Acorn barnacle  (Semibalanus balanoides)

Barnacles are well-known intertidal organisms. They were not firmly accepted to be crustaceans until the 1830s; before this time, the calcareous shell and sessile habit of the adults resulted in confusion with molluscs (2). Our current knowledge of barnacles is largely based on an 8-year period of research by Charles Darwin (3). Adult acorn barnacles are sessile; they have a cone shaped shell-wall comprising a number of calcareous pates. In this species, Semibalanus balanoides, the shell-wall consists of 6 greyish-white plates. The opening at the top of the 'cone' is diamond-shaped, and can be covered by movable plates when the tide goes out. This protects the animal inside from desiccation (2).