Sea Slugs and Snails: Creatures on the Boundary

In the past, shelled gastropods such as Hydatinidae (bubble shells, e.g. Hydatina physis), Aplustridae (e.g. Micromelo undatus), and Bullinidae were treated as cephalaspidean sea slugs and included in sea slug field guides.

However, in Classification and Nomenclator of Gastropoda Families, published by Bouchet and Rocroi (Muséum national d'Histoire naturelle, Paris) in 2005, these animals were reassigned to Heterostropha (the snail side).
This is widely known not only among researchers but also among sea slug enthusiasts.

That said, there are also researchers who, while removing them from Cephalaspidea, still keep them within Opisthobranchia (the sea slug side).
At SEASLUG.WORLD, in response to many requests from our users, we decided to also include the Acteonoidea and Ringiculoidea — historically treated as sea slugs — under the rank of Heterobranchia.

View Heterobranchia at SEASLUG.WORLD

Snail?

Sea slug?

It looks like it will take some time before a clear answer emerges — but the animals themselves go on living their lives, indifferent to human deliberations.