Template:POTD/2017-07-31
A diagram showing the basic anatomy and arrangement of internal organs of the saltwater limpet Patella vulgata. Limpets are aquatic snails with shells that are broadly conical in shape, although the term also refers informally to all gastropods whose shells have no obvious coiling. Members of the family Patellidae, which includes the species shown, are often called the "true limpets" (as opposed to the false limpets and the keyhole limpets). All limpets use their muscular foot for movement and excrete mucus to apply suction and resist wave movement and predation. This image allows the viewer to see the neural torsion common to most snails, wherein the visceral nerves have become "twisted" causing the rectum and kidneys to open near the animal's head. This is the result of once having had an ancestor with a coiled shell— in limpets, the coiled shell has been replaced by a conical one, but the torsion in the Patellidae remains; some other gastropod groups have subsequently "detorted".
Direction of blood flow is indicated in this diagram by small arrows around the circumpallial vein and then into and out of the heart. Not shown are the hundreds of tiny pallial gills which form a ring over this vein and help reoxygenate the animal's blood.Diagram: K.D. Schroeder