The Blastoid Pentremites

Genus Pentremites, of Carboniferous age, belongs to phylum Echinodermata, which includes two main types of echinoderms: those attached (e.g., blastoids, crinoids, cystoid) to the ocean bottom and those that are free-to-move around (e.g., starfish, brittle stars, sea urchins, and sand dollars). All echinoderms have a complex water-vascular system, which is an internal apparatus containing watery fluids. This apparatus resembles nothing else in the animal kingdom. Its primary function is to operate the tube feet, which serve food gathering, locomotion (in mobile echinoderms), and sensory functions, as well as respiratory requirements and burrow building.

The skeleton of echinoderms consists of porous calcite (calcium carbonate) plates, which are spiny and covered by a thin skin. The echinoderm skeleton is a product of internal secretion, like the bony skeleton of vertebrates, and the individual hard parts increase in size during the life of the echinoderm.

A sketch of the side view of a blastoid echinoderm Pentremites, which is characterized by having three main body sections: calyx, column (stem), and roots.

Sketch of the dorsal view of a blastoid showing the position of the mouth and the 5-ray symmetry characteristic of the calyx, with five ambulacral areas and five spiracles. The ambulacral areas are covered by feeding grooves, used for transporting food particles upward to the top of the calyx, where the mouth was positioned. The spiracles were used regulating the pressure of seawater that travelled thorough the porous calyx.

Sketch of the side view of two of the ambulacral areas of a calyx.

Side view of a specimen showing two of the ambulacral areas of a blastoid. Actual specimen is diameter 13 mm. 

Dorsal view of a specimen showing the five actual ambulacral areas and spiracles. Specimen slightly crushed, diameter 14 mm.