FISHES 189 



The gills, as has already been stated, have the 

 same function in fish that lungs have in the air- 

 breathing vertebrates. Most of our common food 

 fishes have four pairs of gill-arches, all those of one 

 side being in a common gill-chamber which has a 

 single external opening. The gill-chamber is situated 

 just back of the mouth and is well protected by bony 

 plates known as gill-covers. 



The gills consist of bony arches to which reddish 

 fringes are attached. Each fringe is provided with a 

 small bloodvessel. The fish, as it swims with open 

 mouth, "drinks" great gulps of water. It, however, 

 does not swallow this water, but lets it flow over the 

 gills and out at the gill-openings. In this way the 

 fish breathes, for the flow of water over the gills 

 means exactly to the fish what inhaling and ex- 

 haling air means to man. The constant flow of water 

 over the gills brings the free oxygen in the water 

 very close to the blood which circulates through the 

 minute bloodvessels present in the almost countless 

 gill-fringes. It is through the thin walls of the gill- 

 fringes that the exchange of oxygen and waste gases 

 takes place. This process, therefore, is very similar 

 to that which occurs in the lungs of air-breathing 

 animals. 



Many fishes, in addition to the gill-fringes, have 

 at least the anterior arch provided with teeth or 

 bristles, called gill-rakers. In those species in which 

 they are numerous and close set, they serve the pur- 

 pose of a screen for removing from the water the 

 small free swimming organisms which constitute 

 their food. 



