The clade Cephalaspidea, also known as the headshield slugs and bubble snails, is a taxon of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia. Bubble shells is another common name for these families of marine gastropods, some of which have thin bubble-like shells.Members of this worldwide clade appear to be the most primitive of the opisthobranchs.Headshield slugs are the most morphologically diverse group of all the opisthobranchs.