The notaspids are primitive opisthobranchs that have a typical molluscan ctenidium rather than a pseudobranch (a "false" secondarily derived respiratory structure). The Pleurobranchidae lack an external shell, or are devoid of a shell altogether.
Some notaspid opisthobranchs have lost their shells (e.g., the preceding species), and others (family Umbraculidae), such as this one, have retained their limpet-shaped shells.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature