The air-bladder is either more or less free in the abdominal cavity, or firmly attached to the vertebral centra and their rib-bearing processes by fibrous extensions passing between the two structures. Not rarely the organ extends from the abdominal cavity into the tail, sometimes penetrating for a short distance into the expanded haemal canal of the anterior caudal vertebrae, or extending unsymmetrically along either the right or left side of the tail. More frequently, perhaps, where the air-bladder is prolonged into the tail, it assumes the form of two bilaterally arranged and symmetrical caeca, which extend backwards for a variable distance internal to the caudal muscles and in contact with the adjacent skeletal elements, as in Notopteridae, and in some Sparidae, Carangidae, and Scombridae. The extension of the air-bladder into the tail is often associated with a short, laterally-compressed trunk, which, if the bladder is to attain its normal degree of development, necessitates its prolongation into the caudal region.
(c) A characteristic feature in the air-bladder of many Teleosts belonging to widely different families is the development of a more or less complex system of simple, or variously branched, caecal outgrowths, which, like the internal septa, are specially characteristic of those Fishes in which the bladder is used as a vocal organ without, however, being peculiar to them.
In some of the Gadidae, as in the Cod (Gadus morrhua), the air-bladder divides anteriorly into a pair of caecal prolongations which extend forwards to the head, and are often curiously coiled. Somewhat similar caeca are also present in species of Berycidae, Sparidae, Siluridae, Clupeidae, and Notopteridae. Caecal prolongations may also be developed from the hinder end of the bladder, and, as already mentioned, extend into the tail; or even from both ends in the same species (e.g. Notopterus).[[340]] In the Silurid, Rita crucigera,[[341]] a long tubular caecum is developed from each side of the heart-shaped bladder, and thence is prolonged backwards to the anus. In certain species of Doras of the same family (e.g. D. maculatus),[[342]] an elegant series of variously sized branched caeca fringe each of the lateral margins of the bladder. It is, however, in the Physoclist family of the Sciaenidae[[343]] that the branching of the air-bladder attains its greatest development in extent and variety.
Fig. 180.—Air-bladder of Otolithus. (From Cuvier and Valenciennes.)
In Otolithus (Fig. 180) two short tubular canals are given off from the antero-lateral angles of the bladder, each subsequently dividing into two elongated, tapering sacs, of which one is directed forwards and the other backwards. In Corvina lobata (Fig. 181) the lateral margins of the bladder are everywhere fringed with a series of tufts of caeca, each tuft being connected by a short common canal with the cavity of the organ. In the "Drum" (Pogonias chromis) (Fig. 182) each side of the anterior third of the air-bladder has a series of digitately branched caecal appendages, the most posterior of which on each side are connected by a tubular canal, also bearing branched caeca, with the corresponding postero-lateral extremity of the bladder.
Collichthys[[344]] has a still more remarkable arrangement. In this Sciaenoid (Fig. 183) twenty-five tubular branches are given off from each side of the bladder, all of which soon subdivide into a dorsal and a ventral division. These still further divide, and their branches either end blindly or are prolonged into a series of arches to the mid-dorsal or mid-ventral line as the case may be, where they become continuous with the corresponding branches of the opposite side. The series of dorsal branches, enveloped in their peritoneal investment, extend between the body of the air-bladder and the roof of the body-cavity, while the corresponding ventral branches, similarly invested, surround that part of the coelom which contains the stomach, intestine, and liver.
Fig. 181.—Air-bladder of Corvina lobata. (From Cuvier and Valenciennes.)
(d) In addition to the subdivision of the cavity of the air-bladder by the externally obvious, transverse, or longitudinal constrictions already described, or by the growth of simple or branched prolongations, the organ is often chambered or sacculated by the development of internal septa or partitions.