Encrustaceans

Louis Agassiz the 19th century biologist and geologist described 
barnacles  as "shrimp-like animals that stand on their heads in limestone houses kicking food into their mouths".


He meant that barnacles don't move around once glued to a surface but when submerged by the sea they wave their feathery feet to catch waterborne plankton.

It wasn't until the 1830s that they were classified as  crustaceans and not molluscs, which makes them relatives of crabs and lobsters.

Charles Darwin  spent 8 years studying barnacles on account of the taxonomic challenges they posed. His study  was filled with specimens  from all over the world that arrived by every post.  
I have told this story before but it's worth repeating:
When Darwin's son George was invited home by a school friend he was surprised to find that his friend's father didn't have a study. "But where does he do his barnacles?"  asked George. 

While I was  looking at the barnacles I spotted a sea slater (extra) which is also a crustacean but in this case a very fast-moving one.