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Show 1894.] TELEOSTEAN MORPHOLOGY. 435 in specimens exhibiting the Cyclopean malformation: behind a line drawn through the preopercular keel the blind side may be almost entirely coloured in Turbot which are otherwise perfectly normal \ N o w the Cyclopean condition may be described as the failure of the anterior part of the dorsal fiu, and of the interneural bones and muscles belonging to it, to become united to the top of the head, accompanied with a partial, if sometimes very slight, arrest of the normal transit of the upper eye. N o one has had an opportunity of watching the development of a Flat-fish so deformed, but the variability in the different phases of metamorphosis which one notices in looking over a collection of partly symmetrical larvae suggests a more or less plausible explanation of the mere mechanical process. Each part appears, as it were, to change independently, so that, assuming a slight want of harmony in the movement of the upper eye and the forward extension of the dorsal (whether by retardation of the first or acceleration of the last), it is conceivable that a condition might arise in which the eye would obstruct the progress of the dorsal, and so compel its anterior extremity to project instead of proceeding as usual along the top of the head. Since the dorsal wall of the upper orbit is formed in normal specimens (of the Turbot) by the base of the dorsal, the failure of the latter to advance in the normal manner would leave the eye devoid of its usual support, so that it would rest, as it actually does in Cyclopean specimens, against the ridge of the skull, and so be more dorsally and less laterally directed than in normal examples. The authors referred to arrive at no conclusion as to why Cyclopean examples should be ambicolorate, but (p. 808) reject Giard's theory that young fish so affected remain pelagic for a longer time than usual, and so expose the blind side longer to the light, whereby it retains its pigmentation instead of becoming colourless. In this I a m inclined to agree with them, but since we know from Mr. Cunningham's o w n observations that metamorphosing Flat-fish, if pelagic, swim in such a position as to keep both eyes in one horizontal plane 2, it would seem to follow that in young Cyclopean examples, the period of pelagic life being the same, the change in position is never carried to quite the same extent as in the case of normal larvae. Hence the side normally colourless in the adult would be a little more exposed to the light, without any elongation of the pelagic period. It can hardly be contended, however, that a difference so comparatively trifling is sufficient to account for so marked a discrepancy in the coloration of all Cyclopean and most normal Turbot. Another abnormality, exemplified by a Turbot and a Brill in m y possession, is referred to by Messrs. Cunningham and M c M u nn (p. 807), from a description of the Brill, at that time living in the Cleethorpes aquarium, communicated by myself. In these specimens the eyes are normal, but a short portion of the anterior end 1 See postscript, p. 446. 2 Nat. Science, vol. i, March 1892. |
