AND PHYSIOLOGY. 



215 



just behind the lower jaw. " These are disposed, in most 

 fishes, in fringed laminae, which are set close together like 

 the barbs of a feather, and are attached on each side of 

 the throat in double rows, to the convex margins of four or 

 five long, bony, or cartilaginous arches, which are suspended 

 from the hyoidean arch." 



487. Air -Bladder. — Another organ which perhaps claims 

 attention here, is the Air-Bladder. This is a small shut sac 

 — sometimes nearly subdivided into two or more sections by 



iTlG. 271. 



Air-Sac of Fish (Carp), a. &. A Divided Form. c. d. A Tube connecting it with the 

 Esophagus o, 



a membranous division — which lies near the middle of the 

 back. In most cases it has no connection with any other or- 

 gan, but sometimes has an opening into the esophagus or 

 stomach. The uses for which it has been supposed to exist, 

 are to enable the fish to alter its specific gravity, and also to 

 aid in respiration in some manner. It has also been conjec- 

 tured that it aids the sense of hearing, since it is in direct 

 connection with the auditory apparatus. It is filled with 

 atmospheric air, with greatly varying proportions of oxygen 

 and nitrogen. Some fishes that leave the water occasionally 

 and crawl over the land, have a cavity in the side of the head 

 for water, which is in contact with a respiratory apparatus, 

 and thus the fish can live for some time out of its native ele- 

 ment. — Wyman. 



48T. Describe the air-bladder. What docs it sometimes communicate with ? Give its 

 probable uses. What is it filled with ? 



