Cirripedia.—These animals have a soft body, enveloped in a membrane, which in some genera is protected only by a horny sheath, but in general is enclosed in a shell composed of various calcareous plates.[432] They have six pairs of feet, terminating in long, slender, articulated tentacula, furnished with cilia, and coiled up like tendrils at the extremities near the mouth. The name of the class (curled-feet) has originated from the appearance presented by the curled tentacula when projecting from the oval aperture of the shell. The testaceous Cirripedes or Barnacles are divided into two groups; namely, the sessile, or those which in their adult state are fixed by the base to other bodies, (Balanidæ,) as the Acorn-shell, or Balanus; and the pedunculated, which have a process of attachment, peduncle or stem, (Lepadidæ,) as the Duck-barnacle, or Lepas. But the young animals of these genera have powerful locomotive organs, and are capable of swimming, by sudden jerks, like some of the crustaceans, to which class, especially in this stage of their existence, they closely approach. But after a short period of freedom, the young Cirripede fixes itself in some locality suitable to its economy, and rapidly undergoes the transformation which results in the sessile adult Barnacle or Lepas.[433]
[432] Until within the last few years the valves of the Cirripedes have been generally figured and described in works on Conchology as being allied to the Mollusca.
[433] See Mr. C. Darwin’s admirable Monograph on the Cirripedia, published by the Ray Society, 1851.
Balanus. [Lign. 167, fig. 1.]—The shell of this sessile Cirripede is of a conical shape and cellular structure. It consists of a thick plate at the base, or place of attachment; of a series of plates, united by sutures, arranged around the body of the animal, and called parietal valves; and of pieces termed opercular valves, by which the aperture is closed. The shell of the Balanus, so common on the rocks of our shores, and on every pile and pier within reach of the tide, is composed of six parietal, and four opercular valves. The fossil Balanus, [Lign. 167]. fig. 1, is from the Crag, a formation containing many shells of this genus: in this example, the six parietal plates of the conical shell only remain, but in some specimens the opercular valves are also preserved. There are about twelve species of Balanus in the Crag; but none have been found either in the older Tertiary, or in the secondary rocks of England. In the newer Pliocene deposits of the Sub-Apennines, and of North America, several species are common.
Lign. 167. Fossil Barnacles and Pholades.
| Fig. | 1.— | Balanus tesselatus. (Sow. Min. Conch.) Crag. Suffolk. |
| 2.— | Loricula pulchella. (Mr. G. B. Sowerby, jun.) Chalk. Kent. a. The situation of the aperture of the shell. | |
| 3.— | A valve (Tergum) of Scalpellum maximum. Chalk. Lewes. | |
| 4.— | Another valve (Carina) of the same species. | |
| 5.— | Pholas priscus[434]; in wood. Lower Green Sand. Sandgate. a. One of the shells seen in profile. | |
| 6.— | Univalve Shells (Neritæ) in hollows formed by Pholades. (Mr. Bensted.) Lower Green Sand. Maidstone. a. The Nerites lodged in the cavities. b. Fragment of wood remaining attached. |
[434] Figs. 5 and 6 are referred to and described at pages [409] and [410], vol. i.