20 BRITISH FRESH-WATER FISHES 
would be of no effect ; but a fish swimming in midwater makes 
use of a superior as well as an inferior keel. It is to be noted, 
however, that whereas the anal fin or inferior keel is usually 
more or less fixed, the dorsal or superior keel is erectile and 
depressible at pleasure. Its formidable armature of spines 
in the perch no doubt serves as a defence against predaceous 
creatures. As the rays of the dorsal and anal fins are arranged 
to correspond with the vertebrae of certain parts of the spinal 
column, they are pretty constant in number, and afford means 
of generic and specific classification. Nevertheless, they are 
subject, at least when very numerous, to a limited amount of 
variation. 
The scales which protect the skin supply an external 
feature almost as characteristic as fins, but not so universal. 
There are many instances of whole families of 
scaleless fish ; other families include both scaled and 
scaleless genera ; but it happens that among the fresh-water 
fishes of Great Britain five only—the sturgeon, the stickle- 
back, the miller’s thumb, the lamprey, and the lampern— 
have no scales. In the first two the want of scales is compen- 
sated for by conspicuous bony scutes or plates, but the other 
three go naked. Even the eel of British waters, member 
of a family containing many scaleless genera, carries rudi- 
mentary scales, although its near relative, the conger, exhibits 
no trace of them. 
In all Teleostean fishes the lateral line forms a very distinct 
feature. This is formed by a continuous row of perforated 
The lateral Scales extending from the scaleless head to the root 
line. of the caudal fin. These perforations are connected 
with the muciferous duct which runs from the head along the 
body, and act as channels for the discharge upon the skin 
of that mucus which gives to the fish its peculiarly slippery 
surface. That seems to be the principal function of the lateral 
line—at all events it is the most obvious one; but it does 
not account for the liberality of the nerve system with which 
Scales, 
ee 
