FISHES 135 



and other viscera. But if the structure of flounder , sand-dab , 

 plaice, sole or any of the true flat-fishes be examined when the 

 animals are being eaten in the ordinary course of events, it will 

 be found that the condition of affairs is here quite different. 

 Here the under surface is the left side of the animal, the upper 

 surface the right side, not the back. In other words, if an 

 ordinary haddock, for example, be laid on the table on its left 

 side, that is, with the right side uppermost, and we then supposed 

 ourselves to flatten it out by slow, long-continued pressure, we 

 should get the conditions of affairs represented in the flounder, 

 which lies permanently on one side. Look again at the flounder, 

 and notice that of the two pectoral fins which a fish possesses, 

 and which in the haddock, for example, lie one on either side 

 of the body, one in the flounder is on the upper, coloured surface 

 and the other on the lower, uncoloured surface. An obvious 

 difficulty is that if this is so, then the flounder should also have 

 one eye on the coloured surface and one on the uncoloured surface, 

 which represent the two sides of the body. This is the actual 

 position of the eyes at an early stage of development, but as 

 development proceeds the eye which is originally on the left 

 side begins to migrate, actually moves round the head, until 

 in the little flounder of the pools, as in the adult, both eyes come 

 to lie close together on the upper side of the body. This migration 

 is one of the most curious facts in connection with the develop- 

 ment of fishes, and can naturally only occur when the bones 

 of the skull are soft, so that it is completed at a very early stage. 

 The result is that the bones are curiously distorted. This dis- 

 tortion is quite noticeable even to persons with no training in 

 anatomy, and the head of a big flounder, plaice or sole should 

 be claimed from the cook, boiled to get rid of the flesh, and com- 

 pared with the undistorted head of cod or haddock prepared 

 in the same fashion. The very young flounders and dabs, in which 

 the change in shape of the body and position of the eye occurs 

 do not live on the sea bottom but swim in the open water, where 

 they are not obtainable without a tow-net. Further, they cannot 

 be kept alive under the ordinary conditions of a school aquarium, 

 so that it is not possible to demonstrate the actual process of 

 metamorphosis . 



