Mike Rubin

Starfish, class Asteroidea
Starfish, class Asteroidea

Diagnostic Characteristics that Define Echinoderms

Echinoderms are slow-moving animals. They're best distinguished by their body's radial symmetry and water vascular system, which provides hydraulic pressure for their unique foot cups. Echinoderms are know for their radial symmetry; however, their larvae have bilateral symmetry. As the larvae mature, their body shape changes, and they settle (JL).Their exterior is covered by small spiny bumps from which they are named ("echin," Greek for spiny and "derma" for skin). Echinoderms have a thin skin over their inner skeletons. (AZ) Echinoderms are also characterized by multiple arms (usually 2 on the right, 1 in the middle, and 2 on the left) which stem from a main body cavity (pentamerous). They body cavity contains 5 segments that each contain their own set of internal organs (they have no heart, brain, nor eyes). (AK) [They are a phylum of marine animals and are found at every ocean depth, from the intertidal zone to the abyssal zone. The first definitive members of the phylum appeared near the start of the Cambrian period. RS]

Echinoderms are split into six main classes:
  • Asteroidea (sea stars) - have five arms radiating from their central disk. Under their arms are suction-like tube feet. Starfish use their tube feet not only for locomotion but also to grasp prey. They are capable of regeneration, growing back lost arms. They can even reproduce by regeneration, growing a whole new starfish from a single arm. (AZ)
  • Ophiuroidea (brittle stars) - have distinct central disks. Their tube feet lack suckers and they move by snake-like lashing of the arms. Some feed by passively floating in the water, while others actively hunt or scavenge. [In these flexible starfish, there is no replication of internal organs. The only set is located iin the central disk. Also, there is no anus in brittle starts, and waste is released through the mouth, which is located in the center of the underside. Lastly, there are ten splitlike openings located at each end of every arm, where breathing and reproduction occurs. (DG)].
  • Echinoidea (sea urchins, sand dollars) - have no arms, but do have five rows of tube feet. The mouth is ringed by complex jaw-like structures for eating seaweed. Sea urchins are somewhat spherical and spiky, whereas sand dollars are disk-shaped and smooth.
  • Crinoidea (sea lilies, feather stars) - have more than five arms, which they use in suspension feeding. They have not changed much over the last 500 million years. Crinoids are pentamerous, or consisting of or divided into five parts, and are stalked, and their five arms are usually branched and featherlike. Most of a crinoid's body consists of an endoskeleton composed of numerous calcareous pieces, called plates or ossicles. The mouth and anus are located on the upper, or oral, surface of the echinoidea. (SW)
  • Holothuroidea (sea cucumbers) - do not resemble other echinoderms on the outside. Their endoskeleton is much reduced. Instead of looking like they have a radial body plan, they are elongated in the oral-aboral axis. However, they do have five rows of tube feet. Sea cucumbers move by means of these tube feet which extend in rows from the underside of the body. The tentacles surrounding the mouth are actually tube feet that have been modified for feeding. (JJF)
  • Concentricycloidea (sea daisies) - were recently discovered and live on waterlogged wood in the deep sea.

external image echinowatervasc.jpg

Acquiring and Digesting Food

Echinoderms have a very short digestive system. It starts at the mouth, which is at the middle of the radial bodies. Then the food goes into the stomach, where the digestive glands secrete enzymes to break down the food; the digestive glands also provide extra surface area for absorption of nutrients. Echinoderms have two stomachs, the cardiac and pyloric stomachs. It varies which stomach digests which food, but some starfish can remove their cardiac stomachs from their main body's in order to externally digest (Walker K.). The undigested food is then secreted through the anus at the top of the body. Starfish can turn their stomachs inside out and insert it into their prey.
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Above is picture of a starfish eating its prey.(CP)

Sensing the Environment

Echinoderms sense their environment by touch; the message goes from their radial nerves to the central disk. They have a decentralized nervous system and have no brain. Radial nerves under the ambulacra (the radial arranged bands) contain most motor neurons and interneurons. The radial nerves are connected by a central nerve ring that surrounds the gut (JM). Also, the starfish is quite sensitive to smell. It's skin contains chemoreceptors that can determine the faintest smell of prey and from which direction it is coming. Starfish, in addition, can tell the difference between light and dark through light-sensing dots (eyes, if you will) on the tip of each arm. [KS]


Locomotion is what echinoderms are most distinguished for. They contract the bulb-like ampulla at the top of the muscular tube foot, which acts like a suction cup to keep it on the ground; each limb has a series of tube feet, all attached to one ring canal. When the echinoderm wants to detach the limb from the ground, it opens its madreporite (its water-filtering opening) to equalize the hydraulic pressure. This whole system is called the water vascular system.
When an echinoderms is turned over or moved from its normal postition, it will exhibit a "righting response" that may consist of using their arms to flip over in a somersault-like movement or burrowing into the sand and then toppling over. (SF)


An echinoderm uses some of the bumps or spines on its surface take in oxygen. Echinoderms also have series of very small gills, which are able to take in oxygen. The respiratory system of echinoderms in poorly developed. (LS)
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example of gills on a echinodermata (LD)

Metabolic Waste Removal

The echinoderm releases its metabolic wastes through the anus, which is at the end of the digestive system. [Echinoderms have a mouth and stomach area on the bottom and top, respectively. In terms of stomach area, there is the oral (cardiac) stomach, which receives ingested food, and the aboral (pyloric) stomach, which connects to the secretory and absorptive structures. Starfish have the ability to extend the larger oral stomach outside their body to extracellularly digest prey. (BY)]


Echinoderms have water pumped through their bodies as part of a very simple circulatory system. This circulatory system is also known as the Hemal System, which is made up of canals that serve to transport water and nutrients throughout the body.(Jake Schwartz)
Shows the central ring canal which all of the other canals lead to.
Shows the central ring canal which all of the other canals lead to.

Self Protection

Echinoderms' skin is covered in a series of spiny protrusions, which is the only system of defense other than unpleasant taste. Sea urchins' spines are long and sharp. Being stabbed by a sea urchin results in painful injury for predators.
Holothuroidea discharge long, sticky tubes when attacked. These are called Cuvierian Tubules. Sea cucumbers have a habit of self-evisceration, meaning that when it is attacked it can expel some if its internal organs. (KD) Also, most Echinoderms are much less protected in their larval stage (as is typical of type 3 survivorship curves) (DRM).

Osmotic Balance

Echinoderms are osmoconformers, which are isoosmotic to their surroundings to maintain osmotic balance. Osmoconformers do not tend to gain or lose water to their environments. Echinoderms are stenohalines; they can't tolerate large changes in external osmolarity.

Temperature Balance

Echinoderms have no special adaptations to maintain a constant body temperature and conform to the temperature of the environment.

Review Questions:
1) Echinoderms are divided into which six groups? Describe them. [Rabya S.]
2) Describe the path that food would travel through the digestive system of an echinoderm. (APS)
3) When echinoderms want to move, what part of the body is responsible for helping them to "release" the ground from their suction? (AS)


Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002.
"DIGESTION." Echinodermata. Web. 25 Oct. 2009. <>.(Walker K.) (JAS) (Jesse Landy) [BY] (KNS) (Sarah Fleming) (AG) (Alyssa Zisk) Donna McDermott) (Jackson Murphy) (Ali Kirsch) (Sara Waugh) Schwartz) (KD) (JAF) (JJF) (Liz Daley)