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Echinoderms - Advanced
Douglas Wilkin, Ph.D.
Jennifer Blanchette
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Printed: December 23, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jennifer Blanchette
www.ck12.org
C HAPTER
Chapter 1. Echinoderms - Advanced
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Echinoderms - Advanced
• Understand the characteristics that distinguish echinoderms from other animals.
• Learn about how deuterostomes, such as echinoderms, are different from protostomes.
Believe it or not, this is an animal. See the mouth and arms?
It is a sea lily, a crinoid echinoderm. Crinoids are essentially a mouth on the top surface that is surrounded by
feeding arms. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have
many more than five arms. Crinoids usually have a stem used to attach themselves to a surface, but many become
free-swimming as adults.
Characteristics of Echinoderms
Echinoderms are marine organisms that make up the phylum Echinodermata. Members of the phylum include sea
stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and feather stars. There are 7000 different
living species of echinoderms and 13,000 identified fossil species. Although the number of echinoderm species is
very small compared to the number of species within the arthropod phylum, Echinodermata is the largest phylum to
lack freshwater and terrestrial species. Another distinction is that echinoderms are among the most distinct organisms
within the animal kingdom. You learned in the Evolution concepts that evolution is not a linear series of steps and
does not necessarily lead to increasing levels of complexity over time. It is based on the response of populations of
species to the adaptive pressures of their environments and favors those that have the greatest reproductive success.
The phylum Echinodermata provides an excellent example of the non-linear, bush-like trajectory of evolutionary
change. Echinoderms are the most closely related phylum to the phylum Chordata, which includes many complex
organisms such as humans. Their shared common ancestor was likely a bilaterally symmetrical organism with a
cephalized (centralized in a head region) nervous system. Yet, as you will learn in this lesson, many details of
echinoderm structure and function are simpler than those of this likely predecessor. The organisms within this
phylum evolved these structures as adaptations to the pressures placed on them by their environments, just as the
various groups within the phylum Chordata did. You will see in this chapter that the results of these two evolutionary
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paths are very different, despite having a fairly recent common origin. Let’s take a look at some of the distinguishing
features of echinoderms, and then we will examine their structures and how they function in more detail.
The word Echinodermata means “spiny skin.” If you have ever seen a sea urchin, like the one shown in the Figure
1.1, then this name will certainly seem fitting. Although these spines may look like components of an exoskeleton at
first glance, echinoderms do not have an exoskeleton. Instead, the spines are extensions of an internal endoskeleton.
The endoskeleton is composed of calcium carbonate plates and spines that are actually covered by a thin layer of
epidermis (skin). These plates and spines are held together by mesodermal tissue.
FIGURE 1.1
A spiny sea urchin species. Notice the
many long spines that look like exoskeletal projections. These spines are actually
part of the echinoderm’s endoskeleton;
a layer of epidermal tissue covers each
spine.
As mentioned earlier, echinoderm species, such as those shown in the Figure 1.2, look very different from most
other invertebrates. This is partly due to their symmetry and their spiny endoskeleton.
FIGURE 1.2
Various types of echinoderms. (a) Feather star. (b) Sea star (starfish). (c)
Brittle star. (d) Sea urchin. (e) Sand dollars. (f) Sea cucumber.
Most of the phyla we have discussed so far (except sponges and cnidarians) exhibit bilateral symmetry, meaning that
they can only be divided into two equal halves by a single cut along the middle of the anterior-posterior (front-back)
axis. Although echinoderms evolved from a bilateral ancestor, adult echinoderms exhibit five-fold radial symmetry
called pentameral radial symmetry, which is depicted in the Figure 1.3. This means that their bodies have five
similar regions positioned around a central axis.
Unlike the adults, echinoderm larvae, such as the one shown in the Figure 1.4, are always bilaterally symmetrical,
reflecting the fact that radial symmetry evolved secondarily from a bilaterally symmetrical ancestor.
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Chapter 1. Echinoderms - Advanced
FIGURE 1.3
A cross section of an apple, illustrating pentameral symmetry.
Pentameral
symmetry in echinoderms refers to the
arrangement of the body parts into five
equal sectors around the mouth or central
region, just like the seeds are arranged
around the core of this apple.
FIGURE 1.4
An echinoderm larva. The larval stages of
echinoderms are always bilaterally symmetrical, like the one shown here, even
though adults are radially symmetrical.
This reflects the fact that radial symmetry
evolved secondarily in echinoderms from
a bilaterally symmetrical ancestor.
One highly unique feature of echinoderms is an organized network of canals throughout the body called the water
vascular system that functions in locomotion, feeding, excretion, and respiration. The water vascular system uses
water pressure generated through muscle contractions to move the organism. We will look at this hydraulic system
in more detail in the next section.
Unlike mollusks, annelids, and arthropods, echinoderms are not protostomes. The species of the phylum Echinoder3
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mata and the phylum Chordata are all deuterostomes. In the Introduction to Animals and Invertebrates chapter you
learned that there are a number of differences between protostomes and deuterostomes, all having to do with their
patterns of embryonic development. These are where differences between the two lie:
•
•
•
•
Pattern of the first several cell divisions.
When the fate of embryonic cells becomes fixed.
Formation of the coelom.
Formation of the mouth and anus.
In deuterostomes, the first several cell divisions are described as radial cleavage because they take place in parallel
to or perpendicular to the axis of the embryo. Another feature of these early cells is that they do not have a set fate.
For example, if one cell is removed from the embryo at this stage (8-cell stage), it is still capable of dividing to
form all of the cells of an entirely new organism. This contrasts the early cell divisions in protostomes that produce
cells with a fixed cell fate. If they are removed from the embryo, they will only be able to divide and differentiate
into a few different cell types, so they are not capable of producing a new organism. Another difference between
protostomes and deuterostomes is the origin of the mouth. In protostomes, the blastopore becomes the mouth. In
deuterostomes, the blastopore becomes the anus, and another opening generated later in development produces the
mouth. The blastopore is the first opening of the digestive cavity that is formed during the gastrulation stage of
embryonic development, as shown in Figure 1.5.
FIGURE 1.5
Blastopore and coelom formation during gastrulation. In deuterostomes, the
coelom begins to form as mesodermal
pouches that fold in from the endoderm,
and the blastopore eventually develops
into the anus. In protostomes, the blastopore develops into the mouth.
The
deuterostome mouth originates as a second opening that forms later in development. Orange = ectoderm, red = endoderm, and blue = mesoderm.
Finally, protostomes and deuterostomes each have different ways of forming a coelom, or body cavity. In protostomes, the mesoderm (middle tissue layer) splits into two sheets of tissue, and the space between these sheets
becomes the fluid-filled body cavity. Deuterostomes form a coelom by invaginating pockets of endodermal (internal
tissue layer) tissue from the developing gut cavity formed during gastrulation (see the Figure 1.5). The tissue
surrounding the pockets differentiates into mesodermal tissue, and the pockets become the coelom.
The echinoderm coelom is well-developed, and it forms two major cavities during development. One cavity becomes
the actual coelom that houses internal organs, while the other develops into the water vascular system. In the next
section we will take a closer look at the water vascular system, the endoskeleton, and other structural features of
echinoderms.
Vocabulary
• blastopore: The opening of an embryo’s central cavity in the early stage of development.
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Chapter 1. Echinoderms - Advanced
• coelom: A fluid-filled cavity formed within the mesoderm; it forms between the digestive cavity and the body
wall.
• deuterostome: An animal in which the first opening formed during development (the blastopore) becomes
the anus.
• endoskeleton: An internal skeleton that provides support and protection.
• gastrulation: The development of different layers of cells in an embryo; in humans, this generally occurs
during the second week after fertilization.
• pentameral radial symmetry: Having the five-radial plan of symmetry, in which the body or organ consists
of five subequal segments, as seen in the echinoderms, certain corals, and jellyfishes.
• water vascular system: A system of fluid-filled tubes used by echinoderms in locomotion, feeding, and
respiration.
Summary
• Members of the phylum Echinodermata include sea stars (starfish), sea urchins, sea cucumbers, sand dollars,
brittle stars, and feather stars.
• Echinoderms are the most closely related phylum to the phylum Chordata, which includes many complex
organisms such as humans.
• Echinoderms are deuterostomes that exhibit pentameral radial symmetry.
• The water vascular system, used for locomotion, is unique to echinoderms.
Practice
Use this resource to answer the questions that follow.
• Echinodermata at http://tolweb.org/Echinodermata .
1. When did echinoderms first appear in the fossil record?
2. What is the endoskeleton of echinoderms made of?
3. What four features distinguish echinoderms from other animals?
Review
1. Although there are not as many echinoderm species as there are arthropods, the echinoderms are still the
largest phylum in which environment?
2. Echinoderms are closely related to the phylum Chordata, which includes humans, even though they look
more closely related to cnidarians. What is one clue that support the theory that echinoderms are related to
chordates?
3. How does early cell development differ between protostomes and deuterostomes?
4. How does the coelom develop in deuterostomes?
5. Unlike other animals, the coelom of echinoderms forms two cavities. What are these two cavities for?
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References
1. NOAA. http://en.wikipedia.org/wiki/Image:Sanc0521.jpg . Public Domain
2. Eric Beach, NOAA, NOAA, Tomasz Sienicki, Tewy, NOAA. http://en.wikipedia.org/wiki/Image:Feather-s
tar1.jpg http://en.wikipedia.org/wiki/Image:Sandstar_300.jpg http://commons.wikimedia.org/wiki/Image:Ree
f2589.jpg http://commons.wikimedia.org/wiki/Image:Woda-5_ubt.jpeg http://commons.wikimedia.org/wiki/
Image:Sand_dollar_%28Dendraster_excentricus%29_01.jpg http://commons.wikimedia.org/wiki/Image:Parast
ichopus_californicus.jpg . Public Domain, Public Domain, Public Domain, CC-BY 2.5, CC-BY 2.5, Public
Domain
3. Julie Harris. N/A . Public Domain
4. Dpbsmith. http://commons.wikimedia.org/wiki/File:Pluteus001.jpg . Public Domain
5. Pidalka44. http://commons.wikimedia.org/wiki/Image:Gastrulation.png . Public Domain
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