1893.] On the Coloration of the Skins of Fishes. 



387 



were undertaken in order to find out what exactly took place when 

 coloration was developed in the lower side of Flounders in certain 

 experiments carried on at the Plymouth Laboratory since the spring 

 of 1890. The first of these experiments was described in the 

 ' Zoologischer Anzeiger,' No. 354, 1891. The method of the experi- 

 ments is to take young Flounders in process of metamorphosis, or at a 

 later stage, and keep them in a vessel with a glass bottom without gravel 

 or sand, and then to direct light from a window on to the lower sides 

 by means of an inclined mirror placed beneath the vessel and opposite 

 the window. The first experiment was not quite conclusive, although 

 some pigment was found on the lower sides of the fish after an ex- 

 posure to light of four months. The second experiment was quite 

 conclusive. Four Flounders were taken on September 17, 1890, from 

 a number reared in the aquarium since the preceding May : they 

 were five to six months old, and 5 to 8 cm. in length. They had been 

 living under ordinary conditions, and were in all respects normal, 

 having no colour on the lower sides. They were placed in the vessel 

 above the mirror. On one of these, two faint specks of pigment were 

 observed on April 26, 1891, one died on the following July 1, which 

 showed no pigment, and one on September 26, 1891. The latter was 

 16- 7 cm. long and showed only a little pigment on the posterior part 

 of the operculum. At this time one of the two survivors had de- 

 veloped pigment all over the external regions of the lower side, and 

 the other had a few small spots. . The first of these two is still alive 

 (March, 1893), being now three years old, and it is now pigmented 

 over the whole of the lower side except small areas on the head and 

 the base of the tail. A drawing showing its condition in November, 

 1891, was exhibited at the soiree of the Royal Society in 1892, and is 

 laid before the Society with this paper. The other specimen died on 

 July 4, 1892. It was then 25 cm. long and had a good deal of pigment 

 in scattered spots on the lower side. This specimen had been exposed 

 about one year and ten months. Several other experiments gave 

 similar results. The Flounders which were reared in the aquarium 

 under ordinary conditions showed no such tendency to develop pig- 

 ment ; a few, as in nature, were found to have small patches of pig- 

 ment on the lower side, but the percentage was extremely small, and 

 the pigment in such cases was constant, not progressive, as in the 

 fish exposed to light. 



The occurrence of abnormal coloration in Pleuronectids is fully 

 considered in the memoir ; a large number of specimens are described, 

 and it is shown that there is no evidence whatever that these speci- 

 mens have been exposed to abnormal conditions. We conclude that 

 these abnormalibies are congenital and not acquired. 



We find that where pigment is developed on the lower side, whether 

 in the experiments or as a congenital abnormality, the argenteum is 



