Subphylum of extinct, stalked echinoderms, ranging from the Silurian to the Permian. The stem (rarely preserved) was surmounted by a bud-like theca with five prominent petaloid ambulacra showing a well-developed pentameral symmetry. The theca had thirteen major plates arranged in three circlets. The ambulacra each had a central food groove, with delicate food-gathering appendages called brachioles to either side. Underlying the ambulacra were the characteristic hydrospires, thought to have functioned in respiration. These were thin-walled, calcareous infoldings (more rarely simple tubes), forming water conduits that communicated with the outer environment via marginal pores in the ambulacra and five apertures around the mouth (the spiracles). Blastozoa are not common as fossils, except locally in shallow-water calcareous rocks, often associated with reef limestones.