OCR Text |
Show 1884.] HYBRIDS AMONG THE SALMONIDCE. 23 On examining an American Charr, Salmo fontinalis, in the parr-stage, and 3£ inches in length, the scales covering the body on their outer surface were found to be covered and bound down to one another by a covering of epidermis (which in its turn was much concealed by a thick layer of mucus); the same appearance was present in a Salmo levenensis 4 inches in length, while in a Salmon-parr, 6 inches long, a change had commenced in some regions of the body. The hind portion of the exposed part of each scale, sometimes to as much as one third or even one half, being silvery, or rather transparent, permitting the silvery lustre to shine through, but not so the basal portion, which retained more of the parr-colour. On placing a needle under these scales, the transparent or silvery part could be readily elevated ; it was no longer bound down by epidermis, as in younger fish, while but very little mucus was present. Among the silvery smolts this apparent denudation of the scales had been carried to a greater extent, the epidermis being seemingly merely present across its base (except where black or orange spots existed), and as a consequence a silvery scale with more or less dark edges was seen. Should the scale of a smolt be raised, it appears as if on the sides and most of the body of the fish it were merely attached at its base and placed in a pocket of the epidermis, and nowhere in the body is any new layer of scales developed, or an increased thickness put on, hut, on the contrary, the epidermal covering seems to become removed in the smolt-stage, thus occasioning their transparent and consequent silvery appearance, which is continued through life. Owing to the epidermis being less removed from the scales of the back, and being present over the scaleless head, these portions retain a darker colour. A hybrid between S. salar and S. levenensis, at 7*4 inches in length, shows a portion of the outer edge of each scale already denuded of epidermis; and although I do not propose in this paper to enter upon the question of colour, I may remark that the lateral bands or finger-marks come into more prominent view when the scales and attached epidermis are removed, rendering it probable that they are found in the cutis. The black spots, or at least some of those which are seen on the body, are in the epidermis, and can be removed with that structure ; while on removing a scale, the epidermal pocket in which its base lies is found of a dark and often black colour. If the epidermis of a young fish is placed under the microscope, it is more or less filled with fine black dots ; consequently the absorption of this structure, irrespective of its leaving the silvery portion uncovered, removes what must add a darker tinge to the fish. In like manner a portion of the red or orange dots may be external to the scales, as in the epidermis or in the structures below and Ganoids. The scales increase in size by apposition, their basal portion forming the matrix being part of the cutis, in a similar way to finger-nails growing out of the nail-fold. There are two main classes of these scales, one retaiuing an embryonic condition, as in some parts of a Mackerel, & c , while those of the Salmon, which attain a larger size, break through the epidermal layer, and thus become partially free." |