368 



SCANDINAVIAN FISHES. 



tliat under certain circumstances, e. g. if" a species were 

 never enabled to exercise it for sevei-al generations, it 

 may entirely disapjjcar. Furthermore, the same stock 

 may eventually l)e found to have given rise to two 

 distinct races, the one deeply and the other faintly 

 coloured, according to the ditt'erent bottom on which 

 each of them has taken up its abode; and lioth of them, 

 through want of jjivicticc, may have lost theii' ancestors' 

 power of changing colour. In this manner we may 

 explain tiie origin of several varieties recognised by 

 zoology, as due merely to the influence of habit. It 

 is even probable that from this point of view we may 

 be compelled to revise many species tliat have been 



selves are generally larger on the blind side. Most 

 often too, the muciferous ducts of the head are more 

 developed on this side than on the eye side. 



The scales of the body vary considerably in de- 

 scription, but are generally well-developed, so much so 

 that in this respect, too, the Flatfishes take the same 

 place among the Malacopterj'gian Physoclysts as the 

 Scale-finned fishes (fain. S(jii(niiiji/ji)ics) among the Acan- 

 thopterygians. Sometimes the scales are middle-sized, 

 but in that case often deciduous, sometimes small and 

 firmly embedded in the skin. As a rule the scaly co- 

 vering of the body ad\ances over the base of the un- 

 paired fins, either on the rays or on the membrane 







sht 



Fi>;-. 104: Schematic figure of tlic copljalic system nf tlic lateral line in a Cud: lig. 105: Similar figure of the head el' a Tiirbot. After 

 Ti'.AQUAU:. .s'rt, suborbital brancli: »/, mandibular brancli; />, frontorostral branch; c, connecting branch between the two frontorostral branches : 



fyt, su|iratem]poral branch. 



established mereh' on tlie strength of characters of this 

 description." 



Tlie sense of sight being now cxchisiveh' confined 

 to one side of thc' body, the coloration of the l)ody 

 follows it, the other side of tlie body becoming albinistic. 

 At the same time too, the great musculaf mass of the 

 body is develo])ed more on the eye side than on the 

 l)lind side. In most cases the paired fins are also re- 

 duced most and fii'.st on the blind side; but sometimes 

 not oidy tiie beginning of the dorsal fin. l)ut aiso the 

 terminations of this tin and of the anal lin cross over 

 to the blind side. The dental c(|iiipment of the jaws 

 is also stronger, as a rule, on tlie jaw-lioncs of tliis side 

 than on those of the eye side, and the jaw-bones tlieni- 



between them; and the head, too, is usuallv covered 

 with scales right out on the snout. In this respect, 

 too, the blind side is generally inferior to the eye side; 

 and the scales are often wanting on the lilind side of 

 the head. Sometimes the scales are cycloid (smooth- 

 margined), sometimes ctenoid (rough-margined), and 

 sometimes a portion of them are changed into s])inous 

 tubercles. In those of the species with ctenoid scales 

 with whose changes of development we are acquainted, 

 the scales are cycloid over the whole of the body du- 

 ring youtii; while in older sjjecimens they are ctenoid 

 on the entire eye side oi- some part thereof, but on 

 the blind side may remain cycloid or lie changed to 

 ctenoid scales there as well. 



