On your discetion specimen of the perch, remove the right operculum to expose the gills. The gills are attached to each of the four pairs of branchial arches. On the anterior surface of the arches are the gill rakers. Each gill has two sets of filaments, one extending anteriorly, and the other posteriorly (Figure 6). Gills with double sets of filaments like these are known as holobranchs. Each gill filament is made of many small thin walled folds called lamellae. The lamellae contain the gill capillaries and exchange of oxygen and carbon dioxide between water and blood occur across the thin walls of the lamellae. In fish, water is constantly being passed over the gills. Water is drawn into the mouth and then into the pharynx while the opercula are closed. Valves in the mouth close, the oral cavity contracts and water is forced past the gill rakers, through the gill slits, over the gill filaments and out behind the posterior free ends of the opercula which are now open. As water passes over the gill filaments (see Figure 6) the exchange of oxygen and carbon dioxide occur. The arrangement of gill capillaries within the lamellae is such that the blood flow is opposite to the flow of water over the gills. This is an example of a counter current system. It is an extremely efficient way to maximize movement of oxygen into the blood.
In addition to their respiratory function, gills also perform important excretory functions. Without exception, all fish primarily excrete their nitrogenous wastes via the gills rather than via kidneys. Marine fish also excrete excess salts via the gills. Freshwater fish absorb needed salts via the gills.
FIGURE 6: STRUCTURE OF A FISH GILL