Crustaceans are a subphylum in the phylum Arthropoda, so they have a tough exoskeleton, a series of jointed appendages, and a segmented body. Crustaceans have three major body parts. They are, front to back: head, thorax, and abdomen. They have two pairs of antennae, and two eyes. The mouth has two mandibles. Most breathe with gills, although sime land crabs have developed lungs. Lobsters and crabs have hard outer skeletons (exoskeleton), and tend to preserve well as fossils. As adults, they moult their shells as they grow in size.