FISHES 



211 



of fleshy filaments attaelied td the posterior and outer border 

 of a slender l)ony arch. On the front and inner side of each 

 arch is a row of teethlike pi-ojections, the gill raker>i. Be- 

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



The blood enters each gill from the lower end, passes 

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

 an artery fi'om the upper end. Thus there is a constant 

 flow of l)lood through each gill. 



Manner of breathing. — Under natural conditions the 

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



Ura 



Spinal cord 



■iSi^ ^^''^i^ Backbone 

 Urei-cr 

 Urinary bladder 



Oull bladder Spleen 



Fig. 148. — Internal structure of a fish. 



alternately. Dining this action water is taken into the 

 mouth, forced through the gill clefts and over the tlouble 

 rows of filaments, and thence out through the gih oisenings. 

 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 simUar to 

 that already described in the crayfish and mussel. 



Alimentary canal. — Tlie moutli and pharynx constitute 

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



