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With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates. Drawing of animal with large mouth, long tail, very small dorsal fins, and pectoral fins that attach towards the bottom of the body, resembling lizard legs in scale and development. Jawless fish lineages are mostly extinct. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.
For this reason, groups such as the “Class Pisces” seen in older reference works are no longer used in formal classifications. The above scheme is the one most commonly encountered in non-specialist and general works. Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. The various fish groups account for more than half of vertebrate species. The final total of extant species may grow to exceed 32,500. Photo of fish with many narrow, straight appendages.
Some are end in points, and others are longer, ending in two or three approximately flat, triangular flaps, each with a dark spot. Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. There are even fish that live mostly on land or lay their eggs on land near water. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. The gills push the oxygen-poor water out through openings in the sides of the pharynx. The fish head is oriented snout-downwards, with the view looking towards the mouth.
Fish from multiple groups can live out of the water for extended periods. Many such fish can breathe air via a variety of mechanisms. A number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water’s oxygen concentration may seasonally decline.
Air breathing fish can be divided into obligate air breathers and facultative air breathers. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators. In most fish, the heart consists of four parts, including two chambers and an entrance and exit. Jaws allow fish to eat a wide variety of food, including plants and other organisms. The intestine completes the process of digestion and nutrient absorption.
Their kidneys return water to the body. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater. Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. Fish brains are divided into several regions. The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Together these structures form the forebrain.