THE FISH 



235 



J^ 



H 



Diagram of the gills of a fish. (H), the 

 heart which forces the blood into the 

 tubes (F), which run out into the gill 

 filaments. A gill bar (G) supports 

 each gill. The blood after exchang- 

 ing its carbon dioxide for oxygen is 

 sent out to the cells of the body 

 through the artery (A). 



covered with a very thin membrane or skin. Into each of these 

 filaments pass two blood vessels ; in one blood flows downward and 

 in the other upward. Blood 

 reaches the gills and is carried 

 away from these organs by 

 means of two large vessels which 

 pass along the bony arch pre- 

 viously mentioned. In the gill 

 filament the blood comes into 

 contact with the free oxygen of 

 the water bathing the gills. An 

 exchange of gases through the 

 walls of the gill filaments results 

 in the loss of carbon dioxide 

 and a gain of oxygen by the 

 blood. The blood carries oxy- 

 gen to the cells of the body 

 and (as work is done by the 

 cells as a result of the oxidation of food) brings carbon dioxide 

 back to the gills. 



Gill Rakers. — If we open wide the mouth of any large fish and 

 look inward, we find that the mouth cavity leads to a funnel-like 

 opening, the gullet. On each side of the gullet we can see the gill 

 arches, guarded on the inner side by a series of sharp-pointed struc- 

 tures, the gill rakers. In some fishes in which the teeth are not 

 well developed, there seems to be a greater development of the 

 gill rakers, which in this case are used to strain out small organisms 

 from the water which passes over the gills. Many fishes make 

 such use of the gill rakers. Such are the shad and menhaden, 

 which feed almost entirely on plankton, a name given to the 

 small organisms found by millions near the surface of water. 



Digestive System. — The gullet leads directly into a baglike stomach. 

 There are no salivary glands in the fishes. There is, however, a large 

 liver, which appears to be used as a digestive gland. This organ, because 

 of the oil it contains, is in some fishes, as the cod, of considerable economic 

 importance. Many fishes have outgrowths like a series of pockets from 

 the intestine. These structures, called the pyloric cceca, are believed to 



