Fig 2.

Nudibranch taxa Xenocratena suecica (A–E) and Cumanotus beaumonti (F–I) with facultative ability to burrow into soft substrata (C, I) compared to the highly simplified obligate infaunal Pseudovermis paradoxus (J–L) and Xenoturbella (M). Note the high external and behavioral similarity between the phylogenetically distantly related Xenocratena (A, C, E) and Cumanotus (F, I) and the drastic external differences between the phylogenetically closely related complex Cumanotus (F, I) and paedomorphic vermiform Pseudovermis (J). Radula (molluscan teeth) features, on the contrary, are very different between Xenocratena (B, note the absence of lateral teeth, lt) and Cumanotus (G, note the presence of lateral teeth, lt) and similar between Cumanotus (G) and Pseudovermis (K, note the presence of lateral teeth, lt). Both Xenocratena (D) and Cumanotus (H) are able to lay egg masses facultatively on soft substrata, whereas Pseudovermis (I) lays egg masses obligately on the soft substrate. Abbreviations: c, cerata (dorsal papillae); ct, central teeth of radula; lt, lateral teeth of radula; ot, oral tentacles; r, area below which radula is placed; rn, rhinophores (chemical sense organs).