ADAPTATIONS 389 



surface. By pricking the air-bladder with a needle through the 

 side of the body some of the air is allowed to escape and then 

 the fish recovers and may live for an indefinite time, whereas if 

 left to itself it would very soon die. 



It is evident from the above that an air-bladder would be 

 unnecessary to a fish which lived on the ground, and a positive 

 disadvantnge to fishes which require to change rapidly from 

 one depth to another. Accordingly we find that in ground- 

 fishes the air-bladder is wanting, having disappeared in the 

 course of evolution. It is entirely absent in the Pleuronectida; 

 or flat-fishes, in the Cyclopteridae which cling to rocks by means 

 of their suckers in shallow water, and in many of the littoral 

 Blenniidae. Its absence in many of the Scopelidae seems to be 

 related to the habit of these phosphorescent fishes of coming 

 to the surface at night and sinking to considerable depths during 

 the day. In the mackerel some species like the common Brit- 

 ish species have no air-bladder, and others possess one, although 

 it is never very large ; these are active predaceous fishes which 

 change their depth very rapidly. 



The gas contained in the air-bladder has been analysed, and 

 found to consist of oxygen and nitrogen with a trace of carbon 

 dioxide. These are the same gases as exist in the atmosphere, 

 but the proportion of oxygen in the air-bladder is much greater 

 than in atmospheric air, and is greater in marine fishes than in 

 fresh-water forms, and greatest of all in certain deep-sea fishes. 

 It has been maintained by Dr. Thilo that the gases in the 

 bladder are originally derived from the atmosphere, and are 

 simply swallowed by the fish, but this could not apply to the 

 adults of those forms in which the bladder is closed, although 

 it would be possible in their young stages, before the closing 

 takes place. There are, however, certain structures known as 

 red bodies and red glands in the wall of the air-bladder which 

 contain a net-work of blood-vessels, and there is good evidence 

 that these secrete gases and exude them into the bladder, while 

 another structure known as the oval is believed to absorb some 

 of the gas when necessary. The red bodies are modifications 

 of the primitive general vascular supply in the walls of the 

 bladder, the blood-vessels having become concentrated in 

 limited areas, instead of being generally distributed. 



The red bodies are situated on the ventral surface of the 



