18 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 
ae the principal organ of propulsion. There are excep- 
and its tions, of course, for the modifications in the structure 
a rae 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, 
