Opisthobranchs,, are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia. The group has also been called Euthyneura referring to the untorted visceral nerve cord. Opisthobranch means gills behind (and to the right of the heart). In contrast Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart. Under the old… (More on Opisthobranchia)