26 A HISTORY OF FISHES 



remarked that the caudal fin is operated primarily by the 

 action of the muscles of the body and tail, but many fishes are 

 capable of moving slowly forward by means of wave-like 

 movements of the fin itself, the waves travelling at right angles 

 to the longitudinal axis of the body. In most fishes the shape 

 of the fins, and more especially that of the caudal, provides a 

 very good index of speed and agility, the same type of fin 

 occurring in quite unrelated groups of fishes whose swimming 

 habits are similar. It is impossible to enter into the mechanical 

 possibilities of the different shapes of caudal fin, but it may be 

 laid down as a general rule that fishes with large tails, the 

 hinder margins of which are square-cut (truncate) or rounded, 

 are comparatively slow swimmers, and, although able to 

 accomplish sudden short bursts of speed, they are incapable of 

 swimming for long periods at a rapid velocity, as are those 

 species provided with deeply forked or lunate tails (Fig. 33). 

 Such fishes have the upper and lower lobes of the fin long and 

 pointed, and the fleshy part of the tail, known as the caudal 

 peduncle, is nearly always very narrow and not infrequently 

 strengthened by one or two fleshy keels on either side as in the 

 "Sword-fishes" (Fig. 6). The Bonito (Gymnosarda) , reckoned 

 to be among the swiftest of all fishes, provides an excellent 

 example of this type of caudal fin — crescent-shaped, without 

 flesh, almost without scales, composed of bundles of rays, 

 flexible, yet as hard as ivory (Fig. 4). Professor Goode writes: 

 "A single sweep of this powerful oar doubtless suffices to propel 

 the Bonito a hundred yards, for the polished surfaces of its 

 body can offer little resistance to the water. I have seen a 

 Common Dolphin swimming round and round a steamship, 

 advancing at the rate of twelve knots an hour, the effort being 

 hardly perceptible. . . . Who can calculate the speed of a 

 Bonito?" 



The other median fins, the dorsal and anal, may also be 

 used by certain fishes as swimming organs, especially in those 

 forms whose bodies have radically departed from the stream- 

 line shape. They may act in conjunction with the caudal fin 

 or as a substitute for it. By appropriate use of the muscles 

 controlling the fin-rays and their supports {cf. p. 58) a series 

 of wave-like movements can be produced in the fin, similar 

 to the wriggUng contortions of the body of the Eel. In the 

 Bow-fin (Amia) of North America, for example, undulating 

 movements of the long dorsal fin are often used to propel the 

 body slowly forward (Fig. ioa), and the Electric Eel {Electro- 



