The "lateral line" is a notable feature in the external appearance of most Fishes. Originally developed in the superficial epidermis of the skin in the form of linear tracts of isolated and often segmentally arranged masses of sense-cells, these organs subsequently become imbedded for protection in the epidermic lining of either an open groove or a closed canal extending along each side of the trunk and tail, and prolonged on to the more exposed parts of each side of the head in the shape of a more {164}complex system of branching grooves or of deeply-seated and externally inconspicuous canals. The course of the lateral line can, as a rule, readily be detected by the naked eye, and, even when not otherwise distinguishable, may be traced by the series of simple or multiple pores through which, at intervals, the canal communicates with the exterior (Fig. 93, A), and often also, in the trunk and tail, by a band of coloration different to that of the rest of the body. COLORATION. Contrary to popular opinion, it may be doubted if any animals, even Insects or Birds, can vie with living Fishes in the brilliancy and changeability of their colours. The nature of their habitat, the rapid fading of the natural tints after death, and the fact that museum specimens, however carefully preserved, afford but a ghostly resemblance to the colours of the living animal, account, no doubt, for much of the prevalent ignorance of the extraordinary extent to which colour-development may proceed in a considerable number of Fishes. Like the generality of northern forms of life, the Fishes of our own seas, rivers and lakes, are less conspicuous for vivid and striking coloration than those of tropical or subtropical climes, although such familiar Teleostean Fishes of our seas and fresh waters as the Mackerel, the Salmon and Trout, the males of the Stickleback and Dragonet, some of the Gurnards (Triglidae) and Wrasses (Labridae), the Opah or King-Fish (_Lampris luna_), and many others, are notable exceptions. Brilliancy of coloration is most conspicuous in the Teleostei: in nearly all other Fishes the colours are more uniform, usually sober and often sombre, with no more variety than is afforded by the presence of dark spots or bands on a lighter ground, or _vice versâ_, or by the lighter colour of the ventral as compared with the dorsal surface. In Teleosts all the resources of colour-formation, pigmentation, reflection, and iridescence through optical interference, in diverse combinations, are employed in the production of the various tints, while the dominant ground colour is often diversified by the presence of stripes, bands or bars, longitudinal or transverse, or of spots of different hues, frequently arranged in striking and intricate patterns. The possibilities of coloration in these Fishes may be briefly illustrated by a few examples:—