Echinoderms include starfish, sea urchins, and sea cucumbers. They live in seawater and have external skeletons. Mollusks are soft-bodied animals, which often live in hard shells. They include snails, slugs, octopus, squid, mussels, oysters, clams, scallops, chitons, and cuttlefish. Mollusks are the second-largest group of invertebrates, with 50,000 living species.
Arthropods are the largest and most diverse of all animal groups.
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