page_title: Limpet
section_title: Anatomy
hierarchical_section_title: Limpet / Behaviour and ecology / Anatomy
context_page_description: Limpets are a group of aquatic snails that exhibit a conical shell shape and a strong, muscular foot. Although all limpets are members of the class Gastropoda, limpets are polyphyletic, meaning the various groups which are referred to as "limpets" have descended independently from different ancestral gastropods. This general category of conical shell is known as "patelliform". All members of the large and ancient marine clade Patellogastropoda are limpets. Within that clade, the members of the Patellidae family in particular are often referred to as "true limpets".
  Other groups, not in the same family, are also called limpets of one type or another, due to the similarity of their shells' shape. Examples include the Fissurellidae family, which is part of the Vetigastropoda clade and the Siphonariidae, which use a siphon to pump water over their gills.
  Some species of limpet live in fresh water, but these are the exception.
context_section_description: The basic anatomy of a limpet consists of the usual molluscan organs and systems:
  A nervous system centered around the paired cerebral, pedal, and pleural sets of ganglia.  These ganglia create a ring around the limpet's esophagus called a circumesophageal nerve ring or nerve collar.  Other nerves in the head/ snout are the optic nerves which connect to the two eye spots located at the base of the cerebral tentacles (these eyespots, when present, are only able to sense light and darkness and do not provide any imagery), as well as the labial and buccal ganglia which are associated with feeding and controlling the animal's odontophore, the muscular cushion used to support the limpet's radula (a kind of tongue) that scrapes algae off the surrounding rock for nutrition.  Behind these ganglia lie the pedal nerve cords which control the movement of the foot, and the visceral ganglion which in limpets has been torted during the course of evolution.  This means, among other things, that the limpet's left osphradium and oshradial ganglion (an organ believed used to sense the time to produce gametes) is controlled by its right pleural ganglion and vice versa.
  For most limpets, the circulatory system is based around a single triangular three-chambered heart consisting of an atrium, a ventricle, and a bulbous aorta.  Blood enters the atrium via the circumpallial vein (after being oxygenated by the ring of gills located around the edge of the shell) and through a series of small vesicles that deliver 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 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
caption: Detailed anatomy of Patella vulgata, a common limpet