Fig. [63].—Diagrams of shells of Operculata. A, Catophragmus (Octomeridae); B, Balanus, Coronula, etc. (Hexameridae); C, Tetraclita (Tetrameridae). C, carina; C.L, carino-lateral; L, lateral; R, rostrum; R.L, rostro-lateral.
The various forms of Acorn-barnacle may be classified according to the number of pieces that go to make up the skeleton; thus starting with the typical number eight (Fig. [63], A), we find that in various degrees a fusion between neighbouring pieces has taken place in the different families.
Fam. 1. Verrucidae.—The ancient genus Verruca, which is still widely distributed in all seas, and is found fixed upon foreign objects on the sea-bottom at various depths, is interesting on account of the asymmetry of its shell, which bears a different aspect according to which side one regards it from. This asymmetry is brought about by the skeletal pieces (carina, rostrum, and paired terga and scuta) shifting their positions after fixation has taken place.
Fam. 2. Octomeridae.—In this family the eight plates composing the shell are separate and unfused (Fig. [63], A). The majority of the species come from the Southern hemisphere, e.g. the members of the genera Catophragmus and Octomeris, but Pachylasma giganteum occurs in deep water in the Mediterranean, where it has been found fixed upon Millepore corals.
Fam. 3. Hexameridae.—This family includes by far the greater number of the Acorn-barnacles, in which only six plates are present, the laterals having fused with the carino-laterals (Fig. [63], B). The very large genus Balanus belongs here, the common B. tintinnabulum of our coasts being found all over the world, and occurring under a number of inconstant varietal forms. Especial interest attaches to certain other genera, from their habit of living parasitically on soft-bodied animals, whose flesh they penetrate.
Coronula diadema and Tubicinella trachealis live embedded in the skin of whales, the shell of the first-named being of a highly complicated structure with hollow triangular compartments into which the mantle is drawn out.
Xenobalanus globicipitis lives attached to various Cetacea, and is remarkable for the rudimentary condition of its skeleton, the six plates of which form a mere disc of attachment from which the greatly elongated naked body rises, resembling one of the naked Stalked Barnacles.
Fam. 4 Tetrameridae.—In this family only four skeletal plates are present (Fig. [63], C). This family is chiefly confined to tropical seas or those of the Southern hemisphere. The chief genera are Tetraclita and Pyrgoma, found in British seas.
Sub-Order 3. Acrothoracica.
Gruvel includes in this sub-order four genera (Alcippe, Cryptophialus, Kochlorine, and Lithoglyptes) the species of which live in cavities excavated in the shells of molluscs or in the hard parts of corals.