Crustaceans are a diverse group of arthropods that includes species such as crabs, lobsters, crayfish, shrimp, krill, barnacles, and many others. They are typically aquatic and have a hard exoskeleton, which they must periodically shed in order to grow. Like insects and other arthropods, they have segmented bodies and jointed limbs. Although they are primarily aquatic, some crustaceans have adapted to life on land, much like some amphibians which are known for their ability to live both on land and in water. Crustaceans fall under the broader category of animals, characterized by their ability to move, reproduce, respond to their environment, and have complex sensory and nervous systems.