more oxygenated blood from the nuchal cavity (the area above the head and neck). Many limpets still retain a ctenidium (sometimes two) in this nuchal chamber instead of the circumpallial gills as a means for exchanging oxygen and carbon dioxide with the surrounding water or air (many limpets can breathe air during periods of low tide, but those limpet species which never leave the water do not have this ability and will suffocate if deprived of water). Blood moves from the atrium into the ventricle and into the aorta where it is then pumped out to the various lacunar blood spaces / sinuses in the hemocoel. The odontophore may play a large role in assisting with blood circulation as well. The two kidneys are very different in size and location. This is a result of torsion. The left kidney is diminutive and in most limpets is barely functional. The right kidney, however, has taken over the majority of blood filtration and often extends over and around the entire mantle of the animal in a thin, almost-invisible layer. The digestive system is extensive and takes up a large part of the animal's body. Food (algae) is collected by the radula and odontophore and enters via the downward-facing mouth. It then moves through the esophagus and into the numerous loops of the intestines. The large digestive gland helps break down the microscopic plant material, and the long rectum helps compact used food which is then excreted through the anus located in the nuchal cavity. The anus of most molluscs and indeed many animals is located far from the head. In limpets and most gastropods, however, the evolutionary torsion which took place and allowed the gastropods to have a shell into which they could completely withdraw has caused the anus to be located near the head. Used food would quickly foul the nuchal cavity unless it was firmly compacted prior to being excreted. The torted condition of the limpets remains even though they no longer have a shell into which they can withdraw and even though the evolutionary advantages of torsion appear to therefore be negligible (some species of gastropod have subsequently de-torted and now have their anus located once again at the posterior end of the body; these groups no longer have a visceral twist to their nervous systems). The gonad of a limpet is located beneath its digestive system just above