The word bivalve means having two shells. The two bivalve shells are hinged together at one place and can be closed tightly by a powerful muscle, the adductor.
Although it is not obvious, the shells of bivalves have an exponential spiral form just like those of gastropods (snails). 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 paralytic shellfish poisoning (PSP) and the unpleasant diarrhoeic shellfish poisoning (DSP). Bivalves are declared unclean in the Bible. (Leviticus 11:10)
- Wile, Dr. Jay L. Exploring Creation With Biology. Apologia Educational Ministries, Inc. 1998