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

on the recent taxonomy see WoRMS:
MolluscaBase (2018). Aplysia  Linnaeus, 1767. Accessed through: World Register of Marine Species
at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=137654 on 2018-04-08
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