Chapter by:

Jonathan R. Hendricks and Jaleigh Q. Pier, Paleontological Research Institution, Ithaca, New York

This chapter was first publicly shared on December 20, 2019.

Chapter citation:

J.R. Hendricks and J.Q. Pier. 2019. Echinodermata. In: The Digital Encyclopedia of Ancient Life.

Chapter contents:

–– 1. Blastoidea
–– 2. Crinoidea
–– 3. Asteroidea
–– 4. Ophiuroidea
–– 5. Echinoidea
–– 6. Holothuroidea

Associated materials:

Virtual Teaching Collection of 3D photogrammetry models of Echinodermata fossils available here!

Above image: Recent preserved Echinodermata specimens. Image by Jon R. Hendricks, licensed under a Creative Commons Attribution-Share Alike 4.0 International License.

Phylum Echinodermata Snapshot

  • Living classes: Crinoidea, Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea
  • Extinct classes: Blastoidea
  • Diversity: ~10,995 extant species, +30,325 extinct species
  • Ecology: exclusively marine, herbivore, carnivore, detritivore, filter feeder
  • Key features of group: 5-part pentaradial symmetry, endoskeleton of CaCO3 ossicles or plates, water vascular system, tube feet
  • Fossil Record: Cambrian to Recent


“Among the echinoderms we find some of the most beautiful of all sea creatures." (Nichols, 1967, pg. 15)

The phylum Echinodermata represents the largest animal phylum of exclusively marine animals and include animals as morphologically diverse as sea stars (Asteroidea), brittle stars (Ophiuroidea), sea cucumbers (Holothuroidea), sea urchins and sand dollars (Echinoidea), and sea lilies (Crinoidea). The name of the phylum is derived from the Greek "echinos," meaning spiny, and "derma," meaning skin. Echinoderms are deuterostomes and are therefore more closely related to chordates (which include ourselves) than they are to almost all other invertebrate phyla.

Image of animal phylogeny, highlighting the deuterostomes

Phylogenetic positions of the bilaterian clade Deuterostomia, characterized by the development of the anus and then mouth during embryonic development. Image by Jonathan R. Hendricks, licensed under a Creative Commons Attribution-Share Alike 4.0 International License.

Adult echinoderms lack a sensory head region; however, they do have a nerve ring and possess all forms of sensory perception. Some groups have a simple one-way digestive system, but that hasn’t stopped them from occupying a great range of feeding modes. They are one of the most successful invertebrate groups and some classes (especially crinoids and echinoids) have substantial fossil records.

'A Success Story Echinoderms' by: Shape of Life (YouTube).


Echinoderms are diverse group with a variety of body forms, making it difficult to understand how the various classes are related. Although early forms arose in the Cambrian, most modern classes have a long geologic record that extends back to the Ordovician.


Image of competing echinoderm phylogenetic hypotheses. A: Asterozoan hypothesis B: Cryptosyringid hypothesis. (Reich et al., 2015; Creative Commons Attribution-ShareAlike 4.0 International License).

Several competing hypotheses of echinoderm phylogeny have been developed over the years. In one, called the Asterozoan hypothesis, Ophiuroidea and Asteroidea are sister taxa and form a clade called Asterozoa, whereas Holothuroidea and Echinoidea form a clade called Echinozoa (with Crinoidea sister to Asterozoa + Echinozoa). In the second, called the Cryptosyringid hypothesis, Ophiuroidea is placed as the sister taxon to Holothuroidea + Echinoidea. Recent molecular phylogenetic analyses (Reich et al., 2015; Telford et al., 2014) have supported the Asterozoan hypothesis, which we similarly favor here.

Image of a simplified Phylum Echinodermata phylogeny

Highly simplified overview of Echinodermata phylogeny based in part on the hypothesis of relationships presented by Reich et al. (2015). Image by: Jaleigh Q. Pier, licensed under a Creative Commons Attribution-Share Alike 4.0 International License.

General Features:

 Pentaradial Symmetry

All adult forms of modern echinoderms have 5-point, or pentaradial symmetry, meaning that they have morphological features that are divisible by five. For example, starfish have five arms (or some multiple of five arms) and the calcareous endoskeletons (or, tests) of sea urchins can be similarly divided into fifths (see image below). Similarly, sand dollars have five "petals" on their dorsal side. What about the strange sea cucumbers? If you think of a sea cucumber as an elongated echinoid "test," they also possess pentaradial symmetry. It is important to note, however, that juvenile echinoderms often have bilateral symmetry (a synapomorphy for bilaterian animals; see here). Further, even though all extant classes of echinoderms have pentaradial symmetry, some extinct fossil forms from the early Paleozoic did not.

Image showing asteroidea and echinoidea pentaradial symmetry

Image of pentaradial symmetry of both Asteroidea and Echinoidea. Image by: Jaleigh Q. Pier, licensed under a Creative Commons Attribution-Share Alike 4.0 International License.

If the concept of pentaradial symmetry seems difficult to grasp, watch the video below, which highlights the different 5-part symmetry of major echinoderm classes.

'Echinoderm Animation: Five-part symmetry' by: Shape of Life

Endoskeleton of ossicles and collagenous tissue

Echinoderms have a true endoskeleton made up of ossicles, or tough plates of calcium carbonate that act as body support and protection. The dried-out skeleton is called a "test" or "theca". Holothuroideans (sea cucumbers) are made up of primarily collagenous tissue because their ossicles are microscopic in size. Ossicles are connected by ligaments which are made mostly of the protein collagen. Echinoderms have specialized mutable collagenous tissue meaning they are able to become very hard and rigid often to prevent predation and also can become soft and flexible to squeeze into tight hiding places. Even though echinoderms do have small muscles, most primarily move around with tube feet powered by their water vascular system.

Images of echinoderm tests and theca examples

Image of echinoderm tests (Ophiuroidea, Asteroidea, Echinoidea) vs echinoderm theca (Blastoidea and Crinoidea). Image by: Jaleigh Q. Pier, licensed under a Creative Commons Attribution-Share Alike 4.0 International License.

Water Vascular System

The water vascular system is important for almost all life functions for an echinoderm including movement, feeding, and respiration. Water pressure is controlled by this system to operate their tube feet, which all classes of echinoderms have. These suction-cupped appendages are permeable to allow transfer of oxygen and waste through the coelom (body wall). Watch the video below, which uses a sea star’s water vascular system as an example of how it works.

'Echinoderm Animation: Sea Star Body Plan' by Shape of Life


A fascinating characteristic of all echinoderms is the process of regeneration. All classes are able to regenerate lost body parts and many are able to reproduce in this way by dividing in half and regrowing limbs to create two new individuals. Both sea stars and brittle stars can regrow arms and also divide in half to produce two new individuals. Some species of sea cucumber are also able to divide in half to create new individuals and also can regrow their entire digestive tract after deploying their defense mechanism (more about this in the class Holothuroidea page!). Sea urchins are able to replace both spines and tube feet as well. Each of these regeneration methods will be discussed in detail on each echinoderm class page.


Photograph of a sea star regenerating at least seven arms (smaller, white-colored rays). Image by: Jon R. Hendricks, licensed under a Creative Commons Attribution-Share Alike 4.0 International License.

Fossil Record:

The Cambrian explosion led to several bizarre early echinoderms; however most early groups did not last very long. It wasn’t until the Ordovician that most of the modern echinoderm classes arose. Due to the delicate nature of their endoskeletons, body fossils are rare for for some echinoderm groups, while others (such as crinoids and echinoids) have very abundant fossil records.


Phanerozoic genus-level diversity of Asteroidea (graph generated using the Paleobiology Database Navigator).

Below is a brief description of what is most likely preserved for each echinoderm class discussed in this chapter:

  • Crinoidea: disarticulated stem ossicles most commonly preserved, in-tact stem and arms preserved under exceptional conditions.
  • Asteroidea: ossicle plates and endoskeleton commonly preserved, especially as molds.
  • Ophiuroidea: central disk and ossicles most likely to be preserved in-tact, arms and body fossil preserved under exceptional conditions
  • Echinoidea: loose spines and test most commonly preserved, body fossils under exceptional conditions.
  • Holothuroidea: rarely preserved, however loose ossicles can be common.

References and further reading:

Boardman, R.S., Cheetham, A.H., and Rowell, A.J. 1987. Fossil Invertebrates. Blackwell Scientific Publications. 713 pp.

Nichols, D., 1967. Echinoderms. Hutchinson University Library, London.

Reich, A., Dunn, C., Akasaka, K., Wessel, G. 2015. Phylogenomic Analyses of Echinodermata Support the Sister Groups of Asterozoa and Echinozoa. PLoSONE:10(3), e0119627. doi:10.1371/journal.pone.0119627

Telford, M.J., Lowe, C.J., Cameron, C.B., Ortega-Martinez, O., Aronowicz, J., Oliveri, P. and Copley, R.R. 2014. Phylogenomic analysis of echinoderm class relationships supports Asterozoa. Proc. R. Soc. B, 281.


Creative Commons License
Unless otherwise indicated, the written and visual content on this page is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. This page was written by Jaleigh Q. Pier. See captions of individual images for attributions. See original source material for licenses associated with video and/or 3D model content.