The Netherlands Nudibranchiate Mollusca

In preparing a small Handbook of the Netherlands Opisthobranchiate Mollusca (Fauna van Nederland, Afl. VIII, Sijthoff, Leiden, 1936), in cooperation with Miss Tera van Benthem Jutting, the present author studied, as his part of the work, the Nudibranchia. As some results of his studies might be of more general interest, they are given here. The southern North-Sea is rather poor in animal life and it is always interesting to note, what species can live under the rather bad ecological conditions it affords. The number of species diminishes further as one enters the Waddenzee or (before 1932) the Zuiderzee. Though as a rule the sandy Dutch coasts are not suitable for Nudibranchs, there are found some on the stone-covered dikes and quays, on the wooden landing-places, shooings and piles in the harbours, whereon they can creep and where their hosts, coelenterates and algae, find a solid underground. Such are the harbours of den Helder and others, the "Hondsbossche Zeewering", the piers of IJmuiden, Scheveningen, the dikes at West-Kapelle and along the "Zeeuwsche stroomen". The shutting off of the Zuiderzee (on the 28th of May 1932) forms from the biological standpoint a most interesting ecological experiment. It now contains almost fresh water, 0.3—1.0 g Cl per 1 (cf. Havinga in: Vakblad voor Biologen, XVII, Dec. 1935, p. 64—73, and in: De Biologie van de Zuiderzee tijdens haar Drooglegging, 4, 1936, p. 5—14; see also: Driemaandelijksch Bericht betreffende de Zuiderzeewerken, XVI, 4, Oct. 1935, p. 7, etc.). Before it was dammed off