35 2 FISHES CHAP. 



however, continues its movement to the left, but now as a flexion 

 stroke, and assumes the curvature and position indicated in B, 

 with a reversal in the direction of both the cephalic and caudal 

 curves, but in the meantime the force of the preceding extension 

 stroke has forced the Fish along the line a b to the new position 

 indicated by B. By a rapid succession of alternating flexions 

 and extensions, during which the tail describes figure - of - 8 

 curves, the Fish travels in an undulating forward course with a 

 maximum of propelling power and a minimum of " slip." In 

 short, the action of the tail precisely resembles the action of the 

 stern-oar in the operation of sculling a boat. 



There are also other considerations which add to the surplus 

 power of the extension stroke by lessening the resistance of the 

 water to the flexion or non-effective stroke. During the flexion 

 stroke the tail fin is less expanded and its area diminished, and 

 by the rotation of the Fish on its long axis the surface of the 

 tail strikes the water obliquely, and further, the tail moves with 

 less rapidity. On the contrary, when the extension stroke is 

 made these conditions are reversed. The caudal fin is expanded, 

 the stroke is more rapid, and by the reverse rotation of the Fish 

 the tail now strikes the water with its flat surface. In other 

 words, the action of the tail during the two strokes may be com- 

 pared to the " feathering " of an oar in rowing. Nor is this 

 all. A Fish in motion through the water produces a suction 

 current behind it. The current offers but little resistance to the 

 flexion stroke, inasmuch as the direction of the two coincide, 

 but during the extension stroke the tail meets the full force of 

 the current, and consequently its grip and propelling power are 

 greatly enhanced. There is a striking analogy between the 

 movements of a Fish's tail in swimming and the action of the 

 screw of a steamer, but as a propelling organ the former is far 

 superior to the latter. As we have seen, the tail of a living 

 Fish can so adjust its shape and surface that it alternately eludes 

 and grips the water in accordance with the needs of particular 

 strokes. 



The curves into which the body of a Fish is thrown when 

 swimming are never less than two, but in long-bodied Fishes, 

 such as the Eels, the number may be increased, and in every 

 case the curves occur in pairs and are complementary to one 

 another. 



