Cephalaspideans (some of which are called bubble shells) are primitive sea slugs because of their external shell (when present), basic gill structure, form of the radula and relatively simple reproductive system – though they are hermaphroditic like all sea slugs. As the name suggests, the Order Cephalaspidea contains the “head shield slugs”, considered descendants of the original burrowing sea slugs.
Generally the head is shaped as a well-developed flattened and thickened shield and it is used for ploughing through the substrate. It is often wedge-shaped over the anterior portion of the body, directing silt and sand away from the mantle cavity and the entrance to the shell (in those that possess an external shell) and may be further developed with posterior lobes. The foot is often enlarged laterally into flaps called parapodia that curl up over the sides of the body or shell. Most cephalaspideans possess a shell borne either externally or internally, but it can be much reduced or even lost altogether.
Although the Haminoeidae and Bullidae have secondarily evolved an algal diet, the greater majority are carnivorous, eating foraminiferans, polychaete and polyclad worms or even molluscs including other sea slugs. Many possess a grinding gizzard for crushing the shells of their molluscan prey.