The flexible tail which terminates the body has the important office of mooring or attaching the fishes to any fixed object. As we see them in the aquarium, they are generally poised, on the tail, as it were; the latter being coiled around a bit of sea-weed, whilst the erect body and head look warily through the waters of their miniature sea. And when they detach themselves, they swim about in the erect position by means of the two pectoral or breast fins, which being placed close to the sides of the neck, project like veritable ears, and assist in rendering the equine appearance of the head of still more realistic nature. These fins move with a quick twittering motion, and propel their possessor swiftly through the water; whilst the back-fin, placed towards the hinder extremity of the body, also assists them in swimming. Some curious points in the internal structure of the Sea-horses warrant a brief notice. As every one knows, the red gills of an ordinary fish are shaped each like a comb, the teeth of the comb being represented by the delicate processes, each consisting in reality of a network of blood-vessels, in which the blood is exposed to the oxygen of the water, and is thus purified. In the Sea-horses, however, the gills do not present this comb-like appearance, but exist in the form of separated tufts or bunches of delicate filaments, which spring from the gill-supports or arches. From this peculiarity, the name 'tuft-gilled,' already alluded to, is derived, and the Pipe-fishes agree in the structure of the gills with the Sea-horses. Then, also, as most readers are aware, the gills of ordinary fishes are covered by a horny plate, appropriately named the gill-cover, and it is by sharply compressing the gills with this cover, that the water used in breathing is ejected from the gills, so as to make room for a fresh supply. In the Sea-horses, however, the gill-cover is not open or free at its under and hinder edges, but is firmly attached all round to the neighbouring tissues, and so rendered immovable. But at one point in its circumference, a small aperture is left, through which the breathing water escapes from the gills.