http://www.medslugs.de/E/Med/Placida_verticillata/Selection.htm
http://siolibrary.ucsd.edu/slugsite/wirtz99/plate_2/wirtz_1n.htm
http://www.ugr.es/~lstocino/placida_verticillata.htm
http://www.medslugs.de/E/Atl-E/Placida_verticillata/Selection.htm
http://sacoglossa.lifedesks.org/pages/34332
http://opistobranquis.info/en/guia/sacoglossa/placida-verticilata/