A 'barnacle' is a small sea creature with a shell, that sticks very tightly and in large numbers to rocks and the bottom of boats

"Barnacles" are a kind of sea creature. Tully is interested in lots of things. It sounds as if she is interested in nature--specifically biology and astronomy. On land, moss and lichen grows on rocks. At sea, barnacles and seaweed grow on surfaces that are in the sea. You find barnacles on wooden structures in harbors. They grow on the hulls of boats. They grow on rocks at the seashore. They are animals, but they can't move. They are "sessile," like oysters. They stay fixed in one place. They are white. What you usually see is their hard, outer shell. I took this picture of some barnacles. A barnacle is about 1 cm. in size. https://imgur.com/cMk9T7j.jpg