THE AIR OR SWIMMING BLADDER OF FISHES. 
381 
air-bladder is entirely absent ; in the Malacopteri we meet with 
fish possessing both a swim-bladder and an air- duct {ductus 
pneumaticus) ; the Anacanthini exhibit in several instances an 
air-bladder, but no ductus pneumaticus ; in the Acanthopteri 
we meet with an air-bladder but no duct ; the same is the case 
with the Plectognathi and the Lophohranchii ; the Ganoidei 
show, in some of the genera of the order, an approximation to 
a luno;-like structure of the air-bladder, which is often cellular 
and provided with an air-duct. The Holocephali have no 
swim-bladder, neither have the Plagiostomi. The Pvotopteri, 
to which the Lepidosirsn belongs, possess a double air-bladder, 
very cellular and lung-like, with air-duct, glottis, and pulmo- 
nary vein. 
"^Tth regard, however, to the existence of this organ in the 
different orders, it must be borne in mind that it is occasionally 
absent in fishes belonging to different genera, and sometimes 
even in fishes exhibiting merely specific differences. We are 
thus, naturally enough, driven to the question of cui hono ? 
what are the functions of the swim-bladder and air-duct when 
present ? Are they such important organs as by some people 
they are said to be? What does their presence indicate, and 
will it serve to throw a gleam of light on that most interesting 
of all riddles, the origin of species? Paley considers that the 
air-bladder of a fish affords a plain and direct instance not only 
of contrivance, but strictly of that species of contrivance which 
we denominate mechanical. It is a philosophical apparatus in 
the body of the animal.” This philosophical apparatus is thus 
described by Eoget in one of the Bridgewater Treatises : ” 
‘^Independently of these instruments of progression (tail and 
fins), most fishes are provided with internal means of changing 
their situation in the water. 
‘‘ The structure by which this effect is accomplished is one 
of the most remarkable instances that is met with of an ex- 
press contrivance for a specific purpose, and of the employ- 
ment of an agency of a class different from that of the me- 
chanical powers usually resorted to for effecting the same object. 
When distended with air it renders the whole fish specifically 
lighter than the surrounding water ; and the fish is thus buoyed 
up and remains at the surface without an effort of its own. 
On compressing the bladder by the action of the surrounding 
muscles the included air is compressed, the specific gravity of 
the whole body is incre^^sed, and the fish sinks to the bottom. 
On relaxing the same muscles the air recovers its former dimen- 
sion, and the fish is again rendered buoyant. Can there be any 
stronger evidence than the placing of this hydrostatic apparatus, 
acting upon philosophical principles, in the interior of the organi- 
