Crustaceans are an extremely diverse group of invertebrates, which includes, among others, the well-known and appreciated shrimps, crabs, and lobsters. They are specialised for life in water, they breathe through gills, which vary in number and shape, and their excretory organs are 1 or 2 pairs of glands associated with the antennae or maxillae.
Their body is covered by a rigid carapace made of chitin, sometimes impregnated with calcareous salts. Chitin is soft at the gaps between segments to allow movement.
The carapace prevents them from growing continuously, which is why they have adopted a curious way of growing: when the body starts to get 'tight' inside the carapace, they break free from it. They then take advantage of a period of time in which they are protected only by a flexible membrane to grow. They then form a new carapace adapted to the new dimensions of the body. This process of replacing the carapace is called moulting.