12 BRITISH FRESH-WATER FISHES 



adult individuals.* Within the bladder, gas, chiefly nitrogen, 

 is secreted, whereby the buoyancy of the fish is regulated. It 

 is said, on the authority of Siebold, that when, as happens in 

 Lake Constance, perch are suddenly brought to the surface 

 from a depth of twenty-five to thirty fathoms, the gas in the 

 bladder, relieved from the enormous superincumbent pressure, 

 expands, and, finding no escape, forces the bladder with some 

 of the viscera out of the mouth of the fish. 



This remarkable organ is present in all Ganoid fish, of 

 which British inland waters only contain one, to wit, the 

 sturgeon. In this order of fishes, it has greater or less con- 

 nection with the respiratory system ; but in the Teleostei, or 

 Bony-skeletoned Fishes, which include the large majority of 

 our native species, it bears no part in respiration, and would 

 be termed with greater propriety the gas-bladder, seeing that 

 its contents consist of nitrogen with an exceedingly small pro- 

 portion of oxygen. In form and position the air-bladder, when 

 present, varies very much. In the perch it is prolonged by 

 two anterior channels into the skull, where it is connected with, 

 and probably assists, the organ of hearing. In the loaches it 

 is partly or wholly enclosed in a bony capsule extending from 

 the vertebrae, and it is wholly absent from some species of 

 bony fish and from the lampreys, which live habitually on 

 the bottom. 



The general buoyancy of the perch being thus secured by 



the air-bladder, whereby it is enabled to move freely at such 



distance from the bottom as it desires, the next point 



requiring explanation is how it maintains a vertical 



position in the water vertical, that is, in regard to its diameter 



from back to belly, for in regard to its length from snout to 



caudal fin its position is horizontal. A fresh-killed perch 



thrown into the water sinks slowly to the bottom ; when the 



* Dr. Giinther is of opinion that a pneumatic connection between the 

 air-bladder and the pharynx exists in all fish possessing an air-bladder at 

 an early stage of their development. 



