The excellent monograph by Westheide (1967) of the hesionid genera Hesionides and Microphthalmus provided the means to study these genera in The Netherlands. Until now no species belonging to either of these genera have been recorded from this country. Nevertheless, several species of Microphthalmus were found during our ecological studies on the polychaetes of the estuarine area of the rivers Rhine, Meuse, and Scheldt. One of these species, M. fragilis Bobretzky, was so far only known from the Mediterranean and the Black Sea. In the present paper three species are recorded and described. The positions of the localities where these species were found are shown on the map (fig. i). Ecological data will be published later. Acknowledgements are due to Dr. W. Westheide (2. Zoologisches Institut, Gottingen, Germany) for his help with the identification of my material, to Prof. Dr. D. J. Kuenen (Zoologisch Laboratorium, Leiden and to Mr. J. van der Land (Rijksmuseum van Natuurlijke Historic, Leiden), for their critical remarks concerning the manuscript.
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