Evolution of the cochlea about 7 mm long. Like in lepidosaurs and archosaurs, it contains a lagena, a vestibular sensory epithelium, at its tip. Only in therian mammals (marsupials and placentals) is the cochlea truly coiled 1.5 to 3.5 times. Whereas in monotremes there are many rows of both inner and outer hair cells in the organ of Corti, in therian (marsupial and placental) mammals the number of inner hair-cell rows is one, and there are generally only three rows of outer hair cells. Amphibians have unique inner ear structures. There are two sensory papillae involved in hearing, the basilar (higher frequency) and amphibian
Cochlea "helicotrema", due to the stiffness-mediated tonotopy. A very strong movement of the basilar membrane due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery often wear earmuffs or earplugs. Not only does the cochlea "receive" sound, it "generates" and amplifies sound when it is healthy. Where the organism needs a mechanism to hear very faint sounds, the cochlea amplifies by the reverse transduction of the OHCs, converting electrical signals back to mechanical in a positive-feedback configuration. The OHCs have