The part of the ear that maintains biological equilibrium is called the vestibular apparatus. It consists of three semicircular canals and two otolith organs, called the utricle and saccule. Any change in the position of the head or body causes the vestibular apparatus to transmit nerve impulses to the brain, causing muscular reflexes that tend to restore the normal position. Hearing structures evolved later in the higher vertebrates. These structures convert sound waves in the air to nerve impulses that are relayed to the brain, where they are interpreted as sound rather than as mere vibrations.
Many invertebrates have specialized sense organs, rather than ears, for hearing and equilibrium.
Crickets and spiders, for example, have membranes much like sounding boards on the legs. Moths have a similar rudimentary ear on the thorax that apparently serves as a warning system for attacks by bats. The structure of the ear. The outer ear, or pinna, is the structure commonly called the ear. It is a skin-covered flap of elastic cartilage projecting from the side of the head and funneling sound into the middle ear.