[5][6] Under the 1931 classification system drawn up by Johannes Thiele, the class Gastropoda was divided into three subclasses; Prosobranchia, Pulmonata and Opisthobranchia.

A phylogenetic study published in November 2004,[11] gave new definitions of the seven main lineages of the Opisthobranchia.

However, in 2005, a study of rRNA gene sequences could not resolve monophyly versus paraphyly of the Opisthobranchia [12] Subsequent taxonomic classification in 2005 overturned the Opisthobranchia as a valid clade, reclassifying it as an informal group within the Heterobranchia.

[14] Order Opisthobranchia Milne-Edwards, 1848 – sea slugs The reduction or loss of the shell, the elaboration of the head, foot or mantle, and the acquisition of chemical defences are evolutionary trends shared by most opisthobranch taxa.

[15] The loss of shell in the group is an example of parallel evolution and has occurred on multiple independent occasions.

[16] Opisthobranchs have undergone detorsion, an evolutionary reversal of the half revolution torsion of their immediate ancestors.

[17] Opisthobranchia represents a morphologically diverse group of gastropods occupying a great variety of ecological niches.

Opisthobranchs have a global distribution, but are restricted almost exclusively to marine habitats with the only exception being few freshwater acochlidians.

[15] Principally soft-bodied marine creatures with a reduced or absent shell and no operculum, opisthobranchs use other methods for protection.

Opisthobranchs are hermaphrodites and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for fertilisation.