Unlike land animals, most fish get their oxygen from water, which contains a certain amount of dissolved oxygen. To get that oxygen, fish gulp water into the mouth and then pump it over gills.

Most fish have four gills enclosed in a gill chamber on each side of the head. Each gill consists of two rows of fleshy filaments attached to a gill arch. Water passes into the gill chambers through gill slits. A flap of bone called a gill cover protects the gills of bony fish. Sharks and rays, however, do not have gill covers; their gill slits are visible openings on the outside of the body.

In each gill chamber, the water passes over the gill filaments, which are closely spaced along the gill arch in two rows.

Each filament has tiny extensions called lamellae. Blood flowing each lamella takes axygen from the water and releases carbon dioxide into the water. The water then passes out through the gill openings, and the process is repeated.

SOURCE
The World Book Encyclopedia Chicago: World Book-Childcraft International, 1978

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This page was last updated on October 30, 2017.