xxii BRITISH FRESHWATER FISHES 



the pelvics far back, near the vent. The Eels (Apodes) 

 differ in that the long dorsal and anal are confluent 

 with the reduced caudal and the pelvic fins are 

 absent. In the Anacanthini the fin-rays are articu- 

 lated, but the pectoral fins are higher up on the sides, 

 and the pelvic fins are placed below or in advance of 

 them. In some, at least, of the Percomorphi and 

 Heterosomata the anterior rays of the dorsal and 

 anal and the outer rays of the pelvic fins are non- 

 articulated spines, typically stiff and pungent, but 

 in the dorsal fin of the Bull-head feeble and 

 flexible; the spinous portion of the dorsal may separate 

 off as a distinct fin. In the two last-named orders 

 the pelvic fins are more or less advanced, and the 

 pelvic bones are often directly attached to the 

 clavicles. The evolution of spinous fin-rays has 

 added a new function to the fins, that of weapons 

 of attack and defence, and pugnacious fishes, like 

 the Sticklebacks, know well how to use them to the 

 best advantage. 



In his interesting book on Concealing Coloration in 

 the Animal Kingdom, Mr. G. H. Thayer has written : 

 " If an object be colored so that its tones constitute 

 a gradation of shading and of coloring counter to the 

 gradation of shading and coloring which light thrown 

 upon it would produce, and having the same rate of 

 gradation, such object will appear perfectly flat; 

 retaining its length and breadth, but losing all ap- 

 pearance of thickness ; and when seen against a 

 background of color and pattern like its own will 

 be essentially indistinguishable at a short distance." 



These words give us the clue to the coloration of 

 our British freshwater fishes, which live in a medium 

 through which light descends from above; conse- 



