Members of the phylum Mollusca are soft-bodied animals, such as the snail, clam, squid, oyster, and octopus. Generally, these animals live in water. Some secrete a hard shell. Each mollusk has a muscular organ called a foot that is used for gripping or creeping over surfaces. The animal has a head with a mouth; a brain or a sense organ; and groups of internal organs for circulation, excretion, respiration, and reproduction. All mollusks have a mantle, a thick fold of tissue that covers the internal organs. The space between the mantle and the body organs is called the mantle cavity.
Unlike the previous invertebrates presented in this chapter, mollusks have a well-developed circulatory system with a chambered heart for pumping blood. The blood passes through gills, where oxygen is obtained from water and carbon dioxide is given off. The circulatory system is said to be “open” because blood passes into open spaces at one point during its circulation.
Most mollusks have well-developed nervous systems with large brains and sense organs. Most of these animals have a closed digestive system with only one opening, and a true coelom. All animals of the phylum Mollusca have bilateral symmetry.
Three major classes make up the phylum Mollusca. The first class includes the gastropods, a group of land snails and slugs. The second class encompasses the bivalves, a group of oysters, clams, and other organisms with two shells. The third class is the cephalopods, a group of animals with numerous arms and suckers that extend from the “head foot”; octopuses and squids are in this group.