THE VERTEBRATE ANIMALS 277 



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

 seems to be biting when no food is present. A reason for this act 

 is to be seen when we introduce a little finely powdered carmine 

 into the water near the head of the fish. It will be found that a 

 current of water enters the mouth at each of these biting move- 

 ments and passes out through two slits found on each side of the 

 head of the fish. Investigation shows us that under the broad, flat 

 plate, or operculum, forming each side of the head, lie several long, 

 feathery, red structures, the gills. 



Gills. If we examine the gills of any large fish, we find that a 

 single gill is held in place by a bony arch, made of several pieces 



of bone which are hinged in such 

 a way as to give great flexibility 

 to the gill arch, as the support 

 is called. Covering the bony 

 framework, and extending from 

 it, are numerous delicate fila- 

 ments of flesh, covered with 



The head of a fish, with the operculum j T , 



cut away to show the gills. a ver y delicate membrane or 



skin. Into each of these fila- 

 ments 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 previously 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 re- 

 sults in the loss of carbon dioxide and a gain of oxygen by the blood. 

 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 

 plants and animals found by millions in the water. 



