Radula
A radula is a toothed chitinous ribbon used to rasp food and found in the mouth of most mollusks (except the bivalves). It is typically mounted on a movable and protrusible muscular mass (called the odontophore) and radula itself is movable over the odontophore. Thus, this organ allows the mollusk to scrape way pieces of food and convey the particles to the digestive tract. The teeth on the radula are chitinous and recurved (point or curve backwards), arranged transversely on a ribbon-like structure. As teeth in the front wear away, new teeth are brought to the fore by a slow forward movement of the ribbon. New teeth are continuously produced within a radular sac behind the odontophore.The number of teeth present depends on the species of mollusc and may number more than 100,000. Indeed, the number, shape, and arrangement of teeth in each transverse row is consistent on a radula, and the pattern can be used as a diasgnostic characteristic to identify the species in many cases. The shape and arrangement of teeth is an adaptation to the feeding regime of the species. Most marine gastropods live by scraping diatoms and other microscopic algae off rock surfaces. Large numbers of teeth in a row (actually v-shaped on the ribbon in many species) is presumed to be a more primative condition, but this may not always be true. The largest number of teeth per row is found in Pleurotomaria (gastropod) with over 200 teeth per row (Hyman, 1967).
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