This strikes us at once in their external form, which, though subject to great variety, being sometimes spherical as in the globe-fish, or cubical as in the ostracion, or expanded as in the skate, or snake-like as in the eel, is generally that of an elongated oval, slightly compressed laterally, a shape which enables the fishes to traverse their native fluid with the greatest celerity and ease. We wisely endeavour to imitate this peculiar form in the construction of our ships, yet the rapidity with which the fastest clipper cleaves the waters is nothing to the velocity of an animal formed to reside in that element. The flight of an arrow is not more rapid than the darting of a tunny, a salmon, or a gilt-head through the water. It has been calculated that a salmon will glide over 86,400 feet in an hour, that it will advance more than a degree of the meridian of the earth in a day, and that it could easily make the tour of the world in some weeks, were it desirous of emulating the fame of a Cook or of a Magellan. Every part of the body seems exerted in this despatch; the fins, the tail, and the motion of the whole backbone assist progression; and it is to this admirable flexibility of body, which mocks the efforts of art, that fishes owe the astonishing rapidity of their movements. Whales and dolphins move onwards by striking the water in a vertical direction, while fishes glide along by laterally curving and extending the spine. In some species, such as the eel, the whole body is flexible; but most of them paddle away with their tail to the right and left, and are thus driven forwards by the resistance of the water. Consequently the power of fishes is chiefly concentrated in the muscles bending the spine sideways, and generally we find these parts so much developed as to form the greatest part of the body. [Illustration: Skeleton of the Perch. A A, Dorsal Fins; B, Caudal; C, Anal; D, Ventral; E, Pectoral.] The fins are the most important auxiliary organs of locomotion in fishes. The dorsal, caudal, and anal fins serve by their vertical position to increase the extent of the rowing surface, and to maintain the animal's balance, while the pectoral and ventral fins, which must be considered as the representatives of the fore and hind limbs of other vertebrata, are, moreover, of great assistance in directing its movements. With the help of these organs, fishes can advance or retrograde, ascend or descend in the water as they please, and it is curious to observe how, alternately extending or contracting one fin or the other, they gracefully plough the liquid element in every direction.