A bivalve is a kind of shellfish that has a soft body protected by a hard shell, which usually is in two parts, hinged together.
Most bivalves live in the ocean, though some live in fresh water, rivers or lakes.
The most familiar bivalves are oysters, clams, mussels, and scallops, because they are very valuable as food to people who live near the ocean.
There are separate articles about these, and about shellfish.
Oysters and mussels attach themselves to rocks; clams burrow in sand or mud; shipworms bore into wood; and scallops swim about freely, and are the only bivalves that have eyes.
Bivalves feed by means of siphons, or tubes.
They suck water in through one siphon, filter it to trap the tiny living creatures or decaying particles on which they feed, and then shoot out the waste through their other siphon.
In this way they are very useful for purifying water.
Each bivalve is both male and female.
As many as two million eggs are released into the water by an individual.
They hatch into tiny creatures that swim by tiny lashing hairs.
After a while this creature grows a flap on each side of its mouth.
Then its shell begins to form, and it becomes too heavy to float, and sinks to the bottom.
Those that find themselves in a suitable place grow; the others die.
From the lines on their shells you can count how many years old they are.
Mussels live to be about ten years old.
The giant clam of Australia may live for sixty or even a hundred years.