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FISHES OF GREAT BRITAIN AND IRELAND. 



B. Anacanthini Pleuronectoidei. 



The structure of the head is apparently unsymmetrical on the two sides. 



Family, IV— PLEURONECTID^, Bisso. 



Heterosomata, Bonaparte. Diprosopa, Latreille. 



Branchiostegals six to eight : pseudobranchise well developed. Gills four. 

 Body strongly compressed, flattened, with one of its sides coloured, the other 

 being colourless or having merely some spots. Both eyes (except in the very 

 young) situated on the upper or coloured surface, sometimes they are rudimentary. 

 The two sides of the head not equally developed, one remaining almost 

 rudimentary. The jaws and dentition may be nearly equally developed on both 

 sides, or more so on the blind than in the coloured. A single, long, dorsal and 

 anal fin. Pectorals when present may be rudimentary. Scales present or absent. 

 Lateral-line on the coloured side single, double, or triple : curved or straight. 

 Air-bladder absent. 



The members composing this family are commonly known as flat-fishes, but it 

 mnst be observed there are two groups of "flat-fishes." The one being as it were 

 flattened from above, as we see in rays and skates, wherein the upper surface is 

 the back. It is not so however among the Pleuronectidos or "side swimmers," 

 which are flattened from side to side, except the head, which appears distorted and 

 likewise flattened. When referring to the pleuronectoids the terms right (dextral) 

 or left (sinistral) are employed with reference to the position of the upper or 

 coloured side : to ascertain this the fish is placed with its tail towards the 

 observer, the dorsal fin above and the anal beneath. Reversed examples are such 

 as have the eyes situated on the side of the body opposite to the one in which it 

 is generally seen. Double examples are those in which both sides of the body are 

 coloured. 



Adams remarked (Voyage of the Samarang) that such as frequent coral reefs 

 often have their tails ornamented with rather vivid colours, and their upper sides 

 marked with somewhat striking patterns ; whereas those that are half buried are 

 as dull and dingy as the surface is where they are found. They are evidently 

 aware how efficacious their colours prove for the purpose of concealment, and 

 when pursued sink down to the bottom, where they lie quite still on the ground, 

 the colour of which assimilates to that of their own upper surface. 



The Pleuronectoids, or flat-fishes, are among the most remarkable of vertebrate 

 animals, as for about a week or more after birth they swim on edge in a way 

 similar to other fishes, having their dorsal fin above, their anal fin below them, 

 and possessing an eye on either side of the head. But as they grow older this 

 erect position becomes lost, their sides become their upper and lower surfaces, 

 while both eyes are on the superior or coloured side of the body. The adult, 

 when at rest or swimming, usually keeps near the bjttom of the water, and 

 progresses by means of a sort of undulating motion of the whole body, and of the 

 unpaired fins. These fish are of a broad, flat shape, and margined in almof-t 

 it. 1 



