The synthesis of siliceous spicules in both demosponges and hexactinellids is enzymatically driven via silicatein. This enzyme exists both intra-spicularly and in the extra-spicular space. It catalyzes the formation of bio-silica constituting the silica lamellae that are formed during the appositional (layer-by-layer) growth of the spicules. The extent of (bio-silica forming) activity of silicatein from the demosponge Suberites domuncula measured in vitro reflects the amount of bio-silica required for the formation of spicules in vivo. It is shown that during growth and maturation of the spicules in demosponges a bio-fusion process occurs that results in an intra-spicular sintering of the silica lamellae to form compact silica rods. The morphological characterization of the globular asters (microscleres) from the demosponge Geodia cydonium revealed that these spherical spicules are formed from a tuft of silicatein filaments which radiate from a common origin. While in demosponges a complete intra-spicular bio-sintering/bio-fusion process occurs, this process is incomplete in hexactinellid spicules. There, only the most inner lamellae of the spicules fuse leaving the more peripheral silica lamellae separate. However, within this class of sponges several families (example Euplectella aspergillum) show a bio-sintering process between individual spicules. There, bio-silica is secondarily deposited onto mature spicules, giving rise to an ordered array of bio-silica bridges. Furthermore we report that for the formation of the strong and stiff bio-silica skeleton of sponges a hardening process is required that is (presumable) driven by cell-membrane bound aquaporin channels which allow the removal of water, released during the bio-silica polycondensation reaction.