The figure is Richard Owen's version (1855) of Charles Darwin's archetypal cirripede (1844). Darwin took as the starting point of his archetype the pelagic crustacean Lucifer. The outlined abdomen suggests the abortion of that region, to yield the first barnacle, which is all head and thorax. Darwin, following Milne Edwards, believed that Lucifer was a stomatopod--thus he has added a joint in the middle of its head which is not actually present. (Today, Lucifer is classified among the eumalocostracans). Darwin emphasizes the eyes and antennae retained in his hypothetical first barnacle. His thoughts on these points can be traced back to a sketch (below) he did in his copy of Martin St. Ange's monograph on cirripedes.
Owen retained the large eyes and antennae of Darwin's archetypal barnacle. He also retains the non-existent cephalic joint in Lucifer. Owen's one change was the addition of a prehensile penis, an oblique nod to Darwin's thorough investigation of hermaphroditism in the Cirripedia.