Editing Octopus (section)

===Digestion and excretion===
The digestive system begins with the [[buccal mass]] which consists of the mouth with the beak, the [[pharynx]], [[radula]] and salivary glands.<ref name=Wells/>{{rp|71–74}} The radula is serrated and made of [[chitin]].<ref name=Courage/>{{rp|40–41}} Food is broken down and is forced into the [[esophagus]] by two lateral extensions of the esophageal side walls in addition to the radula. From there it is transferred to the [[gastrointestinal tract]], which is mostly suspended from the roof of the mantle cavity. The tract consists of a [[crop (anatomy)|crop]], where the food is stored; a stomach, where it is mixed with other gut material; a [[caecum]] where the food is separated into particles and liquids and which absorbs fats; the [[hepatopancreas|digestive gland]], where liver cells break down and absorb the fluid and become "brown bodies"; and the intestine, where the built-up waste is turned into faecal ropes by secretions and ejected out of the funnel via the [[rectum]].<ref name=Wells/>{{rp|75–79}}

During [[osmoregulation]], fluid is added to the [[Pericardium|pericardia]] of the branchial hearts. The octopus has two [[nephridium|nephridia]] (equivalent to vertebrate kidneys) that are associated with the branchial hearts; these and their associated ducts connect the pericardial cavities with the mantle cavity. Each branch of the [[vena cava]] has renal appendages that pass over the thin-walled nephridium before reaching the branchial heart. Urine is created in the pericardial cavity, and is altered by excretion, of mostly ammonia, and absorption from the renal appendages, as it is passed along the associated duct and through the nephridiopore into the mantle cavity.<ref name=Ruppert/>{{rp|358–359}}

[[File:Moving Octopus Vulgaris 2005-01-14.ogv|thumb|A [[common octopus]] (''Octopus vulgaris'') moving around. Its nervous system allows the arms to move with some autonomy.|alt=video of an octopus crawling about, its suckered arms moving]]