FISHES— AQUARIA 



171 



gills will be seen lying in several layers in the cavity. The 

 gills are composed of small red filaments attached to carti- 

 laginous or bony arches. Between these arches are shts that 

 communicate with the back part of the mouth, through which 

 the water is forced. As the water passes over the gills, the 

 fine blood-vessels lying in the surface of the gill filaments 

 absorb the free oxygen in it. This is the way the fish breathes. 



Dissect the fish and 

 find the air-bladder, a 

 long, white sac in the 

 body cavity next to the 

 backbone. This organ 

 is found in most fishes 

 and has an important 

 function. In fish hke 

 the sunfish, bass, and 

 pickerel, it serves as a 

 floating device. The ''"^■"- The Giii of a Fish. 



body of the fish is slightly heavier than the water and 

 naturally sinks. By expanding this bladder the fish can rise, 

 and vice versa. The same principle is applied in our sub- 

 marine boats. Watch a fish in the aquarium as it rises and 

 sinks without moving any fins. It is done by means of the 

 air-bladder. In the dogfish and other fishes, generally of 

 low degree, the air-bladder serves partly as an organ of 

 respiration. It is more or less cellular and lung-like, and 

 is connected with the mouth so that the fish may come to 

 the surface and inhale air. 



Fishes, hke amphibians and reptiles, are cold-blooded ani- 

 mals. That is, their blood is not, as in the birds and mam- 

 mals, at a higher temperature than the medium around them. 



