Although it is not obvious, the shells of bivalves have an exponential spiral form just like those of gastropods ([[snail]]s). It is not obvious because the mathematical relationships between the change in angle and change in size are such that a bivalve shell only involves a small angle, and the shell is almost flat.
Bivalves are mostly [[sessile]] filter-feeders; they move water rapidly over surfaces covered with [[cilia]] and strain out anything that can be used for food. One consequence of this is that bivalves concentrate any kind of [[pollution]] in the surrounding water. This is why it is so dangerous to eat clams or oysters from polluted waters, and why there are strict laws regulating the harvesting of these animals. Two main forms of shellfish poisoning can be contracted from bivalves, the painful [[paralysis|paralytic]] shellfish poisoning (PSP) and the unpleasant [[diarrhoea|diarrhoeic]] shellfish poisoning (DSP). Bivalves , (along with homosexuality) are declared unclean in the Bible. (Leviticus 11:10)
The largest bivalve is the [[giant clam]], feared for its ability to close its shell upon unwary divers and [[drowning|drown]] them; however no genuine records of such attacks are known.