A fish is a creature that is found in the waters all over the world. They come under the classification of vertebrates and they are cold blooded species. There have been classified more than 30 thousands of species of fish in the world.
Most of the fishes have a bony skeleton while some have cartilages. Almost all of them lay eggs for reproduction. They breathe through gills, which is used to take oxygen from the surrounding water. They use fins for their movement. There are many types of fins in a fish. Either the fins are paired or unpaired. Fins like dorsal fin, fat fin, caudal fin, anal fin, pelvic fins and pectoral fins are found in a fish. The pelvic and the pectoral fins are paired fins.
Like other vertebrates fish too have a digestive system, respiratory system, circulatory system, excretory system and the central nervous system. Using their jaws the fish eat any food and the food enters the esophagus from which it enters the stomach. Here the food is broken and sent to pouch like organ where it further broken down with the help of the digestive enzymes. After this process the nutrients of the food are absorbed and the waste is sent to the excretory system.
The gills are the important organs in a fish in the respiratory system. Gills are used to exchange oxygen from the surrounding water and carbon dioxide. Most of the fishes gulp water and pass it to the gills for this exchange to happen. If you think that all the fishes take oxygen from water, you are wrong. There are fishes that get oxygen from air. They come to the surface of the water to gulp air and then go into water. Many catfishes come in this category.
In the circulatory system the heart plays an important role in pumping the blood to the other parts of the body. From the heart the blood is pumped to the gills where the exchange of oxygen is done and then the blood is pumped to the other parts of the body of the fish and then back to the heart. Fishes release ammonia as wastes and the level of ammonia in the body is controlled by the kidneys of the fish. Some of the wastes are released by the gills too. Fishes have very small brains when compared to the other vertebrates. Fishes have well developed eyes with color vision. Although they have ears in their head most of them can’t hear very well. The lateral line of the fish is too sensitive that it can sense the vibrations created by the movement of other fishes. This sense is used to identify other living organisms and prey by its side.
Fish plays an important role in the human life in many ways right from the earlier days till now. Fish is an important food for the human beings. It is also used for entertainment and relaxation. Sports fishing are in practice now in many parts of the globe. Aquariums with different species which are considered as lucky and relaxing are available in many parts of the world. Fishing industry is an important thriving industry in the world.