Editing Inner ear (section)

===Cochlear system===
{{See also|Evolution of the cochlea}}
In [[reptile]]s, sound is transmitted to the inner ear by the [[stapes]] (stirrup) bone of the middle ear. This is pressed against the [[oval window]], a membrane-covered opening on the surface of the vestibule. From here, sound waves are conducted through a short '''perilymphatic duct''' to a second opening, the [[round window]], which equalizes pressure, allowing the incompressible fluid to move freely. Running parallel with the perilymphatic duct is a separate blind-ending duct, the '''lagena''', filled with [[endolymph]]. The lagena is separated from the perilymphatic duct by a [[basilar membrane]], and contains the sensory hair cells that finally translate the vibrations in the fluid into nerve signals. It is attached at one end to the saccule.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 476–489|isbn= 003910284X}}</ref>

In most reptiles the perilymphatic duct and lagena are relatively short, and the sensory cells are confined to a small '''basilar papilla''' lying between them. However, in [[mammal]]s, [[bird]]s, and [[crocodilian]]s, these structures become much larger and somewhat more complicated. In birds, crocodilians, and [[monotreme]]s, the ducts are simply extended, together forming an elongated, more or less straight, tube. The endolymphatic duct is wrapped in a simple loop around the lagena, with the basilar membrane lying along one side. The first half of the duct is now referred to as the [[scala vestibuli]], while the second half, which includes the basilar membrane, is called the [[scala tympani]]. As a result of this increase in length, the basilar membrane and papilla are both extended, with the latter developing into the [[organ of Corti]], while the lagena is now called the [[cochlear duct]]. All of these structures together constitute the cochlea.<ref name=VB/>

In [[theria]]n mammals, the lagena is extended still further, becoming a coiled structure (cochlea) in order to accommodate its length within the head. The organ of Corti also has a more complex structure in mammals than it does in other [[amniote]]s.<ref name=VB/>

The arrangement of the inner ear in living [[amphibian]]s is, in most respects, similar to that of reptiles. However, they often lack a basilar papilla, having instead an entirely separate set of sensory cells at the upper edge of the saccule, referred to as the '''papilla amphibiorum''', which appear to have the same function.<ref name=VB/>

Although many fish are capable of hearing, the lagena is, at best, a short diverticulum of the saccule, and appears to have no role in sensation of sound. Various clusters of hair cells within the inner ear may instead be responsible; for example, [[Osteichthyes|bony fish]] contain a sensory cluster called the '''macula neglecta''' in the utricle that may have this function. Although fish have neither an outer nor a middle ear, sound may still be transmitted to the inner ear through the bones of the skull, or by the [[swim bladder]], parts of which often lie close by in the body.<ref name=VB/>