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Class Polyplacophora

The Polyplacophora are a group of entirely marine molluscs commonly known as chitons or coat-of-mail shells. Despite the fact that they are an ancient group, the fossil record is generally poor. In the North Sea, only 10 species are present. They live in the littoral and sublittoral zone and are herbivorous grazers adapted for living on hard substrata.

Chitons are dorso-ventrally flattened and are bilaterally symmetrical. They have an elongated oval body. Dorsally, the mantle bears eight shell valves. These arched transverse plates interlock along the anterior-posterior margins in such a way that the animal is able to curl up into a ball when disturbed. The mantle skirt extends around the whole periphery of the animal as a girdle (Polyplacophora-transverse sec.). The ventral surface is mainly occupied by the muscular foot, which is surrounded by the mantle cavity. This mantle cavity lies essentially posteriorly and contains the anus, renal pores, genital openings, osphradium and gills. In all chitons, the mantle cavity is continuous with deep channels that extend anteriorly on each side of the animal between the girdle and the foot. The head is small, anteriorly situated, and hidden from dorsal view by the girdle. It is not clearly demarcated from the rest of the body. The head bears no eyes or tentacles but has a median mouth armed with a radula.
The morphology of the shell valves is one of the principal taxonomic characters of the Polyplacophora. The head valve (1), or anterior valve, is typically undifferentiated. The dorsal surface of the intermediate valves (2-7) can be divided into three main areas: a central jugal area, flanked by two median triangles and posteriorly two lateral triangles. The tail valve (8), or posterior valve, is divided into two areas: the antemucronal and the postmucronal areas, defined by the position of the mucro.
The shell valves are composed of four layers: the outer periostracum, which contains most of the pigments; the tegmentum, which is perforated by numerous horizontal and vertical canals; the articulamentum and innermost the hypostracum, a thin layer found only in some regions of the valves. The edges of the valves, where they are embedded in the girdle, are termed insertion plates, while the articulating projections found on the anterior edge of all valves, except the head valve, are termed the sutural laminae. Both these structures are projections of the articulamentum. Some species have notches in the insertion plates. From these notches, rows of holes, the notch rays, perforate the ventral surface of the valve towards the posterior edge of valves 1-7, or towards the mucro of valve 8. The jugal area of valves 2-7 may also be perforated by similar holes. These holes are openings for the aesthete canals. These canals accommodate outgrowths of the mantle epithelium. These outgrowths terminate in apical sense organs, the aesthetes. The sculpturing and morphology of the valves is closely related to the abundance and distribution of the aesthete canals. Each aesthete complex may be associated with a raised papilla on the valve surface or the complexes run together without any sculpturing to separate them.

Source: Jones and Baxter, 1987.