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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.

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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.

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.

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