Respiratory systems as a key to a new superorder division of the class Blastoidea (Echinodermata)

Blastoids have three primary systems providing entrances to blastoid hydrospires, the primary organ for respiration: (1) exposed hydrospire slits formed across the width of the radiodeltoid suture; (2) hydrospire pores formed at the aboral ends of the ambulacra; and (3) hydrospire tubules formed as invaginations along the radiodeltoid suture, becoming openings that pierce the radials and deltoids ontogenetically. Blastoid classification historically divided the blastoids into two groups—the Fissiculata and Spiraculata. The Fissiculata comprised those blastoids that have exposed hydrospire slits or spiracular slits. The Spiraculata had hydrospire pores and spiracles that connect internally to hydrospires. Spiraculate classification focused on the configuration of the spiracles and anispiracle in combination with thecal form. Spiracles are the adoral consequence of the ambulacra infilling the radial sinus and covering the hydrospires by the lancet and the side plates and are found in all spiraculate blastoids. In this revision of blastoid classification, we place primacy on the three mechanisms by which water is drawn into the hydrospires—hydrospire slits open to seawater, hydrospire pores, and hydrospire tubules. Hydrospire tubules are formed along the radiodeltoid suture, a very different ontogenetic position from hydrospire pores, which are formed at the aboral end of the ambulacrum, and a fundamental phylogenetic difference. We herein abandon the term Spiraculata and refer to the spiraculate grade as being the Stomatoblastida, new superorder for spiraculates with hydrospire pores and the Tubuloblastida, new superorder for spiraculates with hydrospire tubules. The Fissiculata is elevated to superordinal status.