MARINE VEETEBEATES 325 



almost pure white colour, while the upper convex side is more or 

 less coloured with pigment produced by exposure to light. The 

 dorsal and ventral fins are both very long ; and, as is usual with 

 bottom fishes, the swimming or air bladder is absent. 



Young flat-fish are at first perfectly symmetrical in form, with 

 one eye on each side of the head, and they swim freely in the water 

 with their bodies in a vertical plane ; but they very soon acquire 

 the habit of swimming on one side, and the eye of that side slowly 

 passes round to the other side of the skull, rotating in its orbit as it 

 moves, till at last both are on the uppermost surface. This, of 

 course, is accompanied by a considerable distortion of the bones of 

 the skull, which is very evident in the skeleton of the adult. The 

 young fish then takes to the bottom, with the result that its under- 

 surface is flattened, while the upper becomes strongly pigmented. 



These fish spend almost the whole of their time on the bottom, 

 only occasionally rising for short intervals, when they swim by undu- 

 latory movements of their bodies and fins ; their food consists of 

 crustaceans, worms, and other small marine animals. 



They furnish very interesting illustrations of protective colouring, 

 the upper surface always closely resembling the ground on which they 

 rest and feed ; and thus they are not only protected from their own 

 enemies, but are enabled to lie unseen by the animals that form 

 their prey. Those which live on sandy shores are finely spotted 

 with colours that closely imitate the sand, while those that lie on 

 mud are of dark and dingy hues. Others, again, are irregularly 

 marked with spots of various sizes and colours that resemble a 

 gravelly bottom ; and most species are still further protected by 

 their habit of throwing sand or mud on the top of their bodies by 

 means of their dorsal and ventral fins. 



SmaJl flat-fishes, especially young Plaice and Flounders, live so 

 close to the shore that they are often left behind in rock pools and 

 sandy hollows by the receding tide,, and it is very interesting to 

 observe the habits of these in their natural conditions. It will 

 generally be noticed that it is most difficult to detect them while 

 they are at rest ; and when disturbed, they usually swim but a 

 short distance, settling down very abruptly, and immediately 

 throwing a little sand over their bodies by a few vibrations of their 

 fins. 



Another peculiarity of some of the flat-fishes is their indifference 

 to the nature of the water in which they live. Flounders may not 

 only be caught in the estuaries of our rivers, but they even ascend 



