Fishes are adapted in such a way that they can breathe underwater. They have gills on each side of their head. These gills carry tiny blood vessels.
The fish gulp in water though their mouths and pass it out through the gills. The gills extract oxygen from the water and pass it into the fish’s blood. Thus, the fish gills have the same function as the lungs but they cannot work without water and, hence, fish cannot survive outside water.
Below Fish info from Wikipedia (Read More):-
A fish is any aquatic vertebrate animal that is typically ectothermic (or cold-blooded), covered with scales, and equipped with two sets of paired fins and several unpaired fins. Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., char and gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish).
Food prepared from fish is also called fish, and it is an important food source for humans. They are harvested either from wild fisheries (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). They are also exploited by recreational fishers and fishkeepers, and are exhibited in public aquaria. Fish have had a role in many cultures through the ages, ranging from deities and religious symbols to the subjects of books and popular movies.
http://en.wikipedia.org/wiki/Fish
