Examples of living echinoderm classes.
(Starfish, sea urchins, sand dollars, crinoids, blastoids)
Name: Echinodermata means "spiny" (echinos) + "skin" (derma).
Chief characteristics: Calcite skeleton with five-part symmetry, superimposed on primitive bilateral symmetry. Echinoderms have a water vascular system with water in a system of tubes within the body. Tube feet are soft, movable parts of the water vascular system which project from the body and are used in locomotion, feeding, respiration, and sensory perception.
Similarity of embryos between echinoderms and chordates suggests that they may be derived from a common ancestral form.Geologic range: Cambrian to Recent.
Mode of life: Exclusively marine. Some are attached to the seafloor by a stem with "roots" called holdfasts; others are free-moving bottom dwellers.
Examples of living echinoderm classes.
Crinoids are animals which resemble flowers - they consist of a calyx with arms, atop a stem of calcite disks called columnals. The crinoid is attached to the seafloor by root-like holdfasts. Some living crinoids are swimmers, and not attached. Over 1000 genera are known.
Geologic range: Middle Cambrian to Recent. Especially abundant during the Mississippian.
Diagram illustrating the major body parts of a crinoid.
Fossil crinoid.
Photo courtesy of Pamela Gore.
Crinoidal limestone, consisting mainly of crinoid columnals.
Reconstruction of crinoids growing on the seafloor during the Mississippian.
Blastoids are extinct animals with an armless bud-like calyx on a stem. About 95 genera are known. A common genus is Pentremites.
Geologic range: Ordovician to Permian - all extinct.
Paleozoic blastoids.
Starfish are star-shaped echinoderms with five arms. About 430 genera are known.
Geologic range: Ordovician to Recent.
Modern starfish.
Photo courtesy of Pamela Gore.
Partially dissected starfish showing parts of water vascular system.
Brittle stars have 5 arms, like starfish, but the arms are thin and serpent-like. About 325 genera are known.
Geologic range: Ordovician to Recent.
Echinoids are disk-shaped, biscuit-shaped, or globular. Viewed from above, they may be circular or somewhat irregular in shape, but with a five-part symmetry. About 765 genera are known.
Geologic range: Ordovician to Recent.
Modern echinoid (dried), with large spines still attached to the globular exoskeleton.
Photo courtesy of Pamela Gore.
Modern sand dollar (dried), with tiny fur-like spines still attached.
Photo courtesy of Pamela Gore.
Sea cucumbers are soft-bodied echinoderms resembling cucumbers. They have microscopic hard parts called sclerites in various shapes resembling hooks, wheels and anchors. About 200 genera are known.
Geologic range: Middle Cambrian?, Middle Ordovician to Recent
A group that was probably ancestral to starfish and sea urchins.
Globular, discoidal, or cylindrical tests (shells), many of which had concave surfaces.
Geologic range: Early Cambrian to Middle Pennsylvanian.
A Middle Ordovician edrioasteroid.
This primitive group had a calyx attached to the seafloor by a stem (like crinoids and blastoids). Distinctive patterns of pores on the plates of the calyx.
Geologic range: Cambrian to Late Devonian. Most common in Ordovician and Silurian.
Echinoderms are closely related to chordates (the group that includes the vertebrates).
The early cell division, embryonic development, and larvae of echinoderms resemble those of chordates, and are different from those of other invertebrates.
Biochemistry of echinoderms is also similar to that of chordates (chemical similarities associated with muscle activity and chemistry of oxygen-carrying pigments in the blood).
CLASS GRAPTOLITHINA (Graptolites)
Name: Graptolite means "write" (grapto) + "stone" (lithos), because they resemble pencil marks on rock.
Chief characteristics: Organic (chitinous) skeletons consisting of rows or lines of small tubes or cups, called thecae. Tubes or cups branch off a main cord or tube called a stem or stipe. Stipes may consist of one, two, or many branches. Entire colony called a rhabdosome. A filament at the lower end of the rhabdosome is called a nema.
Most graptolites are found flattened and carbonized in black shales and mudstones.
Geologic range: Cambrian to Mississippian. (Most abundant in Ordovician
and Silurian.)
Some living organisms which may be surviving descendants (living fossils) have been recovered
in 1989 in the South Pacific and later in Bermuda.
Mode of Life: Planktonic (colonies attached to floats).
Graptolites. Photos courtesy of Pamela Gore.
January 6-12, 2006