former taxonomy:
Class: GASTROPODA  Cuvier, 1797
Subclass: OPISTHOBRANCHIA  Milne-Edwards, 1848
Order: CEPHALASPIDEA  P. Fischer, 1883

on the recent taxonomy see WoRMS:
MolluscaBase eds. (2023). Atys  de Montfort, 1810. Accessed through:
World Register of Marine Species
at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=138052 on 2023-09-04
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