. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. FLYING FISHES AND THEIR HABITS. 509 As almost every oxternal feature is characteristic, so are many internal parts. In connection Avitli the longitudinal arch or convex- ity of the l)ack, so different from the straightness of that of the Gur- nards, a very renuiricable deviation of the air bladder fi-om normal relations is noteworthy; the dorsal curvature, indeed, is a coordinate of an otherwise unexampled position of the bladder. The air,
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. FLYING FISHES AND THEIR HABITS. 509 As almost every oxternal feature is characteristic, so are many internal parts. In connection Avitli the longitudinal arch or convex- ity of the l)ack, so different from the straightness of that of the Gur- nards, a very renuiricable deviation of the air bladder fi-om normal relations is noteworthy; the dorsal curvature, indeed, is a coordinate of an otherwise unexampled position of the bladder. The air, or swimming, bladder is unique in character, as Calder- wood states, in that " it is not situated below, but | niostly J above, the vertebral column, not forming part of the abdominal contents, but situated dorsally in a special cavity [recess] of its own. ^A^ien the abdominal cavity is opened ventrally, and the viscera removed, only the ventral surface of the l)ladder is seen, forming part of the dorsal boundary of the cavity. Seen from this point of view, it is iormed of a broad central portion, white and tendinous, and of two lateral portions strongly muscular.'' The j^hysiological significance of this comes into view when we consider one of the habits or aptitudes of the fish. The structure and position of the air-bladder are adapt- ed for keeping the Dactylop- terid Avith back upward in the air in spite of the form of the body and its relation to the vertebral axis. " The bladder, being prevented from expanding when the pressure from the surround- ing Avater is suddenly re- moved, the high dorsal position of the sccondar}^ portion Ijecomes of the greatest possible ; It helps the fish to emerge from the water and maintain its equilibrium in the air. Such are the most characteristic features of the Dactylopterids com- mon to all the members of the family. The species are few—about half a dozen—and closely related to each other, all being strictly
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