If an eel be taken out of the water, these gill-pouches will be seen to swell out almost immediately, and remain filled with water as long as the fish is kept on land; but when it is returned to its natural element, it will at once discharge the water that kept its gills moist, and which has become foul with the products of respiration, and, with a few vigorous gulps, renew the supply. Eels spend their breeding season, which extends from July to September, in salt or brackish waters; and early in the following summer, the young, which are now called _elvers_, and measure from three to five inches in length, ascend the rivers, travelling enormous distances and overcoming obstacles that we might well expect to be insurmountable. Thus they perform two migrations annually, though it is thought by some observers that the adult never returns to the sea, but dies soon after it has deposited its spawn. The family of Flat-fishes (_Pleuronectidæ_) present many interesting points of structure and habit in which they stand alone, the variations in structure as compared with other fishes being due, of course, to the habits which they have acquired. One of the first features that strike the observer on looking at a flat-fish is the unsymmetrical form of the body. It is very much compressed, and the fish having acquired the habit of lying on the bed of the sea, sometimes on the left and sometimes on the right side, the lower surface has become flattened more, and is of an almost pure white colour, while the upper convex side is more or less coloured with pigment produced by exposure to light. The dorsal and ventral fins are both very long; and, as is usual with bottom fishes, the swimming or air bladder is absent. Young flat-fish are at first perfectly symmetrical in form, with one eye on each side of the head, and they swim freely in the water with their bodies in a vertical plane; but they very soon acquire the habit of swimming on one side, and the eye of that side slowly passes round to the other side of the skull, rotating in its orbit as it moves, till at last both are on the uppermost surface. This, of course, is accompanied by a considerable distortion of the bones of the skull, which is very evident in the skeleton of the adult. The young fish then takes to the bottom, with the result that its under-surface is flattened, while the upper becomes strongly pigmented. These fish spend almost the whole of their time on the bottom, only occasionally rising for short intervals, when they swim by undulatory movements of their bodies and fins; their food consists of crustaceans, worms, and other small marine animals.