1 8 BRITISH FRESH-WATER FISHES 



the miller's thumb, a fish which stands next in the British 

 scale below the perches, and far above the salmon. It retains 

 the fine-run aft, but the head is the broadest part of the 

 animal, broader even than the capacious stomach, which is often 

 distended with food. It has no air-bladder, and although 

 actively predaceous, secures its prey in a very different way 

 from the free-roving perch, for it lurks under stones, whence^ 

 propelled by its powerful tail and large pectoral fins, it 

 launches itself in short, rapid rushes upon passing victims. 

 It is for evolutionists to determine whether, in its departure 

 from the typical form of Spiny-finned Fishes, the miller's 

 thumb has adapted its habits to the form imposed upon 

 it, or has forfeited the form of a free-swimming fish by the 

 practice of skulking habits. There is this to be said for this 

 curious little fish, that, inhabiting as it does shallow and clear 

 brooks, it may have acquired the lurking habit in order to 

 avoid herons and other enemies, against which it would stand 

 an indifferent chance were it to swim abroad. 



The caudal fin (Fig. I., 71), having become separate from 



the primitive continuous vertical fin, now becomes in most fishes 



the principal organ of propulsion. There are excep- 



Locomotion . r h r i ,/- 



and its tions, of course, for the modifications in the structure 

 of fish are innumerable and bewildering ; but it is 

 scarcely necessary, in dealing only with fresh-water species, 

 to describe the peculiar means of locomotion in the rays, 

 blennies, and hippocampi. The action of the caudal fin has 

 been compared with that of a screw propeller ; but the analogy 

 is not perfect. There is no screwing motion ; simply a rapid 

 vibration of the tail from side to side. The flexible rays 

 supporting the membrane of the caudal fin yield towards 

 their points and present a flat, pushing surface of fin, which, 

 acting like the backward stroke of an oar-blade, drives the 

 fish forward. 



The dorsal fin shows more frequent variation than any 

 other. The rays are either spinous, all or some of them, 



