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Fish has specialised structures called the gills to carry out exchange of gases with water, the medium in which they live.
The region between the buccal cavity (mouth) and the oesophagus is called the pharynx. In the pharyngeal region, the wall on either side shows slits which open to the exterior. These slits are called the gill slits. The gill slits are separated by a tissue called the gill arch or the branchial arch. There are four pairs of gill arches separating five pairs of gill slits.
From each gill arch arise two rows of filaments, which are arranged in a V-shaped manner. The gill arch along with the filaments is called a gill.
Each filament is made up of plate-like structures called lamellae, which have a rich supply of blood capillaries. Thus the barrier between the blood capillaries and the water is only few cells thick. The lamellae also serve to increase the surface area greatly.
Along the gill arch run the blood vessels which give off branches into the filaments and the lamellae. The whole arrangement on either side is covered by a movable cover called the operculum (refer to the first diagram in this section). It consists of muscles and thin layers of bone.
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