Why are crabs, lobsters and shrimps called crustaceans?

The word ‘crustacean’ comes from the Latin crusta meaning a hard covering shell or crust. Apart from crabs, lobsters and shrimps, there are thousands of different crustaceans. They live in the sea, except for a few species such as the common woodlouse.

Crustaceans differ greatly in size and shape. Many of them pass through remarkable changes of form (metamorphoses) before reaching the adult stage. All of them, however, have bodies and limbs, which are divided into segments. All, too, are covered with a tough, hard, lime-impregnated coat, or shell, which is pliable at the joints, so that the creature can move its limps.

The coat, or cuticle, cannot grow to fits its wearer. As its owner grows, it is split and cast off. A new coat, still soft and pliable, has been forming underneath, and may take several days before it hardens into a truly protective shell. During this time the creature is defenceless and may fall an easy pray to any enemy.

Vast numbers of crustaceans live in the oceans. They provide food for many kinds of fish and also for the largest living mammals, the whales.