The body structure of malacostracans is highly diverse. In general, it consists of three tagmata (groups of segments) including a head, thorax and abdomen. The head consists of five segments, the thorax has eight segments and the abdomen has six segments.
The head of a malacostracan has two pairs of antennae and two pairs of maxillae. In some speies, there is also a pair of compound eyes that are located at the end of stalks.
Pairs of appendages are also found on the thorax (the number varies from species to species) and some of the segments of the thorax tagma may be fused with the head tagma to form a structure known as the cephalothorax. All but the last segment of the abdomen bears a pair of appendages called pleopods. The last segment bears a pair of appendages called uropods.
Many malacostracans are brightly colored. They have a thick exoskeleton that is further strengthed with calcium carbonate.
The world's largest crustacean is a malacostracan—the Japanese spider crab (Macrocheira kaempferi) has a legspan of up to 13 feet.
- Eumalacostraca - This group includes 40,000 species, which makes it the largest group of malacostracan species. The group includes krill, lobsters, crabs, shrimp, prawns, mantis shrimp and many others. Within this group, the most familiar subgroups include the crabs (a group more than 6,700 species of 10-legged crustaceans that have a short tail and small admomen that lies beneath the thorax) and the lobsters (of which there are several groups—the clawed lobsters, spiny lobsters and slipper lobsters).
- Hoplocarida - This goup includes mantis shrimp, a group of about 400 species of crustaceans whose name reflects their superficial resemblence of the praying mantis (which is an insect and thus not closely related).
- Phyllocarida - This group includes about 40 species of filter feeding crustaceans, the most well-studied of which are the Nebalia.