Barnacles are crustacean related to crabs lobster and shrimp.  Most have a hard limy shell and attach themselves to rocks, seaweeds, logs or live animals like whales, turtles and crabs.

Most are acorn shape with a flat base and cylindrical bodies. They use root like projections to attach themselves and wave hair-like legs through the water to strain out minute plankton organisms which id their diet. 

The  Acorn Barnacle is not edible.