Body Plan Of Mollusks

Figure 35-2 shows that the body of a mollusk is generally divided into two main regions: the head-foot and the visceral mass. As its name suggests, the head-foot consists of the head, which contains the mouth and a variety of sensory structures, and the foot, a large, muscular organ usually used for locomotion. Above the head-foot is the visceral (VIS-uhr-uhl) mass, which contains the heart and the organs of digestion, excretion, and reproduction. As shown in Figure 35-2, the coelom is limited to a space around the heart. Covering the visceral mass is a layer of epidermis called the mantle.

In most mollusks, the mantle secretes one or more hard shells containing calcium carbonate. Although shells protect the soft bodies of mollusks from predators, they also reduce the surface area available for gas exchange. This disadvantage is offset by another structural adaptation: gills. Gills provide a large surface area that is in contact with a rich supply of blood. In this way, gills are specialized for the exchange of gases. Figure 35-2 also shows that the delicate gills of mollusks are protected within the mantle cavity, a space between the mantle and the visceral mass.

Unlike many coelomates, mollusks do not have segmented bodies. Like flatworms and roundworms, most mollusks are bilaterally symmetrical. This symmetry is apparent in the nervous system, which consists of paired clusters of nerve cells called ganglia. The ganglia are situated in the head-foot and visceral mass and are connected by two pairs of long nerve cords. Nerve cells in the ganglia control the muscles involved in locomotion and feeding and process sensory information from specialized cells that respond to light, touch, and chemicals in the environment.

The main feeding adaptation of many mollusks is the radula (RAJ-u-luh). As Figure 35-3 shows, in most species the radula is a flexible, tonguelike strip of tissue covered with tough, abrasive teeth that point backward. Through evolution, the radula has become adapted for a variety of functions in different mollusks.

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