Within the skeletal structure of the medium- and the thick-walled Hexactinellids, the active filtration surface area was increased at expense of the elements of the irrigation system. The ostia and the transversal canal structures, the densities of their occurrences correlate with the values of the wall thickness, the skeleton habitus, the level of modular organization; they are in many ways determined by the parameters of the sponge habitats.
