Blastoids range from Ordovician to Permian in age and are particularly abundant in Carboniferous deposits. They are typically short stemmed or stemless pelmatozoans.
The Calyx: contains the vital organs of the animal and is typically a rigid bud-like structure composed of 13 plates: 3 basals, 5 radials and 5 deltoids. Five exposed food grooves radiate symmetrically from a central mouth or peristome (duct leading to the mouth) and extend well down the theca.
The Arms: minute grooves extend at right angles to the main food or ambulacral grooves which lie on the median line of the lancet plate. Rising erectly upwards from the side plates are the brachioles or slender uniserial structures analagous to the crinoid arms. The food grooves continue along the brachioles. Food particles acquired at the end of the brachioles were swept along the grooves by cilliary action to the base of the brachiole and on to the main food groove ebnding at the mouth. The mouth is surrounded by 5 openings - the Spiracles, one of which was divided to provide an anal opening.
The Stem: supports the animal and served as a means of attachment to the sea bed.
A unique feature of the Balstoids was the presence of a folded elongate vessel suspended into the thecal cavity - the hydrospires which are thought to have served as a respiratory function.