xin 



PHYLUM CHORDATA 



229 



that it is drowned if immersed in water. 

 Siluroid Callichthys anal respiration takes 

 place, air being drawn into and expelled 

 from the rectum. Lastly, in the curious 

 little goggle-eyed Periophthalmus of the 

 Indian and Pacific Oceans the tail-fin 

 seems to serve as a respiratory organ, 

 being kept in the water while the Fish 

 perches on a rock. 



The air-bladder retains its connection 

 with the gullet (rarely with the stomach) 

 in Ganoids and Physostomes ; in the 

 other Teleostei the pneumatic duct atro- 

 phies in the adult and the bladder becomes 

 a shut sac. In Polypterus it consists of 

 two lobes, a large left and a smaller right. 

 The pneumatic duct is always connected 

 with the dorsal wall of the gullet or 

 stomach, except in Polypterus, in which 

 the aperture is ventral. The bladder is 

 sometimes divided into compartments or 

 produced into lateral offshoots : in Amia 

 and Lepidosteus (Fig. 904, a. b.) its wall 

 is sacculated or raised into anastomosing 

 ridges, enclosing more or less well-marked 

 chambers and thus resembling a lung. In 

 Polypterus its lung-like character is en- 

 hanced by the ventral position of the 

 opening and by the blood being conveyed 

 to it (as is also the case in Amia) by a 

 pair of pulmonary arteries given off from 

 the last pair of epibranchial arteries, as in 

 the Dipnoi. 



The air-bladder seems to be capable of 

 acting as a sort of accessory respiratory 

 organ ; it has been found that in a Perch, 

 asphyxiated in stagnant water, the oxygen 

 in the bladder, which normally amounts 

 to 20 or 25 per cent., is entirely absorbed 

 and replaced by nitrogen and carbonic 

 acid. Its normal function, however, is 

 hydrostatic, i.e. it serves to keep the 

 Fish of the same specific gravity as the 

 water. The specific gravity of the Fish 

 as a whole, rising or falling as it must on 

 account of the increase or decrease of 

 pressure at various depths as the Fish 



VOL. II 



In the little armoured 

 ---**' 



st 



sp.v 



FIG. 904. Digestive organs ami 

 air-bladder of Lepidosteus. 

 a. anus ; a. b. air-bladder ; 

 a. b'. its aperture in the 

 pharynx ; b. d'. aperture of bile- 

 duct ; c. pyloric caeca ; g. b. 

 gall-bladder ; hp. d. hepatic 

 duct ; Ir. liver ; py. pyloric 

 valve ; s. spleen ; sp. t>. spiral 

 valve ; st. stomach. (From 

 Wiedersheim's Comparative 

 Anatomy, after Balfour and 

 Parker.) 



descends or ascends 



