E.S.C.O.N.I.

BLASTOIDS--AN INTRODUCTION

Blastoids are small to medium sized stemmed echinoderms that contain distinctive structures known as hydrospires. All members are attached to the substrate for at least part of their lives.

Conventional orientation views the theca from the oral side (summit) with the anispiracle (anal opening) toward the observer. The ambulacra opposite the anal opening is designated "A" with the other ambulacra designated clockwise "B", "C", "D", and "E". The anispiracle (posterior) is thus the "CD" interradial area.

Blastoid morphology consists of the anthus (crown) which includes the theca (calyx) and the brachioles, a stem, and a holdfast system. The theca, which contains all internal structures such as the hydrospires, consists, in all but one genus, of 18-25 major plates arranged in four circlets: 3 basals, 5 radials, 5 lancets, 5-8 deltoids. In the living animal there were also tens (possibly hundreds) of thousands of minor cover and brachiolar plates. The brachioles act as the initial food gathering structures and consist of a median grove with cover plates. The hydrospires, internal semipermeable folds of calcite, served several functions in the living animal. They almost certainly served a respiratory function and may have contributed in reproduction since structures interpreted as eggs have been found in enlarged hydrospire folds to the left of the anispiracle in one species of Pentremites.

The blastoid water vascular system involves hydrospire pores and spiracles, hydrospire slits and spiracles, or hydrospire slits alone. Presumed pathway would be in through the pores or slits to the hydrospires and out through the spiracles or slits. The hydrospires provided the gas exchange capability needed in respiration.

Blastoids were passive suspension feeders (feeding mainly on planktonic organisms) that inhabited clear to silty, moderately agitated waters from shelf to basin. The food gathering system of blastoids was elaborate and consisted of several types of grooves that transported food to the mouth at the summit of the theca. Food entered the brachiolar grooves, transferred to the side food-grooves through the brachiolar pit, then transferred to the main (median) food-groove, and finally entered the mouth. Each of these grooves were 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. Waste elimination was through the anispiracle, an opening formed by the fusing of anus and adjacent spiracles. A series of oral, anal, and spiracle plates prevented mixing of the various fluids.

Blastoids are subdivided into two subclasses: Fissiculata which are characterized by direct entrance to the individual hydrospires by way of slits; and Spiraculata which are characterized by indirect entrance to the hydrospires through canals by way of pores. The earliest blastoid yet found, Macurdablastus from the Middle Ordovician of Tennessee, cannot be classified as either subclass. Blastoids reached their peak in the Mississippian and became extinct at the end of the Permian.

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Last Updated 01/28/2002