All mollusks have a common ancestor in the annelid worm, are primarily aquatic animals, have soft bodies, have hard shells (the exoskeleton), have a mantle (thin tissue) and have an open circulatory system. Mollusks include species such as the octopus, clam, squid, snail, mussel and oyster.
Mollusks also have different body regions: the head, the foot and the mass of the body holding the internal organs. The "foot" is the part of the body that connects to the ground and helps the mollusk to move. Mollusks come in different classes including the class polyplacophora, the class gastropoda, the class bivalvia and the class cephalopoda. Scientists have discovered more than 50,000 different mollusk species.