Final answer:
The term 'Opistobranchia' formerly referred to a subclass of gastropods in Phylum Mollusca known for having reduced or lost shells and gills behind the heart, now often categorized within Heterobranchia.
Step-by-step explanation:
In the classification of Phylum Mollusca, the term used for the subclass of class Gastropoda known as "Opistobranchia" refers to a diverse group of gastropods. Within the Gastropoda class, which is the most diverse class of mollusks, Opistobranchia are characterized by their gill placement behind the heart and a reduction or loss of their shell. However, it is important to note that taxonomic revisions and modern phylogenetic studies have largely abandoned the subclass Opistobranchia, incorporating many of its members into a larger clade within Gastropoda known as Heterobranchia. Gastropods include familiar organisms such as snails, slugs, conchs, and whelks, and they exhibit variations like coiled shells and a distinct developmental process called torsion. This diversity is due to the modification of basic body regions, particularly the foot and mantle, resulting in different species adapted to marine, freshwater, and terrestrial habitats. Gastropods often feature a shell made primarily of calcium carbonate, but in the case of opisthobranchs, the shell is often reduced or absent, reflective of their specific ecological adaptations.