244 CLASSIFICATION OF PLANTS AND ANIMALS 



the bones forming the pectoral and pehdc girdles. The dorsal, anal, and caudal 

 fins are not paired. 



The flattened, muscular body of the fish, tapering toward the caudal fin, 

 is moved from side to side with an undulating motion which results in the 



Dorsdl fin. often two 

 not pa/red 



paired PecforaJ finr. 

 ^ paired 



Fins of a fish. 



:<^A 



rapid forward movement of the fish. It also moves slowly by means of the 

 paired fins. 



A fish, when swimming quietly or when at rest, seems to be biting even if no 

 food is present. Water enters the mouth at each of these biting movements 

 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 oper'cuLum, covering 



these slits on each side, lie several long, 

 feathery, red structures, the gills. The foun- 

 dation of the gill, or the gill arch, is made of sev- 

 eral pieces of bone which are held together in 

 such a way as to give great flexibihty. Cover- 

 ing the bony framework and extending from 

 it are numerous delicate filaments covered 

 with a very thin membrane. In each of these 

 fiilaments are two blood vessels ; in one blood 

 flows downward and in the other upward. 

 Blood reaches the gills and is carried away 

 from these organs b}" means of large vessels 

 which pass along the bony arch previously 

 mentioned. 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. 



The gill arches are guarded on the inner side by a series of sharp-pointed 

 structures, the gill rakers. Teeth, when present, are used to seize and hold 

 prey. In some fishes in which the teeth are not well developed, there seems 

 to be a greater development of the gill rakers, in which case they are used to 

 strain out food or small organisms from the water which passes over the gills. 

 The gullet leads directly into a baglike stomach. There are no salivary 

 glands in fishes. There is, however, a large liver, which in some fishes appears 



"^ Diagram of the gills of a fish. 

 The heart {H) forces the blood 

 into tubes {V) which run out into 

 the gill filaments. A gill bar 

 (G) supports each gill. Trace 

 the course of blood and decide 

 what happens in this gill region. 



