These “Bubble Shells” have been separated from the Cephalaspidea and placed in their own group having a separate and distinct lineage from all the other sea slugs. They are considered a basal category. All have an external shell. The shape and construction of the shells and the accommodation of the animal therein, across the three families, illustrates an evolutionary progression.
The shells within each family have a relative consistency of shape and form, most of which are ornamented with distinctive lines and patterns, and often with spiral striations. The spire varies from prominent to depressed.
All possess a headshield that in some is well-developed extending posteriorly to cover the anterior of the shell and protecting the mantle cavity as all, even if not infaunal, do burrow through the silt/sand at times.
The presentation of the animal varies from almost plain and transparent to flamboyantly coloured. They are all carnivorous feeding on polychaete worms.