Crustaceans are lower animals such as shrimps, prawns and lobsters. They form one of the principal classes of great phylum Arthropods. Crustaceans are mostly aquatic in habitat where other arthropods mostly live on land. Crustaceans are so called because their body is protected by a chiti or sub calcareous `crust` or cover. Many of the small crustaceans such as Daphnia, Cyclops and even small prawns serve as food for fishes and other larger aquatic animals. In typical crustaceans such as crayfish or Spiny Lobster the body is elongate and is formed of segments plus an anterior most terminal known as telson. The body may be divided into regions-a head, a thorax and an abdomen. The segments of the head and thorax are fused to form a rigid `ce thorax`. Behind the cephalothorax is the flexible abdomen composed of six movable segments and the hindermost telson. Each segment bears a pair of appendages.

The entire body of a typical crustacean is protected by a chitinous coating impregnated with lime. Strictly speaking everybody segment or somite is covered by a separate chitinous ring or sclerite. These rings, however, get fused to form a shield or carapace over the rigid cephalothorax. The anterior part of the carapace is produced forwards to form a pointed beak or rostrum bearing a row of spines.

Members of the smaller crustacean groups have as many as 41 to 63 thoracic segments and appendages. In all the highly organized crustaceans like lobster and crabs the carapace completely covers the thorax and they have their characteristic well-developed walking legs, the first pair of which has pinching claws. Crustaceans possess all the important physiological systems of higher animals, viz. digestive, circulatory, respiratory, excretory, nervous, reproductive and sensory.

The principal food of crustaceans includes small animals like snails, insect larvae, eggs and tadpoles, plankton organisms and decaying organic matter. They even prey upon each other. A few of them are wholly plant-eaters. It is during the early morning and at dusk that these animals are most active at catching and eating their prey. Since all crustaceans feed on decaying matter along with other food they are known as scavengers of the sea.

Respiration in crustaceans is performed chiefly by a series of gills contained in a special chamber on either side of the thorax. The circulatory system consists of a heart and several principal arteries and a number of spaces called sinuses into which the blood passes from the arteries. The blood is colourless and contains slightly bluish coloured haemocynin in place of the haemoglobin of higher vertebrates.

The excretory organs of crustaceans are a pair of glands placed laterally on either side of the gullet. The Nervous System is simple like that of a worm. It consists of a brain situated above the oesophagus. The principal sense organs of typical crustaceans are a pair of compound eyes. There are separate males and females in prawns and lobsters. In some other crustaceans both the male and female organs are in one.

Crustaceans form an immense class including groups under 5 sub-classes and 20 orders widely distributed mostly in the sea at depths measuring down to 3000 fathoms. In size crustaceans vary from microscopic forms to those measuring with their legs extended up to 15m long.