CHAP, xix.] FLAT-FISHES. 4G9 



head. STEEXSTRUP states that the variation has, he believes, been 

 observed in all flat-fishes 1 except the Halibut (Hippocjlossus). 



In several but not all cases of this abnormality the eye belong- 

 ing to the lower side was not placed in its normal position on the 

 upper surface, but stood in an intermediate position on the top of 

 the head, so that it could be partially seen in profile looked at from 

 the " blind " side. It seems possible that the pigmentation of the 

 " blind " side is in some way correlated with some abnormal delay 

 in the shifting of the eye and a consequent continuation of the 

 power of receiving visual sensations from this side. 



The abnormality of the dorsal fin is in accordance with this suppo- 

 sition. To understand the nature of this condition it must be remem- 

 bered that the form of the flat-fish is derived from the usual " round " 

 form by two principal changes. (1) By a twisting of the head the eye 

 is brought over from the blind side to the upper side. (2) The dorsal 

 fin is extended forwards above the eye thus shifted ; for as STEENSTRUP 

 and TuAQUAiR 2 have shewn, this anterior extension of the dorsal fin is 

 not in the morphological middle line. It is in fact an anterior repeti- 

 tion of the series of dorsal fin-rays along the new contour-line of the 

 body, and occurs irrespective of the fact that the tissues with which it 

 is there associated are not median at all. 



STEENSTRUP and TRAQUAIR shewed plainly that it is insufficient to 

 suppose that there is a twisting of the head, for this does not explain 

 the presence of the dorsal fin in the position in which it is found, 

 curving along that which was once the side of the head. Traquair sug- 

 gested that these relations could be attained by two processes; (1) a 

 twisting of the head so as to bring over the eye from the future 

 " blind " side, and (2) a forward growth of the dorsal fin along that 

 which is then the upper contour-line of the head. These processes 

 have now been actually seen by AGASSIZ* in several PleuronecticUe. 

 The first observation of a specimen at the stage when the eye is on the 

 top of the head and the dorsal tin is not yet extended, seems to be that of 

 MALM* and there can be little doubt that the normal development 

 proceeds in this way 5 . It has been pointed out by many writers that 

 if the upper eye were to remain in an intermediate position on the top 

 of the head, and the dorsal fin were then to grow forwards, arching 

 over it, the condition of these abnormal forms would be reached. That 

 this is what has actually occurred in them seems likely. 



A number of difficult questions are thus raised as to the histological 



1 The evidence as to the Sole seems to be doubtful (v. infra). 

 '-' TRAQUAIR, Trans. Linn. Soc., 18(55, xxv. p. '263. 



3 AGASSIZ, Proc. Amer. Ac. Sci. 1878, xiv. p. 1, Pis. 



4 MALM, (Efvers. k. Sven. Vet. Ac., 1854, p. 173, see Ann. <nui Maij. N. H. 1865 

 (1), p. 366. 



5 Allusion should be made to the fact that in the genus " Plagusia " the dorsal 

 fin acquires its forward extension at a time before the shifting of the eye occurs. 

 When the time for this change comes the eye of the future blind side passes under 

 the dorsal fin and above the skull, through the tissues from one side of the heml ti> 

 the other. This was first observed by S'TEEXSTRUP, and afterwards by AGASSIZ in 

 great detail and the fact can hardly now be questioned. This mode of development 

 is peculiar to " Plagusia," though when Steenstrup wrote he expected that the same 

 would be found to occur in other Pleuronectidfe. 



