Most bivalves are relatively sedentary organisms; however, many are capable of considerable levels of activity. The former class name Pelecypoda means "hatchet-foot," referring to the laterally compressed foot typically used for burrowing in sand or mud. Bivalves move downward into the substrate by extending the foot into the sediment, anchoring the foot by expanding its tip, and pulling the shell downward toward the anchor by muscular action. Byssally attached bivalves (e.g., Mytilidae, Dreissenidae) can break their byssal threads to relocate, and use the foot to move across a hard substrate in a sequence similar to that used for burrowing. They then produce a new byssus for reattachment. Other bivalves can actively swim by waving the foot or tentacles (e.g., Solenidae, Limidae) or by jet-propelling themselves by rapidly clapping the shell valves together (e.g., Pectinidae).
Although the reduction of the bivalve head has eliminated cephalic eyes and other sense organs, many bivalves (e.g., Ga-leommatidae, Pectinidae) have tentacles and/or photoreceptors along the mantle margins or in the vicinity of the siphons. These structures allow bivalves to respond to changes in light intensity by retracting the siphons and closing the valves. More sophisticated eyes, equipped with retina and lens, are found in several families of epibenthic bivalves (e.g., Cardi-idae, Pectinidae).
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