234 



Part III. — Sixteenth Annual Report 



its earlier (in length) metamorphosis and the assumption of new 

 surroundings — it is an adaptation to life on the bottom. If, however, 

 the pelagic stage is prolonged, as it is for many, the pigmentation is no 

 guide. The presence of a large amount of pigment at an early stage, say 

 12 to 13mm., is a positive index that the specimen is a plaice, because 

 it will accompany a greater degree of transformation ; but its absence is 

 no indication that the specimen is a dab. 



The pigmentation of the young plaice when they come into inshore 

 waters varies with the ground they are on, being a dull grey in those on 

 muddy ground, a light yellow in those from sandy pools. The dull grey 

 colour is also seen in the flounder. This pigmentation is due to a deposit 

 in the epidermal cells, being general all over the body. At this time, 

 also, the first indications of scales may be detected. 



All stages, from the clear transparent forms, with the pelagic pigmenta- 

 tion only, to those with very dense dull grey pigment, were obtained 

 from St. Andrews harbour during May and June. The presence of these 

 two forms, apparently distinct so far as pigment is concerned, might lead 

 one to think that both the dab and plaice were present. The flounder, 

 of course, is quite distinct ; but the presence of transitional forms leads 

 to the conclusion that the clear and transparent forms are the more recent 

 arrivals from deeper waters. Forms resembling Mr. Holt's dabs (figs. 

 112 and 113, I.e.) were obtained, and this first led me to suspect that his 

 separation of these species was not the correct one. 



When these specimens are examined under the lens or microscope a 

 beautiful play of colour will be seen along the lines of the myomeres — red, 

 orange, or yellow. If these colours are dependent on the presence of one 

 substance in the tissues, then an increase of this substance along the 

 lines of the myomeres will appear black, and thus the condition is 

 arrived at which Mr. Holt considered characteristic of the long rough dab. 

 The presence of black along the myomeres has some connection with the 

 free-swimming pelagic stage. It does not appear to any extent in the 

 plaice and flounder; in the dab it is present in some more than in others, 

 depending, apparently, on surroundings. Specimens from the surface, 

 fifteen miles from land, show it very distinctly, while it is scarcely 

 present in the specimens taken in inshore waters; the long rough 

 dab shows it to a marked degree, and it is also present in the 

 lemon dab. One might almost think, therefore, that the early assump- 

 tion of general pigmentation by the plaice, and the presence of the 

 myomeral pigment in the prolonged pelagic stages of the dab, would 

 serve to distinguish these forms, but the possibilities of pigmentation in 

 these flat-fish, shown by Prince and Cunningham, make reasoning from 

 pigmentation alone somewhat precarious. 



The groups of pigment on the bodj^ of the dab in the pelagic stages 

 seem to break up afterwards into a number of scattered spots (fig. 13). 

 This is seen in those which are almost transparent, but as only spirit 

 specimens were examined, the grouping may be present under natural 

 conditions. At the same time, a slight yellowish brown colouration 

 appears, corresponding to the dull grey brown of the plaice, and this later 

 deepens into the reddish brown of the adult, though this varies with the 

 ground the dab is above. 



Summary of the Distinctions between the Plaice and the Dab. 



The endeavour has been to show the limits of the variations in the 

 young stages of these forms. It is quite possible that the variations 



