Acteonoidea
Acteonoidea is an early-branching group of heterobranch sea slugs that still carry a well-developed external shell into which the animal can partly withdraw. It includes the barrel-bubble and bubble shells of the families Acteonidae and Aplustridae (Hydatina, Micromelo, Aplustrum). Many are boldly patterned, burrow in sand and hunt polychaete worms at night.
An obvious, often colourful external shell (globose or barrel-shaped) combined with a broad, frilly or lobed body and a flattened head shield. Unlike nudibranchs they have no rhinophores or dorsal gills; unlike Cephalaspidea the shell is large, thin and boldly banded or lined.
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