Fish are water dwelling vertebrates that breath through gill slits typically on their sides. The gills extract oxygen from water that enters through the mouth and exit through the gills.
Dimensions and definitions
Fishes vary greatly in size from the size or the smallest guppy (about 1 cm), to the largest of Great Whites (roughly 5 to 6 meters in length). Some fish are cold blooded and all are vertebrates, this means that they have a spinal cord and nervous system. For this reason alone, Jellyfish and cuttlefish are not true fish.
All fish also are water dwellers and have gills for extracting oxygen from the water. This also means that dolphins and whales are not fish, but mammals. Fish are in the group paraphyletic.
Fish are capable of eating a wide variety of food, including plants and other organisms. The consumed food is digested initially in the esophagus, then it moves into the stomach to be further broken down.
The next location is the pyloric ceca, which are finger like pouches that absorb nutrients and still further the digestion process. The next step is the food entering the intestine where the digestion is completed and all that remains of benefit is absorbed, before the reset is expelled from the fish.
Oxygen is extracted from the water as it passes through the gills, which are located on either side behind the cranium and on either side of the pharynx. The filaments within the gills extract the oxygen and start the process of distribution of the oxygen throughout the circulatory system. The gills also expel carbon dioxide that is produced from the fish’s body.
Fishes have a no veins or arteries, only one set of tubes that the blood is pushed through while the heart pumps the blood. This is called a closed circulatory system. The blood leaves the heart and winds up at the gills and then throughout the body. Some fishes have multiple gill openings, like sharks, and there are others that have actual lungs such as the beta. These lung fish need to breathe air as well or will suffocate.