Which body features identify an animal as a crustacean?

Crustaceans are members of the subphylum Crustacea within the phylum Arthropoda. While defining the group is challenging due to significant diversity, they are generally identified by the following features:

• Appendages: Crustaceans are distinguished from other arthropods by having two pairs of appendages—the antennules and antennae—located in front of the mouth, followed by paired appendages that function as mandibles (jaws).6
• Body Plan: Many crustaceans, particularly malacostracans, follow a body plan consisting of a head, thorax, and abdomen. In many groups, the head and thorax are fused into a cephalothorax, which is often covered by a protective shield called a carapace.12
• Exoskeleton: They possess a protective exoskeleton composed of chitin and proteins, which is often reinforced with calcium salts. Because this shell is rigid, crustaceans must undergo molting to grow.3
• Sensory Organs: Most possess compound eyes and various setae (hairlike processes) on their exoskeleton that function as tactile or chemosensory receptors.4