Includes crabs, lobsters, horseshoe crabs, krill, etc. Interesting Facts: - The three main types of marine arthropods are horseshoe crabs, sea spiders, and crustaceans. - All arthropods have jointed appendages, which allow for greater mobility. - Barnacles are actually a form of crustacean, which many do not know Arthropod on field guide: Acorn Barnacle (Balanus plandula)