Phylum Mollusca consists of one of the most diverse groups of animals. It has at least 85,000 species in the phylum. Some Familiar organisms in this phylum include snails, octopuses, squid, clams, scallops, and oysters. Mollusks range in size from 20-meter-long giant squid to microscopic milometer organisms. They have relatively soft bodies and are covered with a hard exoskeleton, such as the shell of a snail. Mollusks have a head and foot region that contain sensory organs and muscles, these muscle structures serve for locomotion. They can be found in almost every ecosystem around the globe such as terrestrial mountain tops and in the deep sea.