Fish are aquatic vertebrates that are typically cold blooded covered with scales and equipped with o sets of paired fins and several unpaired fins. Fish are abundant in the sea and in freshwaters with species being known from mountain streams (e.g. char and gudgeon) as well as in the deepest depths of the ocean (e.g. gulpers and anglerfish). They are of tremendous importance as food for people around the world being either collected from the wild (see fishing) or being farmed in much the same way as cattle or chickens (see aquaculture). Fish are also exploited for recreation through angling and fiseeping and fish are monly exhibited in public aquaria. Through the ages many cultures have featured fish in their legends and myths from the "great fish" that swallowed Jonah the Prophet through to the ever-popular half-human half-fish mermaid around which books and movies are still centred (e.g. Splash (film)). Fish have been used as symbols in many different ways from the ichthys used by early Christi through to the allegorical use of a marlin in Ernest Hemingway"s novel The Old Man and the Sea. 图片参考：upload.wikimedia/ *** /mons/thumb/2/23/Geia_Aquarium_-_Giant_Grouper_edit/300px-Geia_Aquarium_-_Giant_Grouper_edit What is a fish? The term "fish" is most precisely used to describe any non-tetrapod chordate i.e. an animal with a backbone but lacking four limbs (or having ancestors that had four limbs). Unlike groupings such as birds or mammals fish are not a single clade but a paraphyletic collection of taxa including hagfishes lampreys sharks rays lungfishes and coelacanths sturgeons gars and advanced ray-finned fishes. [1] A typical fish is cold-blooded; has a streamlined body that allows it to swim rapidly; extracts oxygen from the water using gills; has o sets of paired fins one or o dorsal fins an *** fin and a tail fin; has jaws; has skin that is covered with scales; and lays eggs that are fertilised externally. However to each of these there are exceptions. Tuna and some species of sharks are warm-blooded and able to raise their body temperature significantly above that of the ambient water surrounding them. [2] Streamling and swimming performance varies from highly streamlined and rapid swimmers able to reach 10-20 body-lengths per second (such as tuna salmon and jacks through to slow but more manoeuvrable species such as eels and rays that reach no more than 0.5 body-lengths per second. [3] Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapods gouramis have a structure called the labyrinth an that performs a similar function while many catfish such as Corydoras extract oxygen via the intestine or stomach. [4] Body shape and the arrangement of the fins is highly variable covering such seemingly un-fishlike forms as seahorses pufferfish anglerfish and gulpers.
