The swim bladder in Eucinostomus gula is a closed, retroperitoneal, tubular, single-chambered vesicle extending the length of the body cavity. It represents about 5.5 per cent of the total body volume. Its wall is composed of four layers. Anteriorly the swim bladder has two
horns. On its respective side of the head, each horn enters the auditory bulla through a fenestra shared by the basioccipital and exoccipital bones. Posteriorly the swim bladder projects into a cone-shaped structure formed by the fusion of the first and second interhemal spines. No structures
modified for sound production were identified through dissection. It is suggested that the association of the swim bladder and auditory capsule aids in the location of food organisms buried in the sand. There is a well-developed gas gland-rete mirabile complex in the anteroventral portion
of the swim bladder. The presence of an oval gland in the dorsal region of that portion of the swim bladder projecting into the compound interhemal is discussed. The swim-bladder gas in specimens of E. gula and E. argenteus caught in shallow water and in control specimens
kept in an aquarium had a near-air composition. Gas secretion was stimulated in both species by weighting specimens or subjecting them to increased pressure. In both species the secreted gas is primarily oxygen with only a small percentage being carbon dioxide. Resorption of swim-bladder
gas was stimulated in both species by attaching a float to the middorsal surface. Like the secreted gas, the resorbed gas was primarily oxygen. During asphyxiation a portion of the swim-bladder oxygen in both species is utilized. However, as an oxygen store the swim bladder cannot play
a significant role in these fish.
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