Crabs are in the subphylum Crustacea.
They are decapods (having eight walking legs and two grasping claws), along with lobsters, crayfish and shrimps.
Crabs form an order within the decapods, called the Brachyura.
Their short body is covered by a thick exoskeleton.

They are an extremely successful group, found all over the world.
They are basically heavily armoured shell-breakers.
Most crabs live in sea-water, but there are some who live in fresh water, and some who live on land.
The smallest are the size of a pea; the largest (the Japanese spider crab) grows to a leg span of 4 metres.
About 7,000 species are known.
Crabs are in the subphylum Crustacea. They are decapods (having eight walking legs and two grasping claws), along with lobsters, crayfish and shrimps. Crabs form an order within the decapods, called the Brachyura. Their short body is covered by a thick exoskeleton.
They are an extremely successful group, found all over the world. They are basically heavily armoured shell-breakers. Most crabs live in sea-water, but there are some who live in fresh water, and some who live on land. The smallest are the size of a pea; the largest (the Japanese spider crab) grows to a leg span of 4 metres. About 7,000 species are known.
Crab