http://slugsite.us/bow/pacific/plana.html
this website exists no more:
http://www.biology.ucsc.edu/classes/bio161l/NUDI/hplana.html
http://www.seaslugforum.net/find/hopkplan
http://www.sea-slug.com/species/umiusi271.htm
http://www.ne.jp/asahi/kappa-kuri/miyamoto/OSEHOO/umiushi366.html
http://rfbolland.com/okislugs/hopkplan.html
http://www.nudibranch.com.au/pages/9877d.htm
http://www.umiushi-zukan.com/eng/list_pic.php?ssw=g&kind_id=164
http://www.vibrantsea.net/okenia2375_anilao2013.htm
http://www.Philippine-Sea-Slugs.com/Nudibranchia/Euctenidiacea/Okenia_plana.htm
http://www.medslugs.de/E/Pac-W/Okenia_plana/Selection.htm
http://www.medslugs.de/E/Ind-E/Okenia_plana/Selection.htm