GILLS OF A FISH 



245 



Along each side of most fishes is a line of tiny pits, provided with 

 sense organs and connected with the central nervous system. This 

 area, called the lateral line, is believed to be sensitive to mechanical 

 stimuli of certain sorts. The ear of the fish is under the skin and 

 serves partly as a balancing organ. 



Breathing. A fish, when swimming quietly and when at rest, 

 seems to be biting even if no food is present. Investigation shows 

 us that under the broad, flat plate, or operculum (6^pur'ku-l#m), 

 on each side of the head, lie two pairs of long, feathery structures, 

 gills. The skeleton of the gill, or the gill arch, is composed of 



ondtair- 



filament 

 illrakens 



Explain why a fish in an aquarium is continually opening and closing its mouth. 



several pieces of bone which are hinged in such a way as to give 

 great flexibility. Covering the bony framework, and extending 

 from it, are numerous delicate gill filaments. These delicate 

 structures are guarded on the inner side by a series of teeth-like 

 structures, the gill rakers. In each of these filaments are two 

 blood vessels; one taking blood to the gilte, where it gives up its 

 supply of carbon dioxide, the other vessel taking the blood with 

 ; its load of oxygen back over the body. A thin membrane sepa- 

 rates the blood in the filament from the water bathing the gills. 

 An exchange of gases through the walls of the gill filaments 

 results in a loss of carbon dioxide and a gain of oxygen by the 

 blood. 



f bio — '7 



