Many people think of any animal that lives in the sea as a fish. But only a certain kind of animal with a certain kind of body is a fish. Such creatures as oysters, starfish, jellyfish, and whales aren’t fish.
All fish have a backbone. Oysters, starfish, and jellyfish don’t have a backbone or any skeleton at all. So, they aren’t fish.
All fish are what we call cold-blooded. A fish’s body temperature changes with the temperature of the water it’s in. But a whale is warm-blooded. Its body temperature is always the same. Thus, a whale isn’t a fish.
Most fish have thin folds of skin, called gills, on each side of the head. To breathe, a fish takes water into its mouth. As the water goes over the gills, oxygen is taken out of the water and passed into the
fish’s blood.
Most fish have fins and scaly skins. A fin is made up of thin strips of bone covered with thin skin. Fish use their fins to help them swim, to steer, and to keep their balance.
So, all fish have a backbone and all fish are cold-blooded. Most have gills, fins, and scales. These are the things that make a fish a fish.
a school of silversides