752 FISHES CHAP. 
however, continues its movement to the left, but now asa flexion 
stroke, and assumes the curvaturé 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 db 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 “shp.” 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. 
