The shells of the Cirripeds are similar in general appearance to those of many Acepha lous Mollusca. They are most fully developed in Anatifa, which has five separate plates, four placed laterally in pairs, and, one median. One pair is conside rably larger than the other (c, fig. 332) ; it covers all the anterior part of the animal, and the greater part of the internal organs. The bases of these shells are attached to the car tilaginous peduncle ; the lower halves of their anterior edges form part of the mar gin of the slit-like opening tl 'rough which the arms are protruded (f,g, fig. 332). The inferior pair of shells (d) are of a triangular form ; the smallest side completes the margin of the brachial ori fice ; another side is united by ligament to the upper valve; the third is connected with its fellow by the common intervalvular ligament.
The median piece (e) covers the dorsal aspect of .the animal. It has an elongated lanceolate shape, curved and grooved internally. Its upper point only is inserted into the peduncle.
Its margins are imbedded in the intervalvular ligament. This piece may be compared to the unpaired valve of the shell of Pholas : it oc cupies nearly the same situation. The surface of these shells is generally denuded of epi dermis, excepting just around their margins.
All three are strongly and regularly marked with lines of growth, from which it is seen that the two pairs of lateral valves increase in size, chiefly, by additions to their margins, which look towards one another ; so that the parts first formed are, in the adult animal, re moved to the greatest possible distance from one another. In the upper valve, the umbo or centre of growth is situated in the anterior superior angle, close to the termination of the peduncle; in the lower, it is situated in the anterior-inferior angle; and in the dorsal valve, in the point next to the peduncle. All the shells are thin, diaphanous, of nearly the same thickness throughout, yet much less fragile than shells of Acephalous Mollusca which otherwise resemble them. It has been re marked by Burmeister that the shells of Cir ripeds resemble those of crustaceous animals more than those of Molluscs : to us it appears that they have a greater degree of density, and a more compact crystalline structure than are commonly met with in Crabs ; and that their well-marked lines of growth give them a closer resemblance to shells of acephalous mollusca.
In some genera, as Pollicipes, in addition to the five valves just described, there are other eight smaller calcareous plates arranged around the junction of the peduncle with the shells.
The shells of the Balanids present several striking peculiarities of structure and, in their mode of growth, offer to the Physiologist an interesting subject for investigation. They form truncated cones, the bases of which, without the intervention of peduncles, are fixed to rocks, floating wood, integuments of marine animals, &c. These cones are composed of several pieces, closely cemented together so as to admit of no motion between them, excepting during the process of enlargement of the shell.
In the common acorn-shells (fig. 333), which cover our litto ral rocks and the bottoms of ships, there are seven of these pieces, six form ing the walls, and one dis coid, forming the base. The outer surface of the parietal valves is 'nark ed by the lines of growth in such a manner as to give it the appearance of being com posed of twelve pieces. These may be termed compartnzents. They are all conical. Six of them have their bases applied to the common base of the shell, and the other six are inserted between these, with their apices towards the common base. The first six we shall refer to under the name of the first series of compart ments (a, a, fig. 333); the other six constitute the second series (b, b, fig. 333). The opening in the summit of the cone is closed by an opercule composed of four shelly pieces so arranged as to leave a longitudinal fissure be tween them, through which the arms are pro truded (c, fig. 333). The two series of com partments differ much from one another in their external aspect, owing to the differences in the directions and appearances of the lines of growth. The second series have a smoother surface, and are marked with very delicate lines, both longitudinal and transverse ; they are also less prominent than the first series. The lines on the first series are chiefly trans verse, and correspond with the outline of the base. On the internal surface of the walls there are six deep grooves, in the bottoms of which are seen the openings into certain cham bers, constituting a sort of diploe of the valves, hereafter to be described. These grooves run from the summit to the base of the shell, and are the internal edges of the sutures of the six parietal valves. Around the internal margin of the common base there is a series of holes opening into certain tubes that terminate on the outer margin of the shell. When all the valves are separated at the sutures, it is found that each of four of the six compartments of the first series, as they appear externally, has attached to its dorsal margin one of the second series, and that the union between these two is exceedingly intimate, in fact that they form one piece, notwithstanding their apparent division externally. Two of the second series of compartments are attached to the anterior valve, while the dorsal valve has none. The anteal margins of the lateral valves and both margins of the dorsal valve are marked by transverse depressions corresponding to the numerous partitions of the chambered com partments which are fitted into them ; and, externally, each has a projecting margin. To the upper part of the inner surface of each valve there is attached a laminated process, form ing part of a circle of calcareous plates which gives support to some parts of the mantle.