Figure 5 in Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)

Figure 5. Evolution of special reproductive features in Acochlidia. Strict consensus tree (see Fig. 3, but with outgroups condensed). The evolution of hypodermic impregnation in hedylopsaceans (excluding Tantulum, with plesiomorphic copulation) was concomitant with the production of short-headed sperm. The common ancestor of both Pseudunela and Acochlidiidae evolved an additional, paraprostatic injection system. A potential evolutionary key feature for the radiation of Acochlidium and Palliohedyle is the giant, armed 'rapto-penis'. Within the microhedylacean clade, gonochorism may have been the key to the radiation of marine Microhedylidae s.l. species. An aphallic condition and sperm transfer via spermatophores, correlated at least in Asperspina and Microhedylidae (but still unknown for Hedylopsis ballantinei), may have been necessary evolutionary prerequisites.