Echinoderms, Spiny Skins

The phylum Echinodermata ("spiny-skins") consists of about 6,000 species of exclusively marine organisms which include such well-known animals as starfish and sea urchins. The description "spiny-skins" is derived from the calcareous (chalky) plates that form the endoskeleton of these animals, which in some species is covered with spines. These spines are particularly prominant in sea urchins. The most obvious feature of these animals is their five-rayed, or pentamerous symmetry.

There are six classes of echinoderms: Crinoidea, Asteroidea, Ophiuroidea, Echinoidea, Holothurioidea and Concentricycloidea. All are radial, except for the holothurians, which evolved from a radial ancestor. Among the actinomorphic echinoderms, most are pentameral animals, and the other symmetry forms are derived from pentameral symmetry.

The ancestors of echinoderms, which originated from the Cambrian era, were deuterostomes. Because deuterostomes are all bilateral, we can infer that the ancestors of echinoderms were bilaterians. To adapt to their benthonic habitat and planktonic habitat niches, echinoderms evolved from bilateral symmetry first to asymmetry, then to pentameral symmetry.9


Starfish, or sea stars (class Asteroidea), are te most familiar echinoderms. They consist of a cenral disk from which five arms arise (although some species have more than 5 arms). Tube feet are also used to deal with prey which usually consists of bivalves such as clams and oysters. Within each arm there is also a pair of gonads (sex organs). Reproduction in most echinoderms is a simple process. The sexes are separate and sperm and eggs are released into the water, where fertilization occurs. The larvae that hatch are called bipinnariae and metamorphose gradually into the adult form. In large numbers, starfish can have a devastating ecological effect as some species destroy whole areas of coral reef.


Most known starfish species possess a compound eye at the tip of each arm, which, except for the lack of true optics, resembles an arthropod compound eye. Studies into the visual abilities of the Asteroidea (Echinodermata) have recently shown that species within this class have a more developed visual sense than previously thought and it has been demonstrated that starfish use visual information for orientation within their habitat. Crown-of-thorns starfish has true spatial vision.7,8

Brittle Stars

Brittle stars (Ophiuroidea) are the most diverse group of echinoderms comprising over 2000 species with a global distribution. The name Ophiuroidea is derived from the Greek words ophis, meaning snake, and oura, meaning tail, in reference to the often thin, snail-like winding or coiling arms. The typical ophiuroid body plan shows a pentagonal to round central disc that is offset clearly from the five arms; but a considerable number of species depart from this generalized shape. Species with six, seven and up to ten arms are known.6

The diversity of brittle starsImage source: PLoS One. 2012; 7(3): e31940. doi: 10.1371/journal.pone.0031940 A. Ophiolepis superba, a typical five-armed form with simple arms; B. Ophiacantha enopla veterna, a form with long serrated arm spines and spinelets covering the disc; C, Ophiactis tyleri, a six-armed fissiparous form; D. Euryale aspera, a basket star with branched arms. Scale bars in millimetres.

Species of brittle stars (class Ophiuroidea), which occur at all depths of the ocean, are easily distinguished from starfish by the sharp demarcation of their central disk and by their very long arms. They also differ from starfish in several other ways. Locomotion is achieved not by tube feet but by muscular movements of the arms. Most brittle stars feed on detritus and small organisms but do not have intestine or anus, and indigestible fragments are ejected through the mouth. Starfish and sea urchins breathe by means of skin gills, which are located all over the body surface, whereas brittle stars use respiratory pouches (bursae) which occur near the arm bases. Adult brittle stars are able to regenerate their entire arms.

Sea Urchins

Sea Urchin

Most sea uchins (Echinoidea) have a globular body, with the skeletal plates fused together to form a hard shell (the test). In most species the body is futher protected by sharp spines which may be poisonous. The spines of some sea urchins are also used for boring holes in coral or rock into which the animals wedge themselves to prevent removal. In addition to spines, the skin of urchins and asteroids carries tiny, pincer-like structures on stalks, called pedicellariae, which are used for defense, catching small prey and cleaning the body surface. Sea urchins are omnivorous and often scavenge on organic debris. The mouth is on the underside (oral surface) and the anus on the upper side (aboral surface). Sea urchins possess a set of five teeth which are self-sharpening and which continuously replace material lost through abrasion. They are quite long and slender with the exception of those in the sand dollars.

Adult sea urchins are grazers and have been dominant herbivores in many Caribbean reefs for decades and perhaps millennia. The purple sea urchin, Strongylocentrotus purpuratus, has an innate immune system that is complex and sophisticated with a number of large gene families that provides significant anti-pathogen capabilities in these these animals.

Sea Cucumbers, Sea Lilies

Sea Cucumber
Sea Cucumber
Photo courtesy of E. Eugenia Patten 1999 California Academy of Sciences

As their names suggests, sea cucumbers (Holothuroidea) are sausage-shaped echinoderms, elongated along the oral-aboral axis. They have soft bodies, covered with a glandular skin, and the skeletal plates have been reduced to microscopic ossicles. Sea cucumbers burrow in sand. To breathe, they have tubes known as "respiratory trees" which carry seawater into the body from the anus, so that gaseous exchange can occur internally. The podia and skin surface are also used for respiration.

Sea Lilies
Sea Lilies

The lilies and feather stars (class Crinoidea) are among the most primitive of echinoderms. They resemble a starfish hat has been inverted and (in the case of sea lily) attached to the sea bed by a stalk. The sea lily skeleton which stands on a stalk, is cup-shaped with arms. Tube feet are present in a groove on the upper surface of he arms, but they are used solely for respiration. Food (usually plankton) is collected by the arms and directed to the mouth on the upper surface by cilia. The feather stars resemble sea lilies, and, like them, develop from the larval stage into an attached form. They differ from sea lilies in that they are free-living as adults, either creeping along the sea o swimming.

Pink Starfish

Sea Daisies

Sea Daisies (Concentricycloidea) were discovered only in 1986 in deep seas off New Zealand. These echinoderms inhabit bacteria-rich wood sunk in deep water and apparently ingest bacteria through their body surface. Their small, disk-shaped flat bodies have no arms or mouth. The class includes two species: South Pacific Xyloplax medusiformis and the tropical Atlantic X. turneri.10


  1. Mike Janson and Joyce Pope (consultant editors). The Animal World
  2. CalPhotos Photo Database
  3. U.S. Fish and Wildlife Service Multimedia database
  4. Public Health Image Library (PHIL) Photographs, Illustrations, Multimedia Files
  5. Extraordinary Diversity of Immune Response Proteins among Sea Urchins: Nickel-Isolated Sp185/333 Proteins Show Broad Variations in Size and Charge Lauren S. Sherman,1,¤a Catherine S. Schrankel,1,¤b Kristy J. Brown,2 and L. Courtney Smith1. PLoS One. 2015; 10(9): e0138892.
  6. Global Diversity of Brittle Stars (Echinodermata: Ophiuroidea). Sabine Stöhr,1,* Timothy D. O'Hara,2 and Ben Thuy3. Vincent Laudet, Editor PLoS Onev.7(3); 2012
  7. Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Anders Garm1 and Dan-Eric Nilsson2 Proc Biol Sci. 2014 Feb 22; 281(1777): 20133011
  8. Crown-of-thorns starfish have true image forming vision. Ronald Petie,corresponding author1 Anders Garm,1 and Michael R. Hall2 Front Zool. 2016; 13(1): 41.
  9. Echinoderms Have Bilateral Tendencies. Chengcheng Ji,#1,* Liang Wu,#1 Wenchan Zhao,1 Sishuo Wang,1 and Jianhao Lv2. Hector Escriva, Editor PLoS One. 2012; 7(1): e28978.
  10. Biology. Eldra Solomon, Linda Berg, Diana W. Martin

Keywords: Sea cucumbers, sea lilies, sea urchins, starfish, Seegurken, Seelilien, Lanzenseeigel, Seestern, ophiuroids, crinoids



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