The hypothesis of constancy of size characters of irrigation system module in sponges was mathematically tested. Populations of common sublittoral species Polymastia mammilaris (Demospongiae, Tetractinomorpha) from ecologically different habitats of White Sea were inverstigated. It was shown that increase in body size is not always correlated with the increase in number of oscular papillae, that marks hypothetical modules. On mixed grounds in contrast to the solid ones dominate one-osculum individuals. The greatest variability of body volume for one-osculum sponges was observed exactly on mixed ground. The number of oscular papillae for a volume unit is not constant, but clearly decreases while the volume increases. From the other hand, the volume per one osculum (the volume of irrigation system module) grows up with the increase in body size. Thus, one-osculum irrigation system of P. mammillaris can provides the vital functions of individual of any size. Hypothetical module of the individual is not clearly characterized by size.