Tusk shells

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Scaphopoda (tooth shells, tusk shells; phylum Mollusca) A class of elongate, bilaterally symmetrical, burrowing, marine molluscs which have a tapering shell open at both ends, the larger, apertural end being anterior. Most are also curved, with the convex side ventral. The shell is secreted by the mantle and is composed of three distinct layers of calcium carbonate. Most individuals are 2–5 cm long. The head contains the mouth with a radula, surrounded by food-gathering appendages. There are no eyes. The mantle cavity contains paired kidneys, a liver, a simple stomach, a heart, and a solitary gonad. The nervous system is ganglionic, and there are no gills. The blood-filled foot is used for burrowing. The order contains two families. Scaphopoda are thought to have first appeared in the Devonian, but some have been tentatively reported from the Ordovician in Russia.

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tusk shells See SCAPHOPODA.