Figure 1. The general shell morphology of a sessile barnacle. A) The shell (testa) of a sessile barnacle is made up of a basis that cements the barnacle to the substrate and the wall plates (compartments) (Darwin 1854). Each wall plate consists of a paries (the wall) and two radii or alae lining both edges, or a radius and an ala on each side. B) Having 8 plates is thought to be the ancestral state and when all 8 plates are present, the rostrum usually have alae on both sides while radii line both edges of the rostro-laterals. (Darwin 1854). In A. amphitrite and C. diadema, the rostro-lateral plates have fused with the rostrum, forming a single plate. Consequently, the rostrum is lined both sides by radii. (Adapted from Darwin 1854).