PLEURONECTID^E, OR FLAT-FISH. 49 



ductive powers are enormous ; the roe of a single female 

 having been estimated to contain nine millions of eggs. Nearly 

 allied to the Cod, are the Haddock, Whiting, Hake, Liny, Rock- 

 ling, Coal-Jish, and others. The first of these is considered as 

 the most delicate of the whole family, when fresh ; but it does 

 not take salt well ; and for preservation, the Cod and Ling excel 

 the rest. 



574. The second family of this order consists of the PLEURO- 

 NECTID.E, or Flat-fish. These present several remarkable pecu- 

 liarities of structure ; by which they are distinguished, not only 

 from all other Fishes, but even from all other Vertebrated animals. 

 Their body is extremely compressed, or flattened at the sides ; 

 the animal, however, does not habitually swim with these sides 

 erect in the water, but usually lies flat on the bottom, one side 

 being in contact with it, and the other being directed upwards. 

 The lower side is generally white, whilst the upper is brown ; 

 and the former is commonly (but erroneously) regarded as the 

 belly of the fish, and the latter as its back. The dark colour of 

 the upper surface harmonises with that of the bed on which the 

 fish lie ; so as to enable them to conceal themselves from their 

 foes, or to watch for their prey, without being themselves ob- 

 served. Individuals are occasionally met with, in which both 

 sides are coloured alike ; these are said to be " Doubles." It is 

 usually the coloured side which is doubled ; though it is occa- 

 sionally the white one. By a change in the position of the 

 head, both the eyes are brought round to the light side, so as to 

 look upwards, when the fish is lying on the bottom ; this change 

 may be designated as a sort of twisting-round ; but it also in- 

 volves an unequal development of the bones on the two sides of 

 the head, which shows itself in the mouth. Of the pectoral fins, 

 too, one is usually larger than the other. The dorsal fin is con- 

 tinued along the whole of the ridge of the back, from the head 

 to the tail ; and the anal fin usually forms a like continuous ex- 

 pansion below, sometimes uniting with the ventral fins. These 

 fishes have no air-bladder, and they seem to have little power of 

 rising from the bottom. When disturbed, they will raise them- 

 selves into a vertical position, so as to show their white sides ; and 



