These are of three different kinds; first, the "dorsal," as they are termed by Linnæus, because they form a ridge in the back of the animal. They are composed of eight pieces, or separate valves, placed in a longitudinal series, being joined to each other by inserted lamina, and named Articulata by De Blainville, on that account. The genus Chiton is the only example of this kind of Multivalves.
Fig. 84, 85, Chiton. a, anterior; p, posterior; d, dorsal ridge; l l, lateral areas of the valves; c c, central areas; i i, inserted lamina; m, margin.
The second kind, M. De Blainville terms the lateral bivalves, the pieces being placed in pairs on each side of the animal; these compose the "Pedunculated Cirripedes."
Fig. 86, Anatina.
They differ considerably in the number and arrangement of the valves; the small ones, which are found near the peduncle in some species, are sometimes termed accessary valves; those which form the edge through which the bunch of Cilia protrude, are termed ventral, and those on the opposite side dorsal. The extremity joining the peduncle is the basal, or anterior; and the upper extremity is the apsiral, or posterior. The peduncle is the medium of attachment to submarine substances, to which this well known tribe of shells adhere.
The third kind are termed coronular by De Blainville, and compose the order Sessile Cirripedes of Lamarck; they consist of a number of valves placed against each other side by side in a circle, supported on a plate, or tube, or cup, and closed by an operculum composed of two or more valves.
The basal support is sometimes thick and flat, sometimes forming an elongated tube, and sometimes hollowed out into a cup. In other species it is altogether wanting. The operculum always consists of more than one piece, generally of two pairs: they are either articulated to each other by serrated edges, and placed against each other conically, as in Balanus, or they lie flat in two pairs against each other. Through the ventral pair the cirrhi protrude.
The parietal valves, composing the principal part of the shell, vary in number, form and position. The anterior valves are placed on the same side with the cirrhi; the posterior, those on the opposite side; and those which remain between on each side are the lateral valves. In many cases, particularly in Balanus, each valve is separated into the prominent and depressed areas, and the inserted lamina. In some instances, the parietal portion is formed by a single rounded piece.