As a non-monophyletic taxon Opisthobranchia

aplysia californica, sea hare.

it speculated far 1985 opisthobranchia paraphyletic, based on morphological evidence, , had given rise pulmonata, group of subclass rank.

because pulmonata sibling group opisthobranch taxon, authors argued opisthobranchia therefore not monophyletic group , can no longer accepted valid taxon. included within subclass heterobranchia, although many manuals , websites still use old classification.

a phylogenetic study published in november 2004, gave new definitions of 7 main lineages of opisthobranchia.

however, in 2005, study of rrna gene sequences not resolve monophyly versus paraphyly of opisthobranchia

subsequent taxonomic classification in 2005 overturned opisthobranchia valid clade, reclassifying them informal group within heterobranchia. accordingly, articles no longer use term opisthobranchia, replacing heterobranchia emphasise different concept. 2011, opisthobranchia declared artificial , obsolete , replaced new phylogenetic hypotheses.