242 BIOLOGY FOR BEGINNERS 



like appendages called the gill filaments. These filaments are 

 richly provided with capillaries, so that the blood is brought in 

 close contact with the water over a very large surface. This 

 permits the exchange of oxygen (dissolved in water) and carbon 

 dioxide by means of osmosis. The gill arches have finger-like projec- 









Courtesy oj the A merican Museum of Natural History 



FIG. 86. Skeleton of European Perch, Percaflumatilis, illustrat- 

 ing the bony framework of the higher fishes. After Cuvier. 



The whole fish is adapted for thrusting rapidly forward through the water. 

 The tapering head ends in a sharp prow extending from the nose to the neck. 

 The brain-case is braced on all sides to receive the forward thrust of the many- 

 jointed backbone, which is driven forward by the tail. The fins are spread upon 

 bony sticks or rays, which are supported by bony pieces that are embedded 

 in the flesh. Between the supporting pieces and the fin rays there are usually 

 movable joints. The ventral fins are fastened beneath the pectoral fins, an 

 arrangement which facilitates quick turning. 



The propelling muscles and their bony supports are extended along the 

 sides of the backbone and outside the ribs. The ribs enclose the stomach, 

 intestines and other vital organs. These extract from the food the energy 

 which is given out in muscular exertion. The region of the gills is covered by 

 an elaborate system of jointed plates. 



The mouth is guarded by bony jaws which are attached to the lower side 

 of the skull. 



tions on the side toward the throat called gill rakers, which prevent 

 food or dirt from getting into the filaments and also keep the arches 

 separate to allow free circulation of water. 



The water is taken in at the mouth, which is then closed, forcing 

 it through the gill slits over the filaments and out beneath the 

 operculum ; the forward motion of the fish aids in this process. 



