302 



FISHES 



CHAP. 



(a) The air-bladder is by no means universally present in 

 Teleosts. It is absent in several entire families, 1 such as, for 

 example, the Flat Fishes or Pleuronectidae, the Scopelidae, and 

 the " Lump-suckers " (Cyclopteridae). In a few families, as in 

 the Mackerels (Scombridae), the " Blennies " (Blenniidae) and the 

 Polynemidae, the organ is present in most genera, but absent in 

 a few, or even present or absent in different species of the same 

 genus. Thus, of the three British species of Mackerel, viz. the 

 Spanish Mackerel (Scomber colias), S. pneumatophorus, and the 



common Mackerel (S. scombrus), an 

 air-bladder is present in the first 

 two, but absent in the third. 2 



(&) As might be anticipated, 

 the shape of the air-bladder is 

 extremely different in various 

 Teleosts, and usually conforms 

 to the shape of the body, while 

 FIG. 179. Showing the structure of one differences in relative size are 



of the larger alveoli of the air-bladder 



of Protopterm. l, Central cavity of of frequent occurrence, even in 



the lung ; 2, alveolus ; 3 tubular dosely related sp ecies. Some- 

 cavities communicating with 4, the " 



small terminal saccuii. (From Bald- times the organ is more or less 

 wm Spencer.) tubular, fusiform, ovoid, or heart- 



shaped ; occasionally it is shaped like a " dumb-bell," consisting 

 of two lateral sacs connected by a median tubular portion, as 

 in the Siluroids Clarias and Calliclithys ; or it may be 

 horse-shoe-shaped, as in the Silurid Ailia. 3 Not unfrequently 

 a transverse constriction divides the air-bladder into two inter- 

 communicating sacs, as in most of the Carp family (Cyprinidae), 

 or three such sacs may be formed by two constrictions (e.g. 

 Opliidiuni). In the " Electric Eels " (Gymnotidae) there are two 

 sacs, connected by a slender canal, from which the ductus 

 pneumaticus takes its origin. 4 



The air-bladder is either more or less free in the abdominal 

 cavity, or firmly attached to the vertebral centra and their rib- 

 bearing processes by fibrous extensions passing between the two 

 structures. Not rarely the organ extends from the abdominal 



1 Stannius, Handb. d. Zool. Berlin ii. 1854, p. 220. 



2 Giinther, Study of Fishes. Edinburgh, 1880, p. 457. 



3 Bridge and Haddon, Phil. Trans. B, 184, 1893, p. 209. 



4 Reinhardt, quoted by Stannius, op. cit. p. 225. 



