At larval and adult stage of life cycle, sponges (Porifera) have different flagellated cells that play different roles in their body. The larval epithelium cells serve as locomotion organs, while adult collar cells (choanocytes) have a feeding function. Here, for the first time, we describe detailed structure of the flagellar apparatus (kinetid) in larval cells and choanocytes of Haliclona aquaeductus and Halichondria panicea demosponges, and attach schemes of its organization, based on longitudinal and transversal ultrathin serial sections. The kinetid of larvae has proved to be more complicated than the choanocytes ones. It includes well-developed transverse and longitudinal cytoskeletons that strengthen the larval flagellum. The longitudinal skeleton in H. aquaeductus is represented by unusual structures, which were previously understudied: large tubular rootlets made of oblique-crossed fibers, which connect the kinetosome with mitochondria. We also found out that the kinetid composition varies between different cell types of a highly structured parenchymella of H. aquaeductus. In simply organized parenchymella of H. panicea, flagellated cells are identical and its kinetid includes a permanent additional centriole absent in choanocytes of adult sponges. We tried to evaluate the diversity of sponge larval kinetid organization based on literature data and found it variable within Haplosclerida and quite conservative within other sponge lineages.