Generic and subgeneric division of pill clams (genus Pisidium s.l.) in the base of anatomical characters

Anatomical investigation of pill clams was initiated by N. Odhner, who presented the thorough descriptions of many european species (Odhner 1921; 1929 et al.). The most conspicuous anatomical characters, namely the number of demibranchs in ctenidium and the number of siphons, provided the base for subgeneric division, proposed by the mentioned author and developed then by J. Kuiper (1962). The groups of species, sharing some other anatomical characters (merging of the mantle, the mode of ovisac development) were distinguished later in each subgenus (Meier-Brook, 1970; 1986), but taxonomie status of these groups was not determined.