<P> Fish have a lateral line system that detects pressure fluctuations in the water . Such pressure is non-detectable in air, but grooves for the lateral line sense organs were found on the skull of early tetrapods, suggesting either an aquatic or largely aquatic habitat . Modern amphibians, which are semi-aquatic, exhibit this feature whereas it has been retired by the higher vertebrates . </P> <P> Changes in the eye came about because the behavior of light at the surface of the eye differs between an air and water environment due to the difference in refractive index, so the focal length of the lens altered to function in air . The eye was now exposed to a relatively dry environment rather than being bathed by water, so eyelids developed and tear ducts evolved to produce a liquid to moisten the eyeball . </P> <P> Early tetrapods inherited a set of five rod and cone opsins known as the vertebrate opsins . </P> <P> Four cone opsins were present in the first vertebrate, inherited from invertebrate ancestors: </P>

What is the most significant difference between amphibians and other animals