And--Ah! was that not a slap! How the water boiled and foamed; and what a tail he had! And how the mackerel flew out of the water! Yes.

You are a lucky boy to have seen that.

I have not seen one of those gentlemen show his "flukes," as they call them, since I was a boy on the Cornish coast.
Where is he gone?
Hunting mackerel, away out at sea.

But did you notice something odd about his tail, as you call it--though it is really none?
It looked as if it was set on flat, and not upright, like a fish's.

But why is it not a tail?
Just because it is set on flat, not upright: and learned men will tell you that those two flukes are the "rudiments"-- that is, either the beginning, or more likely the last remains--of two hind feet.