>I have so far been unable to locate any papers on their phylogeny (such
>as determined by cladistic analysis).
Gosliner, T.M. & M.T. Ghiselin 1984. Parallel evolution in opistobranch
gastropods and its implication for phylogenetic methodology.
Systematic Zoology 33(3): 255-274.
Torleif Holthe
Trondheim, Norway