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PHYLUM MOLLUSCA The molluscs include a number of familiar animals, including snails, oysters, clams, octopuses and squids. Many species have a calcareous shell. Molluscs are bilaterally symmetrical, although some have secondarily lost the symmetry in part; they have well-developed excretory, digestive, respiratory and circulatory systems. In all molluscs except the cephalopods, the circulatory system is open. All molluscs are characterized by having three main body regions: a  head-foot , which is the sensory and locomotor part of the body; a  visceral mass , containing most of the organ systems; and a  mantle , which covers the visceral mass and secretes the shell, if one is present. Most molluscs also have  radula , a rasping structure covered with chitinous teeth, which is used in feeding. Molluscs are probably closest phylogenetically to the annelids. Several lines of evidence support this view. Both annelids and molluscs have very similar developmental patte...