Crustaceans are a large, diverse group of arthropods, which include animals like crabs, lobsters, crayfish, shrimp, krill, and barnacles. They belong to a larger family of invertebrate animals known as Arthropods. Despite being closely related to insects and spiders, they are classified under their own subgroup due to distinct features such as having an exoskeleton that they shed in a process called molting and two-parted limbs. Although they are not amphibians, their classification, like that of Amphibia, falls under the broader animal kingdom, and they share similar characteristics like being cold-blooded and having life stages that occur in both water and terrestrial environments.