A fish is any member of a group of animals consisting of all g-bearing aquatic skulls without gills and four fingers. They form a sister group of membranes that together form the sense of smell. The definition includes living ha fish, seven lamp eels, cartilaginous fish and bony fish as well as various extinct related populations. Quadrupeds appear in finned fish, so they are also fish in evolutionary terms. Traditionally, however, fish have been made obsolete or parasitic by excluding quadrupeds (i.e., amphibians, reptiles, birds and mammals all from the same ancestor). In this way, the term "fish" is negatively defined as a symbiotic population, so it is not considered a formal taxonomic grouping in systems biology. Traditional Pisces (also known as Pisces) is considered to be taxonomic rather than phylogenetic.

The earliest organisms that can be classified as fish are the soft notochordia that first appeared during the Cambrian. Although they don't have a real spine, they have chordates, which makes them more agile than invertebrates. Fish will continue to develop throughout the Paleozoic era and diversify into a variety of forms. In the Paleozoic, many fish developed alien armor to protect them from predators. The first jaw fish appeared during the Silurian period, and since then many, such as sharks, have become powerful marine predators, not just the prey of arthropods.

Most fish are exothermic ("cold-blooded" fish), allowing their body temperature to vary with ambient temperature, although some large, active swimmers, such as white sharks and tunas, can maintain a high core temperature. Fish are abundant in most waters. They can be found in almost all aquatic environments, from alpine streams (for example, charcoal fires and fish heads) to the depths of the deepest ocean and even the ocean floor (for example, river eaters and anglerfish). Fish have 33100 described species, with greater species diversity than any other vertebrate species.

Fish is an important resource of human all over the world, especially as food. Commercial and subsistence fishermen fish in wild Fisheries (see fishing), or in ponds or marine cages (see aquaculture). They are also caught by recreational fishermen, nurtured as pets, raised by fishers and displayed in public aquariums. Fish has played a role in culture since ancient times. It is not only the symbol of gods and religions, but also the theme of art, books and movies.

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