REPORT OF COMMISSIONERS OF INLAND FISHERIES. 4 9 



in forward propulsion. The greater efficiency of the swimming 

 apparatus which results from the lateral compression may be a 

 partial explanation of the greater speed in swimming and the greater 

 precision of movement which the flat-fishes shovV as compared with 

 the slow, clumsy, lumbering movements of the skates and rays. 



This sidewise flattening of the fish and the fact that the fish in 

 its natural horizonital position must have one of its sides constantly 

 turned away from the liglit involve considerable modifications in its 

 structure. The unpaired fins increase in extent and area to the 

 degree corresponding to their increased importance in propulsion; 

 the dorsal fin extends forward onto the head nearly down to the 

 snout; the body cavity becomes so shortened that the anus is shifted 

 much further forward, and thus the anal fin is brought to extend 

 along all except a comparatively small fraction of the ventral edge 

 of the fish. The area of these fins is also increased b}' a lengthening 

 of the fin-rays. The paired fins, in most species, since from their 

 position they are of comparatively little use to the fish, have become 

 reduced or wanting. The tail-fin in many species has become in- 

 creased in area and strengthened by a hypural bone. Changes in 

 pigmentation also occur; the pigmented area in most species is 

 entirely confined to the upper or light-exposed side, while the whole 

 under side becomes colorless like the ventral side of other fishes. 

 The color of the upper side is capable of considerable variation from 

 light to dark according to the prevailing shade of the background. 



The mouth parts in some of the species have undergone changes. 

 In the fishes of the Turbot and Halibut tribes the jaws remain 

 unaffected by the change in position of the fish; in these, the mouth 

 is large, symmetrical, toothed around the whole border, and does 

 not partake of the twist of the remainder of the head. In the 

 Flounder tribe {Pleuronectince) , that part of the mouth which belongs 

 to the eyed side has become somewhat contracted and twisted, but 

 on the under side the jaws remain straight and normally toothed. 

 In the case of the soles, the mouth has become very asymmetrical 



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