What is the phylum for a barnacle?

They are actually a crustacean, closely related to crabs and lobsters.
Barnacles are most often seen as roughly circular sessile invertebrates (which means that they cannot move on their own).
Kingdom: Animalia
Phylum: Arthropoda
Class: Crustacea
Order: Maxillopoda
Family: Thecostraca
Genus: Cirripedia

Barnacles are thought to be one of the oldest surviving creatures on the planet as they are believed to date back millions of years.