Crustacea are mainly aquatic arthropods with gills for respiration. Cephalothorax has usually a carapace; appendages biramous but modified for several functions. Head has a pair of antennules, it pair of mandibles, it pair of antennae, and two pairs of maxillae. Development with a nauplius stage, but this map be not present in higher forms. Crustacea involves arthropods most of that have an aquatic existence. The group abounds in species diversity and biomass.
Figure: A generalised Crustacean
It involves crabs, shrimps, lobsters, crayfish, woodlice etc. even though largely aquatic, there are some semi-terrestrial and terrestrial species. Their success on land is not significant. Let us in brief look into the structural peculiarities of crustaceans. The body of crustaceans is categorized into three regions, head, thorax and abdomen. The head and thoracic segments may be fused to make the cephalothorax. The thorax and abdomen may together form a trunk in some forms. The thoracic segments are covered through a dorsal shield known as carapace. The carapace is a posteriorly directed fold of the body wall of the head and fused along with varying number of segments behind it. Carapace may overhang the sides of the body to some extent or in a few cases it may enclose the whole body.