Mollusks and echinoderms, crustaceans and other marine arthropods.
Confused? See below.
Crustaceans: includes crabs, lobsters, shrimp, crayfish, barnacles, and others. Crustaceans have an exoskeleton and usually three body segments. Crustaceans are arthropods related to spiders and insects.
Other Arthropods: such as the horseshoe crab. Included also are some insects and arachnids that are strictly marine. Other insects and arachnids can be found here: Insects & Arachnids.
Mollusks: includes squids, octopus, snails, slugs, mussels, clams, oysters, and others. Mollusks are anatomically diverse, but usually have a shell or mantle, a foot, and a visceral mass containing body organs. Mollusks are mostly aquatic, though some are terrestrial.
Echinoderms: includes starfish, sea urchins, sea cucumbers, sand dollars, and others. Echinoderms are all marine organisms–none are terrestrial and none live in freshwater. Echinoderms have a radial symmetry unusual to bilateral symmetry in most animals.