Representatives of the phylum Porifera are considered as organisms lacking the nervous system. Indeed, the bioinformatic analysis has confirmed the absence of monoamines receptors in the genome of four investigated sponge species. However, in two species we have found genes of 1) the full set of enzymes required for serotonin and dopamine synthesis and 2) transglutaminases required for transamidation of proteins. These data indicate that sponge cells may realize a non-canonical pathway of monoamine action via post-translational protein modification. The HPLC data and immunochemical labeling indicate the prevalence of the dopaminergic pathway over the serotonergic one, though serotonin- and dopamine-immunoreactive elements occur in similar locations within the cells. Using immunochemical markers, we found scattered anti-monoamine immune labels in the apical part of the choanocytes in a few demosponges and in the basal part of the choanocytes in a calcarean sponge. We also found monoamine-positive granules near the flagella in epithelia of larvae of the calcarean Clathrina arnesenae and the freshwater demosponge Eunapius fragilis. Electron microscopic study allows us to conclude that monoamine-positive structures are concentrated in the Golgi apparatus in studied sponge larvae. The obtained morphological and biochemical data indicate a non-canonical intracellular way of monoamines action in the functional activity of sponge flagellar cells. Further studies are required to confirm this suggestion. The study was supported by RFBR grants #19-34-90084 and #18-04-01213.