The most complex gills and gill chambers are found in bony fishes. There are two gill chambers (opercular cavities) in a bony fish. They are situated on each other side of the head immediately behind the mouth. Each chamber is covered by a thin bony plate, the gill cover or operculum (plural: opercula) , which has a free edge. Water from outside can enter the body through the mouth, flow into the pharynx and gill chambers, and pass out of the body from under the opercula. Four gills are situated at the entrance of each gill chamber.
Each gill is a red comb like structure. It is made up if a bony Gill arch bearing
• Two rows of fine finger like gill filaments on the outer side, and
• Gill rakers on the inner side.
The gill filament contains the gaseous exchange surfaces. each filament is made up of a series of transverse plates which is held together along one of the edges. These plates serves to Increase the surface area of the gill filaments a the thus very compact structures with an enormous surface area.
The gill rakers do not take part in gaseous exchange. They act as food strainers and prevent food from entering the gill chambers.
Fishes have a very effective mechanism for circulating water over the gills. Thus, the gills of fishes are very efficient, enabling fishes to be active and attain large sizes.
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