Sclerocytes are spicule

Spicule

Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....

 secreting cells, found in sponges. They secrete calcareous or siliceous spicules which are found in the mesohyl

Mesohyl

The mesohyl, formerly known as mesenchyme, is the gelatinous matrix within a sponge. It fills the space between the external pinacoderm and the internal choanoderm...

 layer of sponges. The sclerocytes produce spicules via formation of a cellular triad. The triad of cells then undergo mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

, creating six sclerocytes. In pairs, the sclerocytes secrete the minerals which create the spicules.

• http://acd.ufrj.br/labpor/4-Bibliografia/Pdfs/Imsiecke95a.pdf