You have a Garveia franciscana or a native species
Garveia franciscana (Torrey, 1902) (=Bimeria f.; =Perigonimus megas Kinne, 1956). Occurs in the Netherlands in brackish waters, Belgium and Germany (Verwoort, 1964; Schuchert, 2007). Origin unknown, but possibly a warm water species as it has also invaded the coasts of Brazil.
Drawing of Garveia franciscana (from Boullion et al. 2004)