A Scanning electron microscopy study of peculiar serpulid tube microstructures was carried out to determine whether this structure is unique to serpulids and to understand its formation and evolution. The isotropic aragonitic tube microstructure of some serpulids, termed spherulitic irregularly oriented prismatic structure (SIOP), is unknown in other animal skeletons. After nucleation, initial spherulitic prismatic aragonite crystals presumably grow in length and width at one end, collide with the surrounding growing crystal fans, and then continue intertwined growth until the secreted carbonate-charged mucus is completely crystallized. Spherulitic irregularly oriented prismati structure presumably evolved either from the IOP or homogeneous rounded crystal microstructures. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.