AbstractBlastoids (phylum Echinodermata; class Blastoidea) pass through a stage during growth in which the oral surface of the minute theca is composed of five oral plates only. The evidence that these thecae, which lack hydrospires, deltoids and lancet plates, are juvenile blastoids is that they occur together with mature blastoids and vice versa; they have growth lines on the exterior of the plates; and the outlines of their thecal plates are congruent with the earliest growth lines of the thecal plates of blastoids. It is proposed that this growth stage should be referred to as the passalocrinid stage, because the juvenile blastoids at this stage have previously been referred to Passalocrinus, originally described as a monocyclic crinoid. Coronoids also pass through a stage comparable to the passalocrinid stage of blastoids. Copyright © 2005 John Wiley & Sons, Ltd.