its foot. It swells and eventually bursts, sending gametes into the right kidney which then releases them into the surrounding water on a regular schedule. Fertilized eggs hatch and the floating veliger larvae are free-swimming for a period before settling to the bottom and becoming an adult animal. True limpets in the family Patellidae live on hard surfaces in the intertidal zone. Unlike barnacles (which are not molluscs but may resemble limpets in appearance) and mussels (which are bivalve molluscs that cement themselves to a substrate for their entire adult lives), limpets are capable of locomotion instead of being permanently attached to a single spot. However, when they need to resist strong wave action or other disturbances, limpets cling extremely firmly to the surfaces on which they live, using their muscular foot to apply suction combined with the effect of adhesive mucus. It often is very difficult to remove a true limpet from a rock without injuring or killing it. All "true" limpets are marine. The most primitive group have one pair of gills, in others only a single gill remains, the lepetids do not have any gills at all, while the patellids have evolved secondary gills as they have lost the original pair. However, because the adaptive feature of a simple conical shell has repeatedly arisen independently in gastropod evolution, limpets from many different evolutionary lineages occur in widely different environments. Some saltwater limpets such as Trimusculidae breathe air, and some freshwater limpets are descendants of air-breathing land snails (e.g. the genus Ancylus) whose ancestors had a pallial cavity serving as a lung. In these small freshwater limpets, that "lung" underwent secondary adaptation to allow the absorption of dissolved oxygen from water. Symbiosis Limpets have a mutualistic relationship with several other beings. Clathromorphum, a type of algae, provides food to limpets, which clean the algae's surface and allow its persistence. The rough keyhole limpet (Diodora aspera) is host to the scale worm copepod Anthessius nortoni, which bites predatory starfish to discourage them from eating the limpet. Homescars Limpets wander over the surface of the rocks during high tide and tend to return to their favourite spot by following a trail of mucus left whilst grazing. Over a period of time the edges of the limpet's shell wear a shallow hollow in the rock called a homescar. The homescar helps the limpet to stay attached to the rock and not