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THE AIE OR SWIMMING BLADDER OF FISHES. 
By rev. W. HOUGHTON, M.A., F.L.S. 
E very fisherman is familiar with some of the forms of 
the organ known as the air-bladder found in many kinds 
of fish, whether tenants of our ponds and rivers or of the 
sea. Sometimes he may remember being startled to hear a 
gurgling sound proceed from the trout from whose mouth he is 
extricating the fiy : this sound is occasioned by the air being 
forcibly pressed by the hand out of the air-bladder through the 
oesophageal orifice. The subject of the swim-bladder of fishes 
is one of considerable interest to the naturalist ; let us take a 
short survey of it. 
The organ in question is found in most osseous fishes* ; it 
extends along the back of the abdomen, between the kidneys 
and the intestinal canal. The forms of this air-reservoir are 
various. Sometimes, as in the perch, it is a simple elongated 
cylinder closed at both ends ; sometimes, as in the carp, tench, 
roach, and other Cyprinidce, the organ is divided crosswise into 
two portions, by a deep constriction wdth a minute orifice 
leading from the one portion to the other ; or these two com- 
partments may have no communication with each other, as in 
Bagrus filamentosus and a species of Gymnotus, Sometimes, 
as in the genera Arius, Polypterus, and Lepidosiren, this 
organ is divided lengthwise into two compartments ; or it may 
be bifurcate ; or divided into four longitudinally succeeding 
portions,” as in the genus Pangasius, one of the Siluridce, 
The sapphirine gurnard {Trigla trirundo) has a swim-bladder 
divided longitudinally into three lobes, of which the middle one 
is the largest ; in other gurnards it is bilobed. Sometimes 
the air-bladder is divided partially, both lengthwise and cross- 
wise, as in Cobitis fossilis, Auchenipterus furcatus, and some 
species of Pimelodus.^^ In Corvina hispinosa the air-bladder 
gives out two somewhat slender processes on each side, one 
extending upwards, the other in a contrary direction. The 
* It is also found in the Cartilaginous Sturgeon. 
