2 ANACANTHINI. 



their entire extent by the dorsal, caudal, and anal fins ; while not only the 

 muscles, but the skin, the gills, gill-covers, and even the pectoral fin-rays are less 

 developed on the blind (or normally under surface) than on the coloured side, the 

 niouth also being, as it were, bent round to this eyeless side, towards which the 

 anterior part of the face seems to be twisted. From a very early age it bad been 

 known that these fishes when first emerging from the ova, and while in a pellucid 

 condition, have an eye on either side of the head ; that by degrees the eye, on 

 what eventually will be the eyeless side, becomes depressed, while at the same 

 time a dark spot appears on the opposite side of the head, so that the fish almost 

 seems to possess three eyes. By degrees this dark spot becomes a distinct eye, 

 while that on the other side gi'adually disappeai^s ; in short, the eye apparently 

 migrates from what is henceforth known as the blind side of the fish. Van 

 Beneden (1853) and many others, considered that this abnormal position of the 

 eyes in adult flat-fishes was due to a greater or less torsion of the entire head on 

 the axis of the body, or else to a twisting of the face alone. Ten years subsequently 

 (1863) Professor Steenstrup, having obtained examples of young flounders, 

 demurred to this explanation. He observed that if such were effected by simple 

 torsion, the nerves and muscles belonging to the upper eye must necessarily pass 

 over the frontal bone of the blind side, and permanently continue in that position, 

 which, however, is not found to be the case, they being at the bottom of the orbit ; 

 and_he considered that the eye must have first passed under the frontal bones, 

 and subsequently upwards and through them. The eye, when leaving its original 

 site, attempts to carry the frontal bone of its own side with it, but the greater 

 portion of that bone resists, remaining in its place. Professor Steenstrup 

 consequently came to the conclusion that the eye on the blind side undertakes a 

 movement deeper and deeper, passing under the half roof formed by the frontal 

 bone of its own side, and is thus brought up through its vault ; so that in order to 

 find room for itself, it partially separates one frontal bone from its fellow, and partly 

 makes its way through the substance of the frontal bone itself ; in short, that the 

 eye, in attaining its final position, first passes obliquely inwards, and then ascends 

 upwards through the head, emerging on the opposite side. Professor Wyville 

 Thompson* considered that the eye of the blind side passes to the coloured 

 or eyed-side of the body, not through the vault of the head, but under the 

 integument, displacing in its progress the frontal bone of its own side ; that the 

 space through which the nervous and vascular connections pass is indicated in the 

 mature skull by the unsymmetrical posterior half of the articulating process of 

 the prefrontal of the blind side, the eye having maintained its normal relation to 

 its associated bone, the frontal, of the coloured side throughout. The eye changes 

 but little in actual position with the growth of the fish, the associated parts 

 being, as it were, developed past it and thus producing this singular obliquity. 

 Traquair (Trans. Linn. Soc. 1866, xxv, p. 263) believed that here we had a real case 

 of asymmetry in the supposed absence of the additional processes of the os frontis 

 anterius and proprium. But Hr. Malmf considered this conclusion to be 

 erroneous, and he observed that the young flat-fish is obliged, owing to the depth of 

 its body, increased by the development of its vertical fins, to remain on one side 

 while resting on the ground, the horizontal fins not being sufficiently developed to 

 sustain it in a vertical position. The eye of the "blind " side has a tendency to 

 turn towards the light, and in doing so carries with it the cartilaginous framework 

 of the skull, which eventually is only apparently asymmetrical. He gives 

 excellent figures of the first stages of these fishes. J Professor Alexander Agassiz 

 remarked§ that the first change and the process is identical, whether we take a 

 right or a left flounder. First there is a slight advance towards the snout of the 

 eye about to be tranferred : so that the transverse axis, passing through the pupil 

 of the eyes, no longer makes a right angle with the longitudinal axis. This 

 movement of translation is soon followed by a slight movement of rotation, so 



* Ann. and Mag. Nat. Hist. 1865, p. 3C1. 



f Svensk. Vet. Akad. Handl. vii, no. 4, 1868. 



j See also Gervais, Arch. Zool. Exp. 1877, p. 193, t. vi, and Lacaze-Duthiers, I.e. p. 305. 



§ A. Agassiz, Proj. Amer. Acad. Arts and Sciences, xiv, 1879, pi. vii-x. 



"*1 



