48 REPORT OP COMMISSIONERS OF INLAND FISHERIES. 



Avhich changes in form and structure of animals may take in order 

 that they may become adapted to the conditions of their environment. 

 The flat-fishes are exchisively bottom fishes; they are somewhat 

 sluggish in habit, feed mostly on bottom invertebrates, and seldom 

 leave the bottom except for relatively short excursions in the pursuit 

 of food or when disturbed. To a fish of this sluggish mode of life, 

 such an inconspicuousness in appearance as will cause it to be over- 

 looked by its enemies and prevent it from frightening its prey will 

 largely increase its chances of survival. In the case of such bottom 

 fishes as the flat-fishes, and the skates and rays, this advantage is 

 secured partly by a protective coloration, perhaps, and partly by 

 an extreme flattening out of the body of the fish. The skates and 

 rays are flattened in a vertical direction, but the flat-fishes are 

 strongly compressed from side to side; therefore we find in the 

 flat-fishes the unique condition that when in the natural position, 

 whether swimming horizontally through the water or resting on the 

 bottom, their upper and lower surfaces correspond in reality to 

 the sides of other fishes. By this sidewise flattening, the d orsal fin, 

 which runs along the middle line of the back, and the anal fin, which 

 runs along the mid-ventral line, are prominently brought out to the 

 "edges" of the fish and by this relative change in position become 

 capable of being used more directly in forward propulsion than is 

 the case with the unpaired fins of most fishes. Thus it would appear 

 that the problem of producing a powerful means of propulsion in a 

 fish which has to meet the requirements of the structural change 

 demanded by an existence confined almost exclusively to the bottom, 

 has been solved by nature in two different ways, viz.: in the case 

 of the skates and rays, by bringing about a great extension of the 

 pectoral fins as a result of a very considerable vertical compression 

 (this is accompanied by an almost complete disappearance of the un- 

 paired fins) ; and in the flat-fishes, by a mechanical adjustment result- 

 ing from an extreme lateral flattening, which gives a great mechanical 

 advantage to the unpaired fins. The whole of the flat posterior 

 portion of the body, as well as the broad, strong tail-fin, is used also 



