In the great majority of fishes, a curious sac or bag known as the swimming or air bladder is found.
The dorsal and ventral fins are both very long; and, as is usual with bottom fishes, the swimming or air bladder is absent.
Isinglass is a sort of glue made from the viscera and air bladder of certain fish, as cod and sturgeon.
So slender and displacing so little water, this fish has no need of the air bladder which supports the thicker fish.
An organ peculiar to fishes is the air bladder—a sac lying under the backbone and communicating by a duct with the stomach.
Each is reached by the auditory nerve from the brain and is also connected with the air bladder in many cases.
The air bladder therefore assists the ear of the catfish as the tympanum and its bones assist the ear of the higher animals.
The habit is also common to some species of mud fishes of the order Dipneusti, in which the air bladder plays the part of lungs.
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