FISHES 



211 



of fleshy filaments attached to the posterior and outer border 

 of a slender bony arch. On the front and inner side of each 

 arch is a row of teethUke projections, the gill rakers. Be- 

 tween the gills are long, sUtUke openings, or clefts (Fig. 147). 



The blood enters each giU from the lower end, passes 

 out into the filaments, and returns to leave the gill through 

 an artery from the upper end. Thus there is a constant 

 flow of blood through each giU. 



Maimer of breathing. — Under natural conditions the 

 perch's mouth and the gUl covers are seen to open and close 



Brrain 



Spinal cord 



Ureter 



Urinary Madder 



Gallbladder Spleen^ ^''^'^ 



Fig. 148. — Internal structure of a fish. 



alternately. During this action water is taken into the 

 mouth, forced through the gill clefts and over the double 

 rows of filaments, and thence out through the giU openings. 

 Thus the gills are being constantly bathed with fresh water 

 laden with oxygen. The current of blood through the fila- 

 ments is separated from the water by a very thin, delicate 

 membrane through which an exchange of oxygen and car- 

 bon dioxide readily takes place. The process is similar to 

 that already described in the crayfish and mussel. 



Alimentary canal. — The mouth and pharynx constitute 

 a single large space that leads to the short, wide gullet. 



