220 



ZOOLOGY 



SECT. 



mucous membrane and capable of retaining water for a consider- 

 able period : the Fish is able to traverse the land, and is even said 



to climb trees, holding on alternately 

 by the spines of its pre-operculum and 

 of its ventral fins. It has become so 

 thoroughly a land animal that it is 

 drowned if immersed in water. In the 

 little armoured Siluroid CaHichthys, anal 

 respiration takes place, air being drawn 

 into and expelled from the rectum. 

 And, 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 connec- 

 tion with the gullet in Ganoids and 

 Physostomes ; in the other Teleostei 

 the pneumatic duct atrophies in the 

 adult, and the bladder becomes a shut 

 sac. The pneumatic duct is always 

 connected with; the dorsal wall of the 

 gullet except in Polypterus, in which 

 the aperture is ventral, and in some 

 Physostomes, such as the Herring, in 

 which it is connected with the stomach. 

 The bladder is sometimes divided into 

 compartments or produced into lateral 

 offshoots : in Amia, Lepidosteus (Fig. 

 843, a. &.), and Polypterus 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 enhanced by its division 

 into two compartments by a longitu- 

 dinal partition, as well as by the ven- 

 tral position of the opening of the 

 pneumatic duct. 



The air-bladder seems able to act 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 



FIG. 843. Digestive organs and air- 

 bladder of Xiepidosteus. . 

 anus ; a. &. air-bladder ; a.b'. its 

 aperture in the pharynx ; b. d. 

 aperture of bile-duct ; c. pyloric 

 eoeca ; g. b. gall-bladder ; hp. d. 

 hepatic duct ; lr. liver ; py. pylo- 

 ric valve ; s. spleen ; sp. r. spiral 

 valve ; st. stomach. (FromWieder- 

 sheim's Comparative Anatomy.) 



