Fish are cold-blooded chordates. This means that the animal’s body temperature varies with the environment temperature. Also, they live in water and have a spine. Most fish breathe with gills, bred by laying eggs and have covered protective scales.
Fish are suitable for underwater life with the help of several specialized physical features. Due to the natural buoyancy of fish they can live in the water and in the process of evolution the fish developed a light, flexible skeleton. Strong muscles are attached to the skeleton, which make possible movement in water. Fins are wing-like formations on the back of the body, belly or tail can act as oars, steering wheel or stabilizers. The inner organs which includes the swim bladder, are in a special body cavity located under the vertebral column.
There are three different kinds of fish. These are jawless fish, cartilaginous fish and boned fish.