Like most echinoderms, blastoids were protected by a set of interlocking plates of
calcium carbonate, which formed the main body, or
theca. In life, the theca of a typical blastoid was attached to a stalk or column made up of stacked disc-shaped plates. The other end of the column was attached to the ocean floor by a holdfast, very much like stalked
crinoids. The stalk was usually relatively short, and in some species, was absent, with the holdfast being attached directly to the base of the theca. The mouth was at the summit of the theca. Radiating like flower petals from the center were five food grooves, or
ambulacra. Each ambulacrum had many long thin fine structures called
brachioles, which were used to trap food particles and bring them to the mouth. Brachioles were delicate structures, and in fossils are not usually preserved in place. A series of five spiracle plates surrounded the star-shaped mouth, which included the
anus, mouth and entrances to a set of five complex, folded
respiratory organs known as
hydrospires. These spiracles prevented mixing of the various fluids. Waste elimination was through the
anispiracle, an opening formed by the fusing of anus and adjacent spiracles. blastoid
Hyperoblastus from the
Milwaukee Formation;
Wisconsin; lateral view of theca. Like crinoids, blastoids were high-level stalked suspension feeders (feeding mainly on
planktonic organisms) that inhabited clear-to-silty, moderately agitated ocean waters from
shelf to basin. The food gathering system of blastoids consisted of several types of ambulacra. Food entered the brachiolar ambulacra, was transferred to the side ambulacra through the brachiolar pit, then transferred to the main (median) ambulacra, and finally entered the mouth. Each of these ambulacra was roofed by cover plates. The cover plates of the brachiolar groove were movable and could open, allowing food to enter, or close as needed. Other cover plates may also have been movable. ==Taxonomy==