A treatise on zoology . lom. Thecommunicating ductus pneumaticus jmsses down the dorsalmesentery to open into the oesophagus. The ductus isshort, and the opening wide in the more primitive forms {Lepi-dosteus, Amia, Acipcnser). But although the bladder is dorsal inthe Actinopterygii, the opening of the ductus varies considerablyin position. In the Chondrostei, Amia, Lepidosteus, and themajority of the Teleostei, it is quite or nearly median and dorsal; AIR-BLADDER but in others, such as Salmo, the Siluridae, Cypriuodontidae, Per-copsidae, and Galaxiidae, it opens more or less on the right. Ont
A treatise on zoology . lom. Thecommunicating ductus pneumaticus jmsses down the dorsalmesentery to open into the oesophagus. The ductus isshort, and the opening wide in the more primitive forms {Lepi-dosteus, Amia, Acipcnser). But although the bladder is dorsal inthe Actinopterygii, the opening of the ductus varies considerablyin position. In the Chondrostei, Amia, Lepidosteus, and themajority of the Teleostei, it is quite or nearly median and dorsal; AIR-BLADDER but in others, such as Salmo, the Siluridae, Cypriuodontidae, Per-copsidae, and Galaxiidae, it opens more or less on the right. Onthe contrary, in the Mormyridae, Notopteridae, Gymnotidae,Cyprinidae, and Characinidae, it opens somewhat on the left (), and in some genera of the last family, such as Eryfl/riinisand Macrodon, the ductus passes down to open quite on the leftside of the oesophagus (Kowntree [375]). The evidence of ontogeny seems to show that the diflerence inthe position of the opening is due rather to secondary shifting than (to. 198. Diagram .showing the relations of tlie oesopliagus, or, the pneumatic duct, , the bilobed air-bladder and its blood-supply in Irotiqiterus, seen from behind, cm, junction of aortic arches todorsal aorta ; LI, left lobe of air-bladder ; l-iho, left pulmonary artery ; , pulmonary vein ;, right lobe of air-bladder ; , right pulmonary artery. to the retention of a more primitive position in the aberrant fish(Moser [3U-i], Piper [330]). With regard to the original condition of the air-bladder,attempts have been made to derive these various organs found inthe Osteichthyes from some single ancestral form. Boas suggested that the bladder was originally dorsal andmedian ; that, together with its duct, it split into right and lefthalves; that these separated, passed round the oesophagus, andreunited ventrally to open finally by a median ventral explanation, however, cannot be applied to the Dipnoi (p. 223), 15 226 OSTEICHTHYES and th
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