Malacostraca

Malacostraca is the largest and most familiar class of crustaceans. This group includes a huge variety of animals, from the lobsters, crabs, and shrimp we eat to krill, mantis shrimp, and isopods (like pillbugs). Their name, which means "soft shell" in Greek, is a bit misleading, as most of them have a very hard, calcified exoskeleton.

Like other arthropods, their bodies are segmented and have jointed appendages. The body of a malacostracan is typically divided into three main parts: a five-segmented head, an eight-segmented thorax, and a six- or seven-segmented abdomen. In many species, the head and thorax are fused together, forming a cephalothorax. They possess two pairs of antennae and a pair of eyes, which are often on movable stalks. Many malacostracans also have specialized limbs, such as the large claws of a crab or lobster, for defense and feeding. They are found in almost all marine environments and have also successfully colonized freshwater and terrestrial habitats.