Lampsilis cardium (Wisconsin Department of Natural Resources / CC BY-ND)

Clams are bivalve mollusks. Two valves connected by a hinge joint form the shell. Like gastropods they have a heart, kidneys, a mouth, a stomach, and a nervous system. However, clams have no heads and are typically blind (except for scallops, which have rudimentary eyes). Most species (oysters and mussels) are attached to substrate and unable to move, while some species (scallops) are able to swim short distances.

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This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.