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This is a female Ctenophora elegans.

See here an article for the identification: https://ccw.naturalis.nl/documents/Oosterbroek_et_al.,_2006.pdf

And this key is in Dutch, but has many pictures: https://ccw.naturalis.nl/documents/05_Tabel_B_Peeters_en_Oosterbroek,_2013e.pdf