Heterobranchia

The Heterobranchs are a large group of snails and slugs. The group includes marine, aquatic and terrestrial gastropod molluscs.

Heterobranchia is one of the main clades of gastropods. It has three informal groups: Bouchet P. & Rocroi J.-P. (eds) 2005. Classification and nomenclator of gastropod families. Malacologia: International Journal of Malacology. 47(1-2).   The details are extremely complicated, and not yet quite worked out. Experts say they have "poorly resolved phylogenetic relationships".Dinapoli A. & Klussmann-Kolb A. 2010. The long way to diversity – phylogeny and evolution of the Heterobranchia (Mollusca: Gastropoda). Molecular Phylogenetics and Evolution 55 (1): 60–76.  

Lower Heterobranchia: a group of rather specialized, highly evolved sea slugs and sea snails.
Opisthobranchia: almost all are marine species, some shelled and some not. They are a group of slugs and snails with grinding gizzard plates. 
Pulmonata. This is a group which  is familiar to most people. It includes most land snails and slugs, many freshwater snails and a few marine species. They breathe air, are hermaphrodite, and usually have no operculum.The operculum is the hard section on the foot which closes up the shell so the animal is safe inside.

 References 

Category:Gastropods