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 vertebras 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 



Scales, 



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 



^°^ 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 



