FLAT-FISHES. 81 



of an air-bladder, and of the structure of their paired fins, [Case 17.] 

 they are unable to maintain their body in a vertical position, 

 resting and moving on one side only. The side turned towards 



Fig. 67. 



Macrurus purallclus : from a depth of 350 fathoms. 



the bottom is sometimes the left, sometimes the right, colourless, 

 and termed the " blind " side ; that turned upwards and towards 

 the light is variously, and in some tropical species even vividly, 

 coloured. Both eyes are on the coloured side, on which side also 

 the muscles are more strongly developed. The dorsal and anal 

 fins are exceedingly long, without division. All the Flat-fishes 

 undergo remarkable changes with age; when quite young, they 

 are perfectly symmetrical, with an eye on each side of the head, 

 and swim in a vertical position like other fishes. To illustrate the 

 gradual change from the symmetrical form of the newly-hatched 

 Flat-fish to the more advanced asymmetrical shape, a series of 

 young Brill (Rhombus lavis) is exhibited in Case 17. Flat-fishes 

 when adult live always on the bottom, and swim with an undulating 

 motion of their body. They occur in all seas, except in the 

 highest latitudes and on rocky precipitous coasts, becoming most 

 numerous towards the Equator; those of the largest size occur in 

 the Temperate zone. Some enter fresh water freely, and others 

 have become entirely acclimatized in ponds and rivers. All are 

 carnivorous. Those most generally known are : The Holibut 

 (Hippoglossus vulgaris), the largest of all Flat-fishes, attaining to a 

 length of 5 or 6 feet, and a weight of several hundredweight ; 

 the Turbot (Rhombus maximus), one of the most valued food- 

 fishes ; the Brill (R. laevis) ; the Plaice (Pleuronectes platessa) ; 

 the Dab (P. limanda) ; the Flounder (P. flesus) ; the Smear-Dab 

 or Lemon-Dab (P. microcephalies) ; the Craig-fluke (P. cyno- 

 glossus)^ and the Soles (Solea). 



