236 



ZOOLOGY 



SECT. 



-a. 



-St 



7/j)J- 



a.li 



mucous membrane and capable of retaining water for a consider- 

 able period : the Fish is able to traverse 

 the laud, and is even said to climb trees, 

 holding on alternately by the spines of 

 its pre-operculuni 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 

 Gallichthys, anal respiration takes place, 

 air being drawn into and expelled from 

 the rectum. Lastly, in the curious little 

 goggle-eyed Pcrioi^lithalmus of the In- 

 dian and Pacific Oceans tlie 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 pharynx or the gullet in 

 Ganoids and Physostomes ; in the other 

 Teleostei the pneumatic ductatropliiesin 

 the adult and the bladder becomes a shut 

 sac. In Poly pterus" it consists of two lobes, 

 alarge left and a smaller right. The pneu- 

 matic duct is always connected with the 

 dorsal wall of the pharynx or gullet except 

 in Polypterus, in which the aperture is 

 ventral, and in some Physostomes, such 

 as the Herring, in which it is con- 

 nected with the'stomach. The bladder 

 is sometimes divided into compartments 

 or produced into lateral offshoots : in 

 Amia, Lepidosteus (Fig. 893, a. h.), and 

 Polypterus its w^all is sacculated or 

 raised into anastomosinof ridges, enclos- 

 ing more or less well-marked chambers 

 and thus resembling a lung. In Poly- 

 pterus its lung-like character is en- 

 hanced by its division into two com- 

 partments by a longitudinal partition, 

 as well as by the venti'al position of 

 the opening of the pneumatic duct and 

 by the blood being conveyed to it by a 

 pair of imlmonary 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 



'\ 



*-'-'-'— 



sp.v 



Fig. 893. — Digestive organs and air- 

 bladder i)f Iiepidosteus. a. 



anus ; a. h. ;iii--lpIadilLr ; «. 1/ its 

 aperture in tlic jiliaryiix ; h. if. 

 aperture of bile-duct. r. pyloi'ic 

 cicca ; f/. Ij. gall-bladder ; hp. d. 

 hepatic duct; Ir. liver ;7)j/. pylor- 

 ic valve; s. spleen ; nji. i: spiral 

 valve; at. stoinach.(Froin\Vieder- 

 sheiiii's Comparatirc Anatomy, 

 after Balfour and Parker.) 



