A taxonomical outline of the Gymnosomata (Mollusca)

This outline is composed to honour Dr. C.O. van Regteren Altena, who, over a long period, has stimulated the author with advice and assistance in his taxonomical studies. Sixty-two valid taxa on species level or lower and eighteen genera, are recognized in the order Gymnosomata at the moment. In 1774 the first species was described, and in 1850 only seven species were known. At the end of the nineteenth century the knowledge of Gymnosomata had increased considerably and in 1900 already twenty-four species were known. The large scientific expeditions from around 1900 contributed much to the knowledge of oceanic plankton and pelagic molluscs. Between 1900 and 1920 another twenty-four species were described, but between 1920 and 1930 only nine taxa were added. The number of taxa recognized as new to science decreased rapidly; in the period 1930-1940 only four new ones were found and after 1950 only one valid taxon new to science was published. This means that, statistically, now only one or two unknown taxa are still expected to exist among the Gymnosomata. New theories and new methods still to be introduced may alter this estimated number. But at the moment we know, in my opinion, enough to establish a revised system for the group. This system is given below with brief references and comments.