The radula and mouth are contained in aproboscis that can be protruded to strikethe prey (Fig. 7.20). Jaws may even bepresent. In these snails, digestion is extra-cellular and takes place in the stomach.Bivalves ingest food particles that arefiltered and sorted out by the cilia on thegills. The radula is absent, and food en-ters the mouth trapped in long strings of mucus. An enzyme-secreting rod in thestomach, the
crystalline style(Fig.7.22b), continually rotates the food tohelp in its digestion. The stomach’s con-tents eventually pass into a large digestivegland for intracellular digestion. Thegiant clam not only filters food but ob-tains nutrients from zooxanthellae thatlive in tiny branches of the gut that ex-tend into its expanded mantle (see Fig.14.34). This extra nourishment may allow them to attain their giant size. All cephalopods are carnivores thathave to digest large prey. The stomach issometimes connected to a sac in whichdigestion is rapidly and efficiently com-pleted. It is entirely extracellular.Molluscs have a circulatory systemthat transports nutrients and oxygen. Adorsal, muscular heart pumps blood to alltissues. Most molluscs have an
open cir-culatory system
in which blood flows outof vessels into open blood spaces.Cephalopods, on the other hand, have aclosed circulatory system in which theblood always remains in vessels and canbe more effectively directed to oxygen-demanding organs such as the brain