The Sea Shore

by William S. Furneaux

Fishes that swim at the surface are generally tinted on the dorsal side with some shade that closely resembles the colour of the water as viewed from above, and are white and silvery below. Such colouring is of course highly protective, for they are not readily distinguished by the sea birds and other enemies that would pounce on them from above, and are almost invisible against the sky to eyes below. This form of protective resemblance is beautifully illustrated in the mackerel, which is barred on the back with black and green, closely imitating the ripples on the surface of the deep green sea, while the under side is of a silvery whiteness that is hardly visible from below with the bright sky as a background.

The flat fish afford other interesting examples, for these live on the bottom, and are coloured above so as to resemble the bed on which they live; the tints being those of mud, sand, or gravel.

But what are we to say of the gaudy colours of the gurnards, rock fishes, &c.? These are certainly not protective in all cases, for we sometimes find brightly coloured species conspicuous among duller surroundings. Such instances, however, are comparatively rare, the gaudy species living principally among the variously coloured rocks, weeds, and corals; and when they do occur it is probable that they serve principally as a means by which the brightly coloured sex—usually the male—attracts its mate. We say ‘usually the male,’ but why so? Because the female requires the protection of a more sombre colour in order that she may with safety deposit her spawn for the perpetuation of her species. Again, the male referred to needs the assistance of his gaudy coat only during the breeding season, hence we find that he assumes the bright colours as a wedding garment, to be cast off when the breeding season is over.

This leads us to the subject of changeability of colours in the same individual. That such changes do occur is well known, and it is still more remarkable that they are produced in rapid succession, apparently at the will of the fish concerned; for its tints will vary as it moves from place to place so as to always harmonise with the surroundings, and also in response to other conditions. The mechanism by which such variations are produced has also been studied and explained:—The colouring matter is held in little vesicles beneath the skin, and these vesicles are capable of being compressed by muscles quite under the control of the fish. When they are globular in form the contained pigment appears dark, but when they are flattened by muscular compression, the pigment is spread over a much larger area, and thus greatly reduced in depth of tint.

As with all vertebrates, the central axis of the internal skeleton of a fish consists of the backbone and the skull. The structure of the latter is so complicated, and its description so full of technicalities, that we deem it advisable to pass it over in a work like this where the scope is so large in proportion to the space available; and this we do with reluctance, because the detailed study of the skull is of real importance to those who would thoroughly understand the principles of classification.

The backbone consists of a variable number of cylindrical vertebræ, united end to end to form a continuous column, both the anterior and posterior faces of each being concave. On the dorsal surface of each vertebra there is a V-shaped arch, surmounted by a spine, the former serving to protect the spinal cord, and the latter giving attachment to the muscles of the back. Some of the vertebræ are also provided with processes for the attachment of the ribs, and those of the tail possess an arch and a spine on the ventral as well as on the dorsal side.

It has already been shown that the pectoral fins are jointed to a girdle. This girdle corresponds with the shoulder-blade of higher animals, and gives direct attachment to the rays of the fin, which may be regarded as the equivalent of the fingers, and thus there is no part of the limb corresponding with the arm. The pelvic fins also are frequently jointed to a pelvic girdle or hip, but this is a very rudimental structure, or is even entirely absent in some species.

The rays of the caudal fin articulate with the extremity of the backbone, but this portion of the fish’s anatomy undergoes such remarkable changes that we must devote a few words to it. It is probably well known to our readers that the tails of fishes exhibit three distinct forms. The first of these is a simple fringe formed by the union of unaltered dorsal and ventral fins; the second is the unsymmetrical or unequally lobed tail so characteristic of sharks, dogfishes, and rays; and the third is the broad symmetrical tail fin, often distinctly forked or bi-lobed, such as we meet with in the majority of our bony fishes. These three kinds are known respectively as the diphycercal, heterocercal, and the homocercal tails.