Vertebrate ear is commonly regarded only as an organ of hearing (phonoreceptor) but, at least, in higher vertebrates it serves the dual function of equilibrium and hearing.
In fishes it is only an organ of equilibrium, in other lower forms it is not known definitely whether it serves only for equilibrium or for both equilibrium and hearing.
The fundamental ear is a membranous labyrinth or internal ear found in all vertebrates with hardly any variations, its cristae and maculae are concerned mainly with equilibrium.
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The portion of the membranous labyrinth which is concerned with hearing is the lagena which begins to develop in fishes and gradually becomes more complex in the evolutionary scale of vertebrate series.
In cyclostomes the membranous labyrinth is degenerate with only one (Myxine) or two (Petromyzon) semicircular canals.
In fishes the membranous labyrinth is complete with three semicircular canals, and in many teleosts it is connected to the air bladder by a duct or a chain of bony Weberian ossicles.
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In tetrapoda the change of environment from water to land necessitated formation of structures to conduct vibrations conveyed through air, thus a middle ear or tympanic cavity was formed from the first gill cleft or spiracle of fishes.
A tympanic membrane covers the middle ear and small bones come to lie in it for transmitting vibrations.
All tetrapods have a middle ear with a Eustachian tube connecting it to the pharynx. In mammals an external ear or pinna is formed, though its beginnings are seen/in some reptiles and birds.
The external ear catches and directs sound waves to the tympanic membrane of the middle ear.