INTRODUCTION xxiii 



quently nearly all of them exhibit a gradation of shades 

 from silvery white below to a dark bluish, greenish, or 

 brownish above. Some, as for example most of the 

 Cyprinoids, depend upon this simple shading to 

 render them inconspicuous, but others are variously 

 marked in harmony with the ground on which they 

 lie or the weeds among which they lurk. 



A remarkable characteristic of fishes as a whole 

 is their power to vary their colour and markings ; 

 some of the fishes of tropical seas can instant- 

 aneously change their colour, sometimes from white 

 to black, or from yellow to red, green, or brown ; just 

 as suddenly they can become spotted, striped, or 

 barred. This inconstancy makes coloration a very 

 doubtful aid to the systematic ichthyologist, and 

 among our freshwater fishes we may cite the Trout 

 as one of the most variable in this respect, whilst the 

 Flounder is unrivalled in its capacity for imitating 

 the ground on which it lies, and the Bull-head is re- 

 markable for its rapid colour changes when excited 

 by greed, fear, or anger ; such instantaneous changes 

 are accomplished by the expansion and contraction 

 of the variously coloured pigment cells in the skin. 



The air-bladder may be mentioned here because 

 of its importance in classification ; it lies between the 

 alimentary canal and the vertebral column, and is 

 connected with the former by a duct in the more 

 generalized types which belong to the first four 

 Teleostean orders enumerated above, but not in the 

 remainder. The air-bladder may be absent (Flounder, 

 Bull-head), or it may become connected with the 

 internal ear by means of caecal diverticula (Shad), 

 or by means of a chain of ossicles which are modi- 

 fied elements of the anterior vertebrae (Ostariophysi} ; 



