well into the visceral mass (digestive glands, intestines, gonad, etc.). The digestive gland and interweaving intestine occupy most of the visceral mass behind the head. At the posterior ventral end is the large gonad organ which, when ripe, bursts and empties its gametes into the right kidney from which they are then expelled directly into the surrounding water. One theory of the function of the osphradia is to sense the release of such gametes by other nearby patellogastropods, triggering a corresponding release in any proximate opposite-sex animals of the same species (see diagram for additional anatomic information). Distribution Representatives of the Patellogastropoda, true limpets, live on the rocky coasts of all oceans. Habitat Some true limpets live throughout the intertidal zone, from the high zone (upper littoral zone) to the shallow subtidal, but other species live in deep sea and their habitat include hydrothermal vents, whalebone (baleen), whale-fall and sulphide seeps. They attach themselves to the substrate using pedal mucus and a foot. They locomote using wave-like muscular contractions of the foot when conditions are suitable for them to graze. They can also "clamp down" against the rock surface with very considerable force when necessary, and this ability enables them to remain safely attached, despite the dangerous wave action on exposed rocky shores. The ability to clamp down also seals the shell edge against the rock surface, protecting them from desiccation during low tide, despite their being in full sunlight. When true limpets are fully clamped down, it is impossible to remove them from the rock using brute force alone, and the limpet will allow itself to be destroyed rather than stop clinging to its rock. This survival strategy has led to the limpet being used as a metaphor for obstinacy or stubbornness. Life habits Feeding Most limpets feed by grazing on algae which grows on the rock (or other surfaces) where they live. They scrape up films of algae with a radula, a ribbon-like tongue with rows of teeth. Limpets move by rippling the muscles of their foot in a wave-like motion. In some parts of the world, certain smaller species of true limpet are specialized to live on seagrasses and graze on the microscopic algae which grow there. Other species live on, and graze directly on, the stipes (stalks) of brown algae (kelp). Homing behaviour Some species of limpets return to the same spot on the rock known