Hearing may initially have been important to vertebrates as a mechanism to alert them of any dangerous activity around them. It also became important in the search for food and mates, and in communication. Sound results when pressure waves transmit energy through some medium-air or water. Hearing in air i.e., on land, poses serious problems for vertebrates, since their middle ear transformers are sound pressure sensors, but in air, sound produces less than 0.1% of the pressure as compared to water. Adaptation to hearing in air resulted from the evolution of an acoustic (pertaining to sound) transformer that incorporates a thin, stretched membrane, called as ear drum, tympanic membrane, or tympanum, that is exposed to the air.
The tympanum, first evolved in amphibians as a modified integument stretched over a cartilaginous ring. It vibrates in response to sounds and transmit these movements to the middle ear, a chamber behind the tympanum. In anurans, touching the tympanum is an ossicle called the columella or stapes. The opposite end of the columella touches the membrane of the oval window, which stretches between the middle and inner ears which contain sensory receptors for sound.
Why don’t fishes need either an outer or middle ear?
In bony fishes, receptors for equilibrium, balance, and hearing are in the inner ear, and their functions are similar to those of other vertebrates. Semicircular canals detect rotational movements, and other sensory patches help with equilibrium • and balance _ by detecting the direction of gravitational pull. Fishes lack outer and middle ‘ear found in other vertebrates. Vibrations pass •from the water through the bones of the skull to the inner ear. A few bony enter the fishes have chains of bony ossicles pharynx are transmitted to gas in the swim bladcler, (Weberian ossicles) that abut on causing it to expand and contract at frequencffis and one hand with swim bladder and amplitudes corresponding to the incoming sound with inner ear at its posterior part on waves,forward a chain the other end. Vibrations that strike of bones (ossicles) and then to a fluid-filled sac the fish body are thus amplified by connected directly to the inner ear. the swim bladder and sent through the –ossicles to the inner ear in skull. Fig. 2.22