The skeletal ring is roofed over by a pair of terga at the carinal end and a pair of scuta at the rostral end; these four plates make up the operculum by which the animal can shut itself completely up in its shell, or between the valves of which it can protrude its limbs for obtaining food. [Illustration: FIG. 62.—Diagrammatic section of the growing shell of _Balanus porcatus_. _C_, Canals; _Ct_, cuticle; _H_, hypodermis (= epidermis); _H′_, part of shell secreted by the hypodermis; _Hl_, hypodermal lamina; _M_, part of shell secreted by the mantle. (After Gruvel.) ] The relation of the animal to its shell is shown in Fig. 61. The shell in the Operculata is not merely secreted as a dead structure on the external surface of the epidermis, but represents a living calciferous tissue interpenetrated by living laminae (Fig. 62, Hl) derived partly from the external hypodermis and partly from the lining of the mantle. The hard parts of the shell usually also contain spaces and canals (C). [Illustration: 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.