Opisthobranchia (phylum Mollusca, class Gastropoda) A subclass of marine organisms, which are often ‘naked’, including the sea slugs and pteropods, ranging from the Cretaceous to the present day. They have only one gill and are thought to represent an evolutionary grade between the prosobranch gastropods (Prosobranchia) and the pulmonate gastropods (Pulmonata).

Opisthobranchia (phylum Mollusca, class Gastropoda) Subclass of marine organisms, which are often ‘naked’, that includes the sea slugs and pteropods, and ranges from the Cretaceous to the present day. They have only one gill and are thought to represent an evolutionary grade between the prosobranch (single-gilled) gastropods and the pulmonate gastropods (i.e. gastropods in which the mantle cavity functions as a lung).