OCR Text |
Show 1894.] TELEOSTEAN MORPHOLOGY. 437 pelagic stage, yet such, in fact, is the case. In almost all flatfish there is a tendency, even before the yolk is absorbed, towards an arrangement of the pigment into several series of patches, transverse to the long axis of the body. Each series consists mainly of a group of chromatophores on the dorso-lateral and ventro-lateral regions of the trunk, and is completed by corresponding groups on the dorsal and ventral parts of the marginal fin \ The pigment is, of course, equally developed on either side, and, as the body of the larva increases in depth and the basal ridges of the median fins appear, the chief colour-patches extend on to these, and ultimately come to be confined almost entirely to these areas. Between the primary patches of each ridge, secondary markings commonly make their appearance and shortly become little inferior to the original. The process is illustrated, incidentally, in the drawings of almost every author who has studied the development of flat-fish. The Brill, so far as I a m acquainted with its ontogeny, exhibits these spots as conspicuously as any metamorphosing larva, and more so than the Turbot, in which the early development of a diffuse body-pigment tends to mask them somewhat. Nevertheless they are easily visible in the later pelagic stages of that form, and can be made out, even on the blind side, in a specimen the eye of which has arrived at the ridge of the head, and in which, of course, the pigment of the blind side is considerably less abundant than that of the coloured side. On the ocular side of a Brill at a similar stage of cephalic metamorphosis they are shown clearly enough by Cunningham in his ' Treatise on the C o m m o n Sole' (pi. xv. fig. 5). In later life in both the species mentioned, as also in most other flat-fishes, the dark spots cease to be conspicuous, the lighter intervening areas being the only markings which attract attention in half-grown and adult fishes (on this part of the body). In the Topknots, however, the markings remain visible throughout life in Rh. punctatus and Phr. unimaculatus, while in full-grown Rh. norvegicus they are as conspicuous as in the younger stages of any Pleuronectid. The Brill of which w e have been speaking less commonly exhibit another marking on the blind side, in addition to the rows of spots on the interspinous ridges. This is situated on the lateral line rather behind the middle of the body. A mark is very usually present in this position in metamorphosing flat-fish of other species, and in the Brill it persists as a rather large roundish black spot on the ocular side in half-grown and perhaps also in full-grown fishes, though I have not myself observed it in the latter. It corresponds in position to the ocellus of Ph. unimaculatus, to the posterior spot of Rh. punctatus, and to a similar marking in Rh. norvegicus. 1 Such an arrangement of the larval pigment is by no means confined to the Pleuronectidse, and any deductions that may be drawn as to the ancestral significance of such pigmentation may probably be capable of a wider interpretation than that which, for the purposes of the present paper, I have thought necessary to suggest 1'or them. PROC. ZOOL. Soc-1894, No. XXIX. 29 |