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Fig. 3. Coordinate system used for elongated body theory calculations. The solid
line represents a midline at one time, while the dotted line represents it at
a later time. Perpendicular and parallel velocity,
v⊥ and v||, are shown as vectors at
the point (xb, yb). The arc length
s is shown along the midline and the eel is swimming into a flow
U. L is total body length.