When a fish sleeps, it exists in a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most fish can dart away. A few kinds of fish lie on the bottom to sleep. Most fish do not have eyelids, so they cannot close their eyes to sleep.

The organs of hearing in fish are entirely internal, located within the skull, on each side of the brain, and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body to be transmitted to the hearing organs.

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Q: What is a fish?

A fish is any of approximately 34,000 species of vertebrate animals. The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group.

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Written by Stanley H. Weitzman, Lynne R. ParentiSee All

Fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Feb 22, 2024 • Article History

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Category: Animals & Nature

Key People:: David Starr Jordan; Louis Agassiz; Eugenie Clark; Constantine Samuel Rafinesque; Étienne de La Ville-sur-Illon, comte de Lacépède

Related Topics:: chondrichthyan; eel; lungfish; shark; agnathan

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What is a fish?

How do fish sleep?

How do fish hear?

fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded. The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits ...(100 of 15612 words)

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