Flexi Says: Overall, mollusk species can be found almost anywhere on earth. Cephalopods are exclusively marine inhabitants. Gastropods and bivalves can be found in marine and freshwater environments, although gastropods can also be found on land. Many species have important relationships with humans (for example, as food). Mollusks are mostly bilaterally symmetrical and often show cephalization. Their basic body plan consists of three parts: a visceral mass, where most of the internal organs are; the mantle, which wraps around the visceral mass and produces the shell if there is one; and the foot, that protrudes out of the mantle and is modified for different uses in the different groups. The radula is a specialized muscular feeding organ that contains teeth made of a carbohydrate (chitin) substance. It is located in front of the mouth in the head region of all mollusk classes except Bivalvia.