A very diverse phylum of invertebrates which have a common body plan modified in various ways. There is usually a shell, secreted by a series of tissues (called the mantle), and beneath this there is the body, which contains a space (mantle cavity) in which lie the gills. The body plan is modified to give the major classes of the phylum. One class, the Monoplacophora, shows signs of internal segmentation. The Amphineura (chitons) have a bilaterally symmetrical shell of eight plates. Scaphopoda have tapering, curving shells which are open at both ends. Bivalvia have the soft parts enclosed between two shells. Gastropoda usually possess a coiled univalve shell. The most advanced of the molluscs are the Cephalopoda, which possess either internal or external, chambered shells. Other groups are also assigned to the Mollusca, but only cephalopods, bivalves, and gastropods have a good geologic record.