360 THE FLOUNDER FAMILY. 



has even reached the dorsal ridge marking the separation 

 between the two sides. The pigment is still symmetrically 

 arranged on both sides, the black spots being principally 

 located upon the ventral margin of the trunk but in some they 

 extend up the sides and the hind-dorsal region. 



These little post-larval pleuronectids are distinguished as 

 such from the gadoid and other families by the great depth of 

 the body and of the marginal fin. They are found in great 

 numbers in the surface and mid-water and appear to move 

 further down towards the sea-bottom as age advances. Along 

 with the downward migration appears a tendency, coincident 

 with the change in the eyes, to move more and more upon one 

 side until, by the time the little fish takes up its habitat upon 

 the sandy bottom, it rests, as already indicated, upon its left 

 side. The pigmentation on this side gradually disappears, 

 leaving a pearly white surface, so familiar in the adult. This 

 disappearance of pigment seems to be the direct effect of the 

 reaction of the organism to the absence of light. 



At a length of 10 mm. (or about inch) the little pelagic 

 plaice (Plate XV, fig. 2) show signs of commencing metamor- 

 phosis 1 . The left eye appears on the dorsal ridge of the head 

 in its migration over to the right side. Holt 2 gives the fol- 

 lowing diagnostic characters for the young plaice at this size. 

 " The head is contained four times, and the greatest height of 

 the body (without fins) three times, in the total length, without 

 the caudal fin.... Very little black pigment is present on the 

 body. The dorsal interspinous patches are merely marked by a 

 few chromatophores. Rather more pigment occurs ventrally, 

 but it is somewhat diffuse, except along the pre-anal ventral 

 outline. A little is present internal to the otocysts, on the 

 sheath of the notochord, and along the renal region." There 

 seemed to be no coloured pigment. The dorsal fin showed 

 62 fin rays and the anal 48. We may add that the black 

 pigment tends to collect in three or four patches dorsally and 

 ventrally. 



1 Mobius and Heincke found that the transference of the eye in an aquarium 

 took about 4 weeks. 



a Transactions of the Royal Dublin Society, 1893, p. 77. 



