What is "heterobranchia"

Heterobranchia, the heterobranchs (meaning "different-gilled snails"), is a taxonomic clade of snails and slugs, which includes marine, aquatic and terrestrial gastropod mollusks.

Heterobranchia is one of the main clades of gastropods. Currently Heterobranchia comprises three informal groups: the Lower Heterobranchia, the Opisthobranchia and the Pulmonata, also known as the lower heterobranchs, the opisthobranchs and the pulmonates.