Fig. 261.—Valves of a Chiton separated to show the various parts (anterior valve uppermost): a, a, articulamentum; b, beak; j, jugum; pl, pl, pleura; t, t, tegmentum.
Fig. 262.—Valves of Chitonellus separated out (anterior valve uppermost): a, a, articulamentum; t, t, tegmentum. × 2.
The Polyplacophora are characterised, externally, by their usually articulated shell of eight plates or valves, which is surrounded and partly kept in position by a muscular girdle. These plates overlap like tiles on a roof in such a way that the posterior edge of the first, cephalic, or anterior valve projects over the anterior edge of the succeeding valve, which in its turn overlaps the next, and so on throughout. Seven-valved monstrosities very rarely occur.
A certain portion of each valve is covered either by the girdle or by the valve next anterior to it. This portion, which is whitish in colour and non-porous in structure, forms part of an inner layer which underlies the rest of the substance of the valve, and is called the articulamentum. The external portion of the valves, or tegmentum, is generally more or less sculptured, and is largely composed of chitin, impregnated with salts of lime, thus answering more to a cuticle than to a shell proper. It is very porous, being pierced by a quantity of minute holes of two sizes, known as megalopores and micropores, which are connected together by minute canals containing what is probably fibrous or nerve tissue, the mouths of the pores being occupied by sense organs connected with these nerves. The tegmentum of the six intermediate valves is generally divided into three triangular areas by two more or less prominent ribs, which diverge from the neighbourhood of the median beak or umbo. The space enclosed between these ribs is known as the median area or jugum, the other two spaces as the lateral areas or pleura. The ribs terminate with the edge of the tegmentum, and are not found on the articulamentum. In certain genera these areas are either non-existent, or are not distinctly marked. The sculpture of the lateral areas (which is, as a rule, much stronger than that of the median area) will generally be found to resemble that of the anterior valve, which has no proper median area. In the posterior valve the median area is very small, while the sculpture of the rest of the valve corresponds to that of the lateral areas generally (see Fig. [261]).
Fig. 263.—First, fourth, and eighth valves of a Chiton, showing l.i, laminae of insertion; n, n, notches; s.l, s.l, sutural laminae. × 2.
The articulamentum of the intermediate valves is divided into two equal parts in the middle of the anterior edge, opposite to the beak, by a sinus. Each of the portions thus formed is again divided by a notch or suture into two unequal parts, the anterior of which is known as the sutural lamina, and is more or less concealed by the valve in front of it, while the lateral part, or lamina of insertion, is entirely concealed by the girdle. The articulamenta of the anterior and posterior valves are either simple or pierced by a series of notches (Fig. [263]).
The girdle of the Chitonidae varies considerably in character. Sometimes its upper surface is simply corneous or cartilaginoid, with no other sculpture than fine striae, at others it is densely beset with spines or bristles, or tufted at intervals with bunches of deciduous hairs; again it is marbled like shagreen or mossy down, or covered with serpent-like scales. The width of the girdle varies greatly, being sometimes very narrow, sometimes entirely covering all the valves (Cryptochiton). As a rule, its outer edge is continuous, but in Schizochiton it is sharply notched over the anus.