The document discusses the three main classes of mollusks: gastropods which include slugs and snails, bivalves like clams that have two shells, and cephalopods such as squid and octopus. It provides details about their anatomy including that they have a foot, visceral mass containing organs, and mantle that secretes shells for most. It also describes how different types eat, their circulatory systems, and nervous systems. Facts noted are that most live in oceans, range in size from 1mm to 18m, and cephalopods have the most advanced nervous system of invertebrates.