Darwin’s Cirripedia project was an exacting exercise in systematics, as well as an encrypted study of evolution in action. Darwin had a long-standing interest and expertise in marine invertebrates and their sexual arrangements. The surprising and revealing sexual differentiation he would uncover amongst barnacles represented an important step in his understanding of the origins of sexual reproduction. But it would prove difficult to reconcile these findings with his later theorizing. Moreover, the road to discovery was hardly straightforward. Darwin was both (...) helped and hindered by the tacit expectations generated by his transformist theorizing, and had to overcome culturally-embedded assumptions about gender and reproductive roles. Significant observational backtracking was required to correct several oversights and misapprehensions, none more so than those relating to the chronically misunderstood “Mr. Arthrobalanus.” With careful attention to chronology, this paper highlights some curious and overlooked aspects of Darwin’s epic project. (shrink)