Fissiculata, which are characterized by direct entrance to the individual hydrospires by way of slits; and Spiraculata, which are characterized by indirect entrance to the hydrospires through canals by way of pores. The earliest blastoid yet found, Macurdablastus from the Middle Ordovician of Tennessee, cannot be classified as either order. References External links Blastoids at UC Berkeley Museum of Paleontology Palaeos.com article Drawings and color reconstruction of Pentremites godoni blastoid at www.emilydamstra.com Three Dimensional Structure and Fluid Flow though the Hydrospires of the Blastoid Pentremites rusticus Blastozoa Ordovician echinoderms Devonian echinoderms Permian echinoderms Ordovician first appearances Permian extinctions Lopingian extinctions