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Echinoderms
Jessica Harwood
Douglas Wilkin, Ph.D.
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AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
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Printed: July 15, 2015
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Jean Brainard, Ph.D.
Sarah Johnson
Jane Willan
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C HAPTER
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Chapter 1. Echinoderms
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Echinoderms
Give examples of echinoderms.
Discuss the major features of echinoderms.
Define radial symmetry.
Describe the water vascular system.
Summarize eating in echinoderms.
What is a sea cucumber?
A sea cucumber is not a vegetable! It is an invertebrate animal found in the ocean. Note the spines on this sea
cucumber. The spines are a key feature of echinoderms.
What are Echinoderms?
You’re probably familiar with starfish and sand dollars ( Figure 1.1). They are both echinoderms. Sea urchins
and sea cucumbers are also echinoderms. What’s similar between these three organisms? They all have radial
symmetry. This means that the body is arranged around a central point.
Echinoderms belong to the phylum Echinodermata. This phylum includes 7,000 living species. It is the largest
animal phylum without freshwater or land-living members.
Characteristics of Echinoderms
As mentioned earlier, echinoderms show radial symmetry. Other key echinoderm features include an internal
skeleton and spines, as well as a few organs and organ systems. Although echinoderms look like they have a
hard exterior, they do not have an external skeleton. Instead, a thin outer skin covers an internal skeleton made of
tiny plates and spines. This provides rigid support. Some groups of echinoderms, such as sea urchins ( Figure 1.2),
have spines that protect the organism. Sea cucumbers use these spines to help them move.
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FIGURE 1.1
A starfish (left) and a keyhole sand dollar
(right), showing the radial symmetry characteristic of the echinoderms. Starfish are
also known as sea stars.
FIGURE 1.2
Another echinoderm, a sea urchin (Echinus esculentus), showing its spines.
Echinoderms have a unique water vascular system. This network of fluid-filled tubes helps them to breathe, eat,
and move. Therefore, they can function without gill slits. Echinoderms also have a very simple digestive system,
circulatory system, and nervous system. The digestive system often leads directly from the mouth to the anus. The
echinoderms have an open circulatory system, meaning that fluid moves freely in the body cavity. But echinoderms
have no heart. The echinoderm nervous system is a nerve net, or interconnected neurons with no central brain.
Many echinoderms have amazing powers of regeneration. For example, some sea stars are capable of regenerating
lost arms. In some cases, lost arms have been observed to regenerate a second complete sea star! Sea cucumbers
often release parts of their internal organs if they perceive danger. The released organs and tissues are then quickly
regenerated.
How do Echinoderms Eat?
Feeding strategies vary greatly among the different groups of echinoderms. There’s no one food or technique that’s
shared by all echinoderms. Different eating-methods include:
1. Passive filter-feeders, which are organisms that absorb suspended nutrients from passing water. Some echinoderms use their long arms to capture food particles floating past in the currents.
2. Grazers, such as sea urchins, are organisms that feed on available plants. Sea urchins are omnivorous, eating
both plant and animals. The sea urchin mainly feeds on algae on the coral and rocks, along with decomposing
matter such as dead fish, mussels, sponges, and barnacles.
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Chapter 1. Echinoderms
3. Deposit feeders, which are organisms that feed on small pieces of organic matter, usually in the top layer of
soil. Sea cucumbers are deposit feeders, living on the ocean floor. They eat the tiny scrap particles that are
usually abundant in the environments that they inhabit.
4. Active hunters, which are organisms that actively hunt their prey. Many sea stars are predators, feeding on
mollusks like clams by prying apart their shells and actually placing their stomach inside the mollusk shell to
digest the meat.
How do Echinoderms Reproduce?
Echinoderms reproduce sexually. In most echinoderms, eggs and sperm cells are released into open water, and
fertilization takes place when the eggs and sperm meet. This is called external fertilization. The release of sperm
and eggs often occurs when organisms are in the same place at the same time. Internal fertilization takes place in
only a few species. Some species even take care of their offspring, like parents!
Vocabulary
• echinoderm: Invertebrate, such as a sea star or a sand dollar, that is characterized by a spiny endoskeleton,
radial symmetry as adults, and a water vascular system.
• nerve net: Interconnected neurons with no central brain.
• radial symmetry: Symmetry of a body plan in which there are no head or rear ends, so the body can be
divided into two identical halves at any point, like a pie.
• sea urchin: Echinoderm with a round, soft body enclosed in a shell covered with long spines.
• water vascular system: A network of fluid-filled tubes found in echinoderms.
Summary
• Echinoderms show radial symmetry and have an endoskeleton and a unique water vascular system. Some have
spines.
• Echinoderms generally reproduce by external fertilization; regeneration is fairly common among echinoderms.
Explore More
Use the resource below to answer the questions that follow.
• Echinoderms: The Ultimate Animal at http://vimeo.com/37295088 (13:54)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57297
1. What can sea star muscles do that our muscles cannot? How would this trait help sea stars living in the
intertidal zone?
2. How are deep-sea sea cucumbers like earthworms?
3. Why are brittle stars often found with their arms raised in the water current?
4. What organ system do echinoderms possess that is not seen in any other animal group?
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5. Where do sea stars have their "eyes"? How does this arrangement help them coordinate movement?
6. Some sea stars evert their stomachs to digest their prey, but Pycnopodia can do something else as well. What
does Pycnopodia do to some of its prey?
Review
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3.
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5.
List three examples of echinoderms.
What is radial symmetry?
What are two important characteristics of echinoderms (other than radial symmetry)?
How do sea urchins eat?
Give an example of an echinoderm that is an active hunter.
References
1. Starfish: Courtesy of Dr. James P. McVey, National Oceanic and Atmospheric Administration; Sand dollar:
Flickr:pics by Stefanie. A starfish and sand dollar are echnioderms. Starfish: Public Domain; Sand dollar: CC
BY 2.0
2. Gordon Milligan. Another echinoderm, a sea urchin, showing its spines. CC BY 2.0
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