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During our studies of this first part of the brittle-stars collection of the Snellius Expedition (1929-1930) in Indonesia, 37 species were found. Among these, a new genus and a new species of Amphiurid, Paramphichondrius tetradontus, are described; Ophiothrix picteti Loriol is transferred from the subgenus Acanthophiothrix to Ophiothrix s.s.; 17 other species, either rare or of taxonomically confused status, are discussed, i.e., Astroceras elegans, Ophiocentrus koehleri and O. vexator, Macrophiothrix belli, M. longipeda, M. rhabdota, and M. sp. aff. variabilis, Ophiothrix contenta, O. savignyi and O. trilineata (with a new synonym: O. tristis), O. (Acanthophiothrix) armata, O. (A.) spinossissima, O. (Keystonea) martensi, O. (Placophiothrix) hybrida and O. (P.). striolata, Ophionereis thryptica and O. variegata.

INTRODUCTION AND METHODS During an investigation of distribution and ecology of the Archiannelida of the estuarine area of the rivers Rhine, Meuse and Scheldt in the southwestern part of The Netherlands, three species of polychaetes new to the Dutch fauna were found. Because these species have not been reported very frequently from other localities, the finds are described in some detail. The first two species were found in samples taken with a o.i m2 Van Veengrab; the last one originated from a core sample taken in the beach. After collecting the samples have been treated in the way described by Boaden (1963). RESULTS AND DISCUSSION Hesionura augeneri (Friedrich, 1937) (fig. 2) This species was found at two localities in the coastal water (fig. 1) at depths of s and 8 m, respectively. The bottom consisted of medium sand. The salinity at these localities normally is over 16.5°/00 CI-. Description (fig. 2). — Length up to 20 mm =135 segments; diameter without parapodia up to 120 μ. Prostomium bluntly conical, but elongated in longer animals, distally with four antennae. Four small, sometimes barely visible eyes dorsally on the prostomium. First segment with one pair of ventral cirri, second segments with one pair of dorsal cirri as well. Parapodia of the third segment bearing setae and a ventral cirrus; all other parapodia with a dorsal cirrus as well. Ventral cirri longer than the dorsal ones; especially in larger specimens the ventral cirri have a deep brown colour. Parapodia uniramous with acicula and four setae, three being compound and one simple. Pygidium with two long cirri.

During the years 1968-1970 a study has been made of the soft-bottom macrofauna (Coelenterata, Mollusca, Echiuroidea, Polychaeta, Archiannelida, Crustacea Malacostraca) of the estuaries of the rivers Rhine, Meuse, and Scheldt in the southwestern part of The Netherlands and of the adjacent part of the North Sea. 1751 samples were taken with 0.1 and 0.2 m² Van Veen grabs; in the intertidal zone several hundreds of samples were dug out, whereas further samples were taken for the analysis of interstitial polychaetes and archiannelids. Of the environmental factors especially salinity, sediment composition, current velocity, water temperature, oxygen saturation, turbidity, water pollution, exposure to waves, and the influence of ice have been investigated. Salinity shows a gradient from over 19.0 promille Cl in the offshore waters to below 0.3 promille Cl in the freshwater tidal area. Sediments mostly consist of medium, fine and very fine sands, but coarse sands, muds, and peat banks also occur. Current velocity is in general below 1.5 m/sec., whereas water temperatures range between about 1 and 22°C in the course of the year. Oxygen saturation is low in the rivers (0-70%) and shows in the brackish water areas a gradient to values over 100% reached in the marine parts of the estuaries. Water pollution shows the reverse pattern, but turbidity is highest in the brackish parts of the rivers. The larger part of the shores of the estuaries is sheltered against wave-action, but the beaches along the North Sea may be considered as semi-exposed. The influence of ice is most severe in the freshwater tidal area. Food is abundant in the estuaries, but less in the offshore parts of the North Sea. Taxonomical remarks have been made on, among others, Polinices polianus, Venus gallina striatula, Eteone foliosa, Nereis virens, Nephtys caeca, Aricidea minuta, Spio martinensis, Streblospio shrubsolii, Tharyx marioni, and Ophelia borealis. For 2 coelenterates, 19 gastropods, 44 lamellibranchs, 1 echiurid, 76 polychaetes, 4 archiannelids, 6 leeches, 15 decapod crustaceans, 2 mysids, 8 cumaceans, 1 tanaid, 9 isopods, 31 amphipods, 8 echinoderms, and 1 species of Branchiostoma the distribution is compared with the environmental data mentioned above or earlier papers treating this have been enumerated. For many species this comparison was carried out by means of a maximum-likelihood method. The following general trends in the distribution of the species studied over the estuarine area were observed. The biological limit of the estuarine environment at the seaward side could be placed where the level sea-bottom changes into a topography of tidal channels and shallows. At the landward side no clear-cut limit could be demonstrated. The brackish-water minimum in the species distribution demonstrated first by Remane (1934) could be confirmed for the estuaries of the Delta area. It appears that nearly all species occurring in the brackish water area also live or may live in either the more marine parts of the estuaries or in fresh water. Hence, nearly all may probably be considered as euryhaline marine or freshwater species. The fauna investigated could be divided in a relatively stenotherm component occurring only offshore and a relatively eurytherm component occurring offshore as well as in the estuaries. Below low water level no species could be demonstrated to be restricted by hydrostatic pressure to a certain depth range. Only three species could be shown to be restricted to the intertidal zone. Several species proved to be restricted to a certain sediment range. Of the species occurring in the brackish water area most appeared to prefer muddy sediments. Therefore, species considered as brackish-water species probably often are really euryhaline inhabitants of mud-bottoms. No species could be shown to be governed directly in its distribution by the exposure to wave-action, but many proved to be so indirectly by the nature of the sediment. Some species probably were inhibited in their occurrence by a high turbidity. Pollution and low values for the oxygen saturation could be shown to be inhibiting for several species in the freshwater tidal area, but for only very few in the brackish tidal area. Shortage of food probably is the reason for the absence from the offshore waters of some species common in the seaward parts of the estuaries. It proved to be impossible to apply any biological brackish water subdivision to the estuaries of the Delta area. The subdivisions proposed earlier could be correlated to hydrographical irregularities of the estuaries. The species characteristic for the brackish water stretch of the estuaries of the Delta area are demonstrated to occur in nearly all estuaries and brackish waters along the Atlantic coasts of Europe. Some species also occur in the western part of the Mediterranean. This European estuarine brackish-water fauna, however, is nearly completely different with regard to species composition from similar brackish water faunas in South Africa, S.E. Asia, New Zealand, and N. Siberia, and along the Pacific as well as along the Atlantic and Gulf coasts of N. America. Hence, a similarity of brackish-water faunas all over the world does not exist. The species diversity proved to be lowest in the brackish parts of the estuaries and in the intertidal zone of the freshwater tidal area. Especially intertidal sand-bottoms show a gradient from a high number of species in the beaches along the North Sea to nearly no species in the sand flats of the freshwater tidal area; intertidal mud bottoms and subtidal sand bottoms show a comparable pattern. Because the brackish water areas lodge a relatively high number of species introduced from other parts of the world, they are supposed to be undersaturated with regard to the number of species occurring. Omnivores are relatively abundant ,in the brackish parts of the estuaries, so the few species occurring apparently occupy wide niches. The percentage of species with pelagic larvae decreases strongly from the sea to the river, whereas the number of species with a non-pelagic development shows the reverse pattern. Pollution, turbidity, size of brackish waters and the critical salinity hypothesis of Khlebovich are ruled out as an explanation for the species minimum in brackish waters. Instability of temperature and salinity partially explain this phenomenon, but a satisfactory explanation for the exceptions only is provided by the time-stability hypothesis of Sanders. It is shown that the history of the N. W. European estuaries during the Pleistocene glaciations sufficiently explains the poverty of brackish water species in these estuaries. Also for other brackish water faunas, geological history probably forms a reasonable explanation for their poverty.

INTRODUCTION During his investigation of the amphipod fauna of the sandy beaches of the Netherlands, Mr. W. J. M. Vader, formerly of this institute, met with two species of Eurydice Leach, 1815. Besides the common Eurydice pulchra Leach, 1815, he found a second species in smaller numbers. Because Giordani Soika (1955) as well as Renaud-Debyser & Salvat (1963) recorded Eurydice affinis Hansen, 1905, and E. pulchra from comparable beaches in northern France, Mr. Vader asked Dr. B. Salvat for some specimens of the former species for comparison. When these specimens arrived, Mr. Vader had left our laboratory. He was kind enough to entrust me with the further study of the Eurydice problem, for which I am most indebted to him. Further I want to thank Dr. L. B. Holthuis, Rijksmuseum van Natuurlijke Historie, Leiden, and Dr. B. Salvat, Muséum National d'Histoire Naturelle, Paris, for material, and Messrs. A. J. J. Sandee and L. de Wolf for their help in the field-work. The genus Eurydice is well known from the southern part of the North Sea. As early as 1778 Slabber described his "Agaat Pissebed" from the Dutch island of Walcheren. From the figure that he gave we can conclude with certainty that his animal belongs to E. pulchra. Holthuis (1956) still mentioned this as the only species of the genus from the coast of the Netherlands. In another publication Holthuis (1950) reported the occurrence of Eurydice spinigera Hansen, 1890, in the southern North Sea. E. affinis is here recorded for the first time from the Netherlands and the North Sea. The species was already known from Wissant (Pas de Calais) in France, just at the southern entrance of the North Sea (Giordani Soika, 1955

INTRODUCTION 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. 1). Ecological data will be published later. Acknowledgements are due to Dr. W. Westheide (2. Zoologisches Institut, Göttingen, 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 Historie, Leiden), for their critical remarks concerning the manuscript. Microphthalmus fragilis Bobretzky, 1870 Microphthalmus fragilis — La Greca, 1950: 9-13. Material : Geul van Bommenede near buoy GB 16, N. E. of Brouwershaven, island of Schouwen, province of Zeeland; depth 5.5 m; 25 September 1963; grab sample no. 592; leg. P. de Koeijer, C. de Rooy, A. J. J. Sandee, W. J. M. Vader and L. de Wolf; about 30 specimens. Description. — Specimens of 70 and 75 segments are about 10 mm long. The prostomium bears five tentacles, whereas the three following segments each bear four tentacular cirri. The whole anterior part is very short, the