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Show 1894.] TELEOSTEAN MORPHOLOGY. 439 ventral abdominal region is flattened, it is nevertheless rather darkly pigmented, and to m e it certainly seems more probable that the Pleuronectidae of the present day began to take on their asymmetrical characters as compressed and uniformly coloured forms than in the condition of ordinary round fish. A feature which appears to m e to be of the highest importance is the fact that ambicolorate fish appear to be always what one may term " ambiciliate " also. That is to say, whenever pigment is found on the blind side of a fish, the scales in that region are as rough as those of the other side (in such forms as exhibit asymmetry in this respect), or, at all events, are rougher than on the blind side of normal examples of the same species. The Turbot, for instance, which normally has no spines on the blind side, always possesses them if ambicolorate. They are usually confined to the pigmented region, but may occur also on the white part of the skin, but only, according to m y own experience, in specimens which exhibit some pigmentation or other of the blind side 1. I a m not aware that it has been contended that the action of light can have any effect on the development of the spines or scales, and Mr. Cunningham's experiments yield no information on the subject, since in his most perfectly coloured Flounders (op. cit. pi. 53) practically no pigment manifested itself in the regions corresponding to the site of the only tubercles which British examples of PI. flesus possess. If we admit the inefficacy of light in this respect, it becomes evident that w e are dealing with a phenomenon in which the pigmentation is not the only element of importance. The reversion, in fact, if such it is, extends to the derma generally, and not merely to its power of producing the various elements of coloration. I have shown2 that the Turbot in its early pelagic stages is possessed of a very powerful cephalic armature, and that the larvae of another sinistral form, perhaps the Brill, are equally well armed, though in a different manner3. It is possible to regard these cephalic spines either as protective structures, in essential relation to the pelagic period of existence, or, perhaps with greater probability, as of merely ancestral significance. In a future communication I hope to be able to show that the dermal spines or tubercles of our British Turbot are undergoing, or have undergone, a reduction in number, so that the condition of the 1 I am speaking now of British examples only, since Turbot from the Norwegian Fjords not uncommonly possess spines on the colourless blind side. Norwegian Turbot, however, are much more profusely spined on the ocular side than their British allies, and the spinulation of the blind side, when present, is always much inferior to that of the other. I have never seen an ambicolorate example from Norway; but only a few hundred fish have come under my notice, and it is possible that "double" specimens may have been withdrawn by the consignors, since such are not supposed to have a high market value, except for naturalists. Besides being more spinous, these Turbot appear to be also considerably smaller than our own. a Journ. M . B. Assoc. 1892, p. 402. 3 Trans. R. Dubl. Soc. v. 1893, pi. xii. 29* |
