Most mollusks have an elongate, unsegmented body with a distinct head. The internal organs are held between a muscular foot, a modified lower part of the body, and a calcareous shell secreted by an underlying tissue known as the mantle (Fig. 9.1). The mantle tends to overhang the body forming a chamber at the posterior, the mantle cavity. This cavity contains the gills. The mouth opens anteriorly, at the other end of the mollusk. Sensory organs, such as eyes and tentacles, are concentrated in the head.
Shell morphology is extremely diverse and it performs a range of functions for different groups of mollusks. Shells may be coiled or straight, chambered or undivided, singular or two-valved. Primarily, shells provide protection but they also may be used in burrowing or boring, or to enable buoyancy. The evolutionary development of some mollusks, such as octopus and squid, has tended to result in the loss of the shell.
In other groups, such as the snails, the shell has become emphasized with time. Mollusks with a shell are more likely to fossilize and therefore this chapter will focus on such groups.
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