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 Te/eostei, 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 vertebra, 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 
Fins, 
* 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. 
