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
Stirrup remained in use until the late Heian period when a new stirrup was developed. The "fukuro abumi" or "musashi abumi" had a base that extended the full length of the rider's foot and the right and left sides of the toe cup were removed. The open sides were designed to prevent the rider from catching a foot in the stirrup and being dragged. The military version of this open-sided stirrup ("shitanaga abumi") was in use by the middle Heian period. It was thinner, had a deeper toe pocket and an even longer and flatter foot shelf. This stirrup stayed in