The two subdivisions of this large clade are quite diverse:
Opisthobranchia are virtually all marine species, some shelled and some not, and comprise about 25 families and 2000 species of the bubble shells, the seaslugs, as well as the sea hares.[2] The internal organs of the opisthobranchs have undergone detorsion (unwinding of the viscera that were twisted during torsion).
The Pulmonata comprises about 20000 species, includes the majority of land snails and slugs, many freshwater snails, and a small number of marine species. The mantle cavity of the Pulmonata is modified into an air-breathing organ. They are also characterized by detorsion and a symmetrically-arranged nervous system. The pulmonates almost always lack an operculum and are hermaphroditic.
The families currently included in Heterobranchia have historically been placed in many different parts of the taxonomic class of gastropods. Earlier authors (such as J.E. Gray, 1840) considered Heterobranchia to consist of only marine gastropods, and conceptualized it as a borderline category, intermediate between the Opisthobranchia & Pulmonata, and all the other gastropods.[7]
The (sometimes recognized) category Heterostropha within the Heterobranchia, which includes such families as Architectonicidae, the sundial or staircase snails, is primarily characterized by a shell which has a heterostrophic protoconch, in other words the apical whorls are coiled in the opposite plane to the adult whorls. The classification of this group was revised by Ponder & Warén in 1988.[8]
The graph of neighbor-joiningphylogenetic tree shows that there is no clade-supporting pattern for the monophyly of Opisthobranchia (green) or of Pulmonata (yellow) based on datasets by Jörger et al. (2010).[9]
^Haszprunar G. (1985). "The Heterobranchia ― a new concept of the phylogeny of the higher Gastropoda". Zeitschrift für zoologische Systematik und Evolutionsforschung. 23 (1): 15–37. ISSN0044-3808.
^Ponder, W. F.; Warén, A. (1988). "Classification of the Caenogastropoda and Heterostropha- A list of family-group names and higher taxa". Malacological Review Supplement. 4: 288–317.
Dinapoli A. (2009). Phylogeny and Evolution of the Heterobranchia (Mollusca, Gastropoda). Thesis, Frankfurt am Main, 176 pp. PDF[permanent dead link].
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