The tunics of the majority of air-bladders are an extremely fine internal one, frequently shining silvery, containing crystalline corpuscles, sometimes covered with a pavement-epithelium; and a thicker outer one of a fibrous texture, which sometimes attains to considerable thickness and yields isinglass. This wall is strengthened in many fishes by muscular layers for the compression of the whole organ or of some portion of it.
A distinction has been made between air-bladders which communicate by a duct with the intestinal tract and those which are entirely closed. However, it is to be remembered that at an early stage of development all air-bladders are provided with such a duct, which in a part of the fishes more or less completely obliterates, being then represented by a fine ligament only. In young Lucioperca of six to eight inches in length the duct may be found still open for a considerable distance; and, on the other hand, in adult Physostomi, that is Teleosteous fishes with a ductus pneumaticus, not rarely the whole duct is found very narrow, or, for some part of its length, even entirely closed.
Fig. 61.—Air-bladder of Otolithus sp.
Fig. 62.—Vertical section through abdominal cavity of Collichthys lucida. b, air-bladder; l, liver; s, stomach; epp and ipp, external and internal laminæ of peritoneum parietale; epv and ipv, external and internal laminæ of peritoneum viscerale; dv, dorsal air-vessels; vv, ventral air-vessels.
Air-bladders without duct are found in Acanthopterygians, Pharyngognaths, Anacanths, and Lophobranchs. They may consist of a single cavity or divided by constrictions into two or three partitions situated behind one another; they may consist of two lateral partitions, assuming a horseshoe-like form, or they may be a single sac with a pair of simple or bifid processes in front or behind (Fig. [61]). The families of Sciænidæ and Polynemidæ possess air-bladders with a most extraordinary development of appendages rising from each side of the air-bladder. In the Sciænoid (Fig. [63]) fifty-two branches issue from each side, each branch being bifurcate and bearing smaller appendages. In Pogonias chromis (Fig. [64]) the sides of the anterior half is provided with irregular broad-fringed appendages, the hindmost of which communicates by a narrow duct with the posterior extremity of the air-bladder. In Collichthys lucida (Fig. [62]) twenty-five appendages issue from each side; the anterior ones are directed towards the front, but the lateral assume a more posterior direction, the nearer they are to the posterior extremity of the air-bladder, where they form an assemblage giving the appearance of a cauda equina. All these appendages soon bifurcate in a dorsal and ventral stem; these stems bifurcate again and again, and either terminate after the first or second bifurcation or are so far prolonged as to reach the median line of the ventral and dorsal sides, anastomosing with the branches of the other side. The branches being enveloped in laminæ of the peritonæum, form a dorsal and ventral sac of beautiful appearance, caused by the regular arrangement of the air-vessels. The dorsal sac is situated between the air-bladder and the roof of the abdominal cavity without being attached to the latter. The ventral sac receives within its cavity the intestine, liver, and ovaries.—A peculiar mechanism has been observed in the air-bladder of the Ophidiidæ, the anterior portion of which can be prolonged by the contraction of two muscles attached to its anterior extremity, with or without the addition of a small bone.
Fig. 63.—Air-bladder of a Sciænoid.
I. Visceral surface opened at b, to show openings of the lateral branches.