The Malacostraca are the largest group (class) of crustaceans with more than 25,000 species. They include the Decapods - crabs, lobsters, and shrimp, the Stomatopods - mantis shrimp, the Euphausiids - krill, . the Amphipods (sandhoppers) and the Isopods which are found on land (woodlice).

Their major structural features are:

• A head (with 5 segments, a thorax with 8 segments andan abdomen with usually 6 segments;
• The head has 5 segments, with a pair of antennules and a pair of antennae, as well as 3 mouthparts, comprising the mandibles, the maxillula and the maxilla;
• 8 thoracic segments. The cephalothorax is covered by a hard exoskeleton (carapace) by the fusion of 3 segments;
• They usually have 8 pairs of thoracic legs of which the first pair or several pairs are often modified into feeding appendages called maxillipeds. The first pair of legs behind the maxillipeds is often modified into pincers;
• The abdomen has 6 segments . The appendages are called pleopods and are usually used for swimming;
• Genital openings on the sixth segment for the female and on the eighth segment for the male;
• They have compound stalked or eyes not on stalks;
• They have a two-chambered stomach and internal gills;
• They have a centralized nervous system with a large brain and ventral nerve cord
  

Diversity in the Malacostraca

Decapod external anatomy