Homoscleromorpha, that was recently recognized as the fourth class of Phylum Porifera, lacks
reliable diagnostic morphological traits. In the present study we analyzed and compared the kinetid
structure (flagellar apparatus) of 10 Oscarella species. In sponges the kinetid organization is a
perspective phylogenetically important character with approximate resolution at level of orders.
Investigating the choanocyte kinetids of Oscarellidae sponges under transmission electronic
microscope and analyzing the literature data, we revealed that species of this genus form two
phylogenetically and morphologically distinct groups. The first group includes O. lobularis, O.
tuberculata, O. bergenensis, O. viridis, O. rubra, O. malakhovi, O. microlobata, O. carmela, and O.
filipoi and O. zoranja, these species have nuclei located in basal position without any connection to
the kinetosome. The second group includes O. balibaloi, O. nicolae, O. kamchatkensis, O. pearsei,
O. imperialis and Pseudocorticium jarrei, whose nuclei are connected with the kinetosome. In
addition, this second group also shares mesohylar cells harboring paracrystalline inclusions. Such
division corresponds to the presented phylogenetic reconstruction of the Oscarellidae family. The
kinetid structure of choanocytes seems to be a diagnostic trait that distinguishes two clades of
Oscarellidae, and should be included in further descriptions of new species. This new diagnostic
character might also help to revise the subdivision of the family Oscarellidae and call to transferring
a number of Oscarella species to the genus Pseudocorticium.