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Show 422 MR. E. W. L. HOLT-STUDIES IN [May 1, abdominal cavity of a flat-fish, we find that the peritoneum of the ocular side is beset with dark pigment, while that of the blind side is not. It seems clear enough that whatever cause governs the presence or absence of external pigment in these fishes is equally potent with regard to the dark pigment of the internal region \ If this cause be the light, we have in the condition of round-fishes an apparent anomaly, not in the absence of pigment from the peritoneum in littoral fishes, since the difference of position involves a much greater interruption of the light from above, but in its conspicuous development in the deep-sea fishes, the peritoneum of which can be in no way subject to any action of the sun's light. In conclusion it may be remarked that M. abyssorum is a form such as might well be descended from a parent stock not greatly differing from the C o m m o n Ling of the present clay, and that the extensive vertical range of that species would appear to offer facilities for the establishment of such an abysmal offshoot2. II. ON THE EECESSUS ORBITALIS, AN ACCESSORY VISUAL ORGAN I N P L E U R O N E C T I D FISHES. In making an examination of the cephalic anatomy of the 1 Since this was written, I find that Messrs. Cunningham and MacMunn, in their monograph " O n the Coloration of the Skins of Fishes " (Phil. Trans. 1894, p. 809), consider that the " difference can only be explained as the effect of light falling on the upper side of the fish, and not on the lower." I omit further reference to their results, since they are discussed at some length in No. II. of this series of papers. 2 It is interesting to find that Professor Smitt (op. cit. p. 525) arrives at a precisely opposite conclusion, viz., that " from a systematic and genetic point of view, M. dipterygia must be regarded essentially as a predecessor of the C o m m o n Ling, though the former has evidently adopted in certain respects a distinct direction of development from the common original type which we are entitled to assume." " This," he remarks, appears from an appended table of averages of certain measurements in the two species. N o further explanation being forthcoming, I a m unable to say in what way the table maybe supposed to support his conclusions, nor a m I inclined to attach any great value to a series of averages based in no case on more than three specimens. The right of the author to form any opinion he chooses, as to the relative antiquity of the types of structure exhibited by the two species, is undoubted ; but it seems impossible to limit its application, and the logical conclusion is, that if Smitt holds this opinion in the present instance, he also regards all Gadoids which exhibit an abysmal type of structure as more primitive than their littoral brethren. This view is, of course, in direct opposition to that held, as I suppose, by all other ichthyologists, viz., that the abysmal forms are descended from littoral ancestors. Elsewhere (p. 520) Smitt notes that the trace of division in the second dorsal fin is more marked in the Birkelange than in the C o m m o n Ling, but it does not appear that this was taken into consideration in formulating the conclusion quoted above. To most ichthyologists it would appear as rather important evidence as to the greater antiquity of the type exhibited by the commoner species. .Again, when dealing with Gadus esmarkii, the author considers that the persistence of the barbel brings that species nearer to the common origin of the Cods than the Pollack, which has no barbel (p. 499), so that the same facts would appear, in the hands of Professor Smitt, to be capable of diametrically opposite interpretations. |
