The family Chydoridae embraces about 170 reliable species . These species were combined in genera according to the form of their shell, postabdomen, antennule, and antenna (see e .g . KURZ, 1875). These and other features allow to outline genera, but give no idea of the real phylogeny of this family. In a number of cases the similarity of the form turned out to be superficial and after a more detailed study species were relocated to other genera .
The structure of the head pores was the first morphological basis for the phylogenetically justified taxonomy of the Chydoridae . According to the structure of the head pores FREY (1959, 1962) distinguishes four sections in the family Chydoridae . The Eurycercus section includes Eurycercus species having one large median head pore and lateral pores very close in. The Chydorus section includes species with two separate major head pores on the median line of the head shield and two minor pores between them (genera Chydorus, Dunhevedia, Pleuroxus, Peracantha, Alonella, Anchistropus) . The Alona section embraces species possessing two or three major head pores along the median line of the head shield and the minor pores in the lateral position (genera Alona, Oxyurella, Alonopsis, Acroperus, Camptocer- cus, Kurzia, Graptoleberis, Rhynchotalona, Leydigia) . In a separate section Monospilus is placed possessing one large median pore .
The descriptions of Chydoridae species, except 17 species, do not contain information on the trunk limb structure . Meanwhile these