrum; and indeed, at first appear to be more intimately connected with the labrum than with the mandibles. The palpus on its upper and exterior surface, is in direct continuity with the square thickened piece of membrane, on which the mandible is articulated, and likewise with that side of the upper or free portion of the mandible which faces the labrum. This face of the mandible, beneath the toothed edge, is hollowed out or arched (Pl. [26], fig. [5], p), owing to the above-mentioned continuity of its membrane with that of the palpus. On the lower surface, the palpus is firmly united to the lateral corners of the labrum; or indeed the corners of the labrum may be almost said to be formed by the soft, swollen bases of the palpi: the point of union, when viewed from the outside, is seen to form a knob on the shoulder of the labrum, beneath the level of its crest, and at this knob (Pl. [26], fig. [3], close to d′) several thickened bands in the surrounding membrane unite. The free portion of the palpus stands out transversely behind (i. e. anteriorly to, in a homological sense) the labrum. I suspect that the palpus possibly may consist of two segments, of which the terminal one is free, and the lower one confluent with the labrum.