. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 39^ Biology of the Vertebrates over on one side in the course of development and assume their lazy adult habit of life, the swim bladder degenerates. Fishes which possess swim bladders in the adult form may be divided into two groups according to the character of the swim bladder, namely, the physostomi, having a pneumatic duct leading from the swim bladder to the alimentary tract; and the physoclisti, having the outlet duct closed or atrophied. To the fir
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 39^ Biology of the Vertebrates over on one side in the course of development and assume their lazy adult habit of life, the swim bladder degenerates. Fishes which possess swim bladders in the adult form may be divided into two groups according to the character of the swim bladder, namely, the physostomi, having a pneumatic duct leading from the swim bladder to the alimentary tract; and the physoclisti, having the outlet duct closed or atrophied. To the first group belong the bony ganoids, the dipnoans, and the soft-rayed teleosts. In the latter group are included all the spiny-rayed fishes. The pickerel, Esox, represents a simple type of physostomous fish having a single sac for a swim bladder, with a pneumatic duct at the anterior end opening into the esophagus (Fig. 331a). In some fishes the single swim bladder is made up of two connecting sacs (Fig. 331b). Pneumatic Duct Swim ^^Swlm Bladder^ \ Red Gland Bladder //^-><^//.Pneumertic Duct \«s,. Esophagus Esophagus Esophagus A. PICKEREL B. CARP C. EEL Fig. 331. Diagrams of swim bladders. The eel shows the posterior chamber forming by the enlargement of the pneumatic duct. (After Tracy.) In physoclistous fishes, upon the degeneration of the pneumatic duct, the swim bladder becomes a closed sac having two chambers separated by a partition through which there is a sphincter-like opening, regulated in size by both circular and radiating muscles similar in arrangement to those in the iris of the eye. The posterior chamber is formed by an enlargement of the pneumatic duct that is no longer needed for its original function. This is quite apparent in the eel, Anguilla, a physostomous fish on the verge of becoming physoclistous, in which the duct is caught in the very act of enlargement into a separate chamber (Fig. 331c). The formation of the posterior chamber by the enlargement
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