Page:British Reptiles, Amphibians, and Fresh-water Fishes.djvu/71

INTRODUCTION fertilising purposes. This accomplished, no further heed is paid to what happens, and fishes seem to resemble insects in this respect. Exception must be made, however, in the case of that plucky little "soldier," the Stickleback, who not only builds a nest, but jealously guards it and his wife against invasion. It is a notable exception of great interest, and will be referred to later when the life-story of this favourite species is being unfolded.

Of the external appearance of a fish it need only be said here that, whilst some have large scales—like the Roach and Rudd—others bear very small ones—like the bottom-loving Gudgeon—and others again—like the Eel and Tench—are practically scaleless as compared with their heavily-plated brethren. The tapering body, that is, having the head and hind quarters narrower and more torpedo-shaped than the deeper central parts, is well known, and its use in permitting the owner to propel itself through the water with the least resistance is equally familiar. Not that all our fishes are thus shaped, for the Eel and Lamprey are snake-like in appearance, and certain marine species are far different in form, but the typical example will serve our purpose. The body bears upon it fins equivalent to the limbs possessed by a quadruped, or the wings and legs of a bird. If we take that voracious and social fresh-water inhabitant, the Perch, as a basis for explanation, we find that along the back it has two fins, the one in front being called the first dorsal, and the one immediately behind is the second 55