54 



PIGMENTS OX THE LOWER SIDES OF FLAT FISHES. 



In the sole there was very little pigmentation on the lower side, only 

 a little at the base of the tail, and on the lower surface of the tail, but 

 in the plaice and flounders the pigmentation extended continuously 

 over the marginal region of the body and the proximal part of the 

 dorsal and ventral fins. The extent of the pigment in the smaller 

 ilounder is shown in Fig. 1. The pigmentation of the outer half of the 

 marginal fins was slight. In the larger flounder the pigmented area 

 on the surface of the body was broader ; the central area was unpig- 

 mented, with the exception of a narrow patch about the middle of the 

 lateral line, as in the smaller specimen. In the two plaice the pig- 

 mentation was not quite so intense and not quite so extensive. In both 

 it was absent, or very nearly so, from the surface of the marginal fins, 

 and from the anterior part of the body, with the exception of a small 



Fig. 1.— Under side of Flounder, showing pigmentation. 



patch on the bony ridge of the head in the smaller specimen. In both 

 it was present in the regions of the interspinous bones in the posterior 

 three-fourths of the body, both dorsally and ventrally, and also over 

 the whole lower surface of the caudal fin, absent from the central 

 region of the body entirely ; there was no patch on the lateral line, as 

 in the flounders. 



It will be clear, therefore, that there was a most remarkable similarity 

 in the distribution of the pigment in these four specimens, which 

 suggests a common cause acting in all of them, and not indefinite 

 " variation." This common cause was access of light to the pigmented 

 areas. There was no sand or gravel at the bottom of the tank, and the 

 fish were resting on the bare slate. The lower surfaces of the fish were 

 not perfectly flat, and therefore certain areas were, when the fish were 

 at rest, in contact with the slate, while other areas were separated from 



