46 ANIMAL LIFE 



adjustments to overcome the tendency to float upon 

 its back. As a help towards steadying the body it 

 has developed fins along the middle line of the back 

 and tail. These ' median ' fins act as keels or centre- 

 boards. They are supported by a series of spines, 

 which can elevate or depress the membrane they 

 support. The first of these spines is often stronger 

 than the rest, and acts as a cut-water. Moreover, to 

 diminish the resistance of such fins in a turning move- 

 ment the spines can be lowered, so that the fin collapses, 

 and raised again when the new tack is begun. Thus 

 rapidly can a shark unstep the peg that stretches 

 his dorsal fin and ship it again in the notch at its 

 base. 



If such complex adjustments are needed by active 

 fish, we do not wonder that a change in the shape 

 or attitude of the body has been developed in those 

 which seek their food on the bottom, or wish to 

 escape the constant exertion of a mid-water life. 

 Of these, the flat-fish and skates are the most remark- 

 able. Connected with the dory family by many ties 

 of structure and mode of occurrence, the sole and 

 plaice, turbot and dab, agree with their relative, 

 the John Dory, in being flattened, but differ in resting 

 on or in the sand instead of hovering over it. It 

 might be thought that this result was merely due 

 to a compression of the body parallel to the ground, 

 but examination shows that both eyes in a flat-fish 

 appear on the upper coloured side, that the mouth is 

 awry, and that the skull-orbits are also skew-whiffed. 



