302 FISHES ‘(CHARS 
(a) The air-bladder is by no means universally present in 
Teleosts. It is absent in several entire families,’ 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. pnewmatophorus, and the 
common Mackerel (S. scombrus), an 
air-bladder is present in the first 
two, but absent in the third.” 
(b) 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 
Fic. 179.—Showing the structure of one differences in relative size are 
of the larger alveoli of the air-bladder 7 
of Protopterus. 1, Central cavity of of frequent occurrence, even 1n 
eee a eee closely related species. Some- 
small terminal sacculi. (From Bald- times the organ is more or less 
alia) 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 Callichthys; or it may be 
horse-shoe-shaped, as in the Silurid Ailia.2 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. 
Ophidium). In the “ Electric Eels” (Gymnotidae) there are two 
sacs, connected by a slender canal, from which the ductus 
pneumaticus takes its origin.* 
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 
Stannius, Handb. d. Zool. Berlin ii. 1854, p. 220. 
Giinther, Study of Fishes, Edinburgh, 1880, p. 457. 
Bridge and Haddon, Phil. Trans. B, 184, 1893, p. 209. 
Reinhardt, quoted by Stannius, op. cit. p. 225. 
ee (SO eS 
