72 COLOUR, FORM, AND MOVEMENT. 



Paired Fins. There are two pairs of these, and we may 

 note in passing their interest as the precursors of the fore 

 and hind limbs of other vertebrate animals. They are 

 placed, one larger pair upon the sides near the head and 

 the other smaller pair further back and lower down on the 

 body. 



In some fishes, e.g. members of the cod family, the hind 

 pair has been transferred to a position in front of the fore 

 pair. This arrangement doubtless assists in keeping up 

 the head end, which is necessary owing to the position of 

 the centre of gravity and which is one of the uses of the 

 pectoral fins. 



Both pairs also act as side keels, maintaining steadiness 

 and preventing overturning. This is not difficult to under- 

 stand in the light of what we now know of the other 

 adjustments in the fish body. 



Another important service performed by these fins, which 

 may be discovered by watching the fish, is that of steering. 

 It will be noticed how frequently the fins of one side act 

 independently of those of tha other. A backward stroke 

 of a pectoral fin turns the fish towards that side, and other 

 turning movements are similarly effected. Lastly they are 

 in some cases used as propelling organs. Sticklebacks 

 may be observed using their pectoral fins for this purpose. 



Air Bladder. An interesting structure possessed by 

 many fishes is the air bladder. This sac lies below the 

 backbone ; it is very variable in form and size ; it may 

 open into the gut or be closed. It is undoubtedly a hydro- 

 static organ in the first instance, though in many cases it 

 has other functions. 



Its use seems to be to adjust the specific gravity of the 

 fish to that of a particular plane in the water, which is 

 called the plane of least effort. If the fish rise far above 

 this plane, or go far below it, special effort will be required 

 to maintain the new position until by an internal adjust- 

 ment of the amount of gas in the bladder a new specific 

 gravity is attained, and the bladder is contracted or dis- 

 tended. This adjustment is effected by blood glands in 

 the air bladder, and also by the duct liberating gas in 



