Fishes cannot shut their eyes, having no true lids; but the eyes
themselves are very well developed and the vision is acute, especially
for moving objects. Except in gristly fishes, the external opening to
the ear has been lost, so that sound-waves and coarser vibrations must
influence the inner ear, which is well developed, through the
surrounding flesh and bones. It seems that the main use of the ear in
fishes is in connection with balancing, not with hearing. In many cases,
however, the sense of hearing has been demonstrated; thus fishes will
come to the side of a pond to be fed when a bell is rung or when a
whistle is blown by someone not visible from the water. The fact that
many fishes pay no attention at all to loud noises does not prove that
they are deaf, for an animal may hear a sound and yet remain quite
indifferent or irresponsive. This merely means that the sound has no
vital interest for the animal. Some fishes, such as bullhead and
dogfish, have a true sense of smell, detecting by their nostrils very
dilute substances permeating the water from a distance. Others, such as
members of the cod family, perceive their food in part at least by the
sense of taste, which is susceptible to substances near at hand and
present in considerable quantity. This sense of taste may be located on
the fins as well as about the mouth. At this low level the senses of
smell and taste do not seem to be very readily separated. The chief use
of the sensitive line or lateral line seen on each side of a bony fish
is to make the animal aware of slow vibrations and changes of pressure
in the water. The skin responds to pressures, the ear to vibrations of
high frequency; the lateral line is between the two in its function.