Last summer, on the way back in from a surf, I had an amazing encounter with a couple of American horseshoe crabs (Limulus polymephus) doing their thing.
If you look really closely you can see how the pusher leg works. The last leg has a really cool articulated shape that splays out when the animal pushes it against the sand (like an inside-out umbrella).
|The pusher leg with foot closed...|
|...and open (as when pushing against sand)|
I always wondered about why it had that shape and now I see!! In the video you too can see how well the pusher leg works. Awesome, beautiful, and WAY COOOL!