The species of earthworms included in the Lumbricidae family have a very complex esophageal organ called calciferous or Morren gland, being the representatives of the genus Lumbricus the individuals where that gland is better developed. Previous studies have shown the secretion produced in the glandular structure is not crystalline but amorphous in the form of amorphous calcium carbonate (ACC) spherulites. Afterwards the spherulites undergo a crystallization process to form rhombohedral calcite crystals that finally result in macroaggregates of these crystals. This crystallization process can go through two intermediate states, vaterite or aragonite. These changes happen on the surface of the amorphous phases.