Mollusks are related (distantly) to the common garden snail. In the sea they are larger, more diverse, faster and smarter, from almost invisibly tiny limpets to colossal squid 46 feet long.
Crustaceans are related to the common garden sow bug. In the sea they are also more diverse and larger with crabs and lobsters that weigh over 20 pounds.
Echinoderms include the starfish, sea urchin and sea cucumber.