fish

fish Cold-blooded, aquatic vertebrate animal characterized by fins, gills for breathing, a streamlined body almost always covered by scales or bony plates on to which a layer of mucus is secreted, and a two-chambered heart. Fish are the most ancient form of vertebrate life, with a history of about 450 million years. They reproduce sexually, and fertilization may be external or internal. The eggs develop in water or inside the female, according to species. Fish have lateral line organs, which are fluid-filled pits and channels that run under the skin of the body. Sensitive fibres link these channels to the central nervous system and detect changes of pressure in the water and changes of strength and direction in currents. About 75% of all fish live in the sea; the remainder are freshwater species that live in lakes, rivers and streams. A few fish, such as the salmon and eel, divide their lives between salt and freshwater habitats. The classification of fish varies. They are usually divided into three classes: Agnatha, which are jawless fish, including the hagfish and lamprey; Chondrichthyes (cartilaginous fish), which includes shark, skate, ray and chimaera; and the much more numerous Osteichthyes (bony fish), including subclasses of soft-rayed fish (lungfish and lobefin) and the very successful teleost fish, such as salmon and cod. There are more than 22,000 species of bony fish, and they represent c.40% of all living vertebrates. They divide into 34 orders and 48 families.