Why are crabs, lobsters and shrimps called crustaceans?

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 limbs. This coat or cuticle cannot grow to fit 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 prey to any enemy. Vast numbers of crustaceans live in the oceans. They provide food for many kids of fish and also for the largest living mammals the whales.

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