Sea Star Dissection
Objectives:
 To study the external and internal anatomy of an echinoderm.
 To be able to identify the major characteristics of an echinoderm on a preserved
sea star specimen.
Background:
The phylum Echinodermata includes sea stars, sea urchins, sand dollars, sea
cucumbers, brittle stars, sea lilies, and feather stars. As their name implies echinoderms
have spiny skin to protect themselves from predators. These organisms have no head,
but have a well developed digestive system. In addition, they also employ a unique
water vascular system which they use for locomotion, circulation, and excretion. All of
these organisms exhibit pentamerous radial symmetry. The phylum Echinodermata
contains over 6000 species and is entirely marine.
In this laboratory, you will be examining some of the features of the common sea
star Asterias forbesi. These sea stars are abundant along the Atlantic Coast from the
Gulf of Maine south to the Texas coast. These sea stars can range in color from tan or
brown to orange and pink. The madreporite or sieve plate is usually orange. They can
grow to just over 10 inches in diameter.
Materials:
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Preserved sea star specimen
Dissecting pan
Scalpel
Probe
Scissors
Paper towels
Binocular dissecting microscope
Procedure:
External Anatomy
1. Place your sea star oral side down in the dissecting pan. Note many bumps and
spines on the aboral surface of the sea star. How many arms (Fig.1 #2) does the sea
star have?
2. Locate the madreporite (Fig.1 #3) or sieve plate which allows water to enter the water
vascular system of the sea star. It is small, off-white in color and is located on the
central disk (Fig.1 #1) of the sea star off-center between two arms.
3. Locate the anus (Fig.1 #8) of the sea star. It is small and can be found on the aboral
surface in the center of the central disk.
ACTION: Make a sketch of the aboral surface of the sea star. Label an arm, the central
disk and the madreporite.
4. Using the dissecting microscope, examine a small area on the aboral surface of the
sea star. Locate the pedicellariae which are minute pincers surrounding the spines
which have tiny jaws that remove wastes and particles from the skin of the sea star.
Distinguish between the pedicellariae and the papulae or dermal branchiae which are
thin, hollow, soft projections that serve as gills.
ACTION: Sketch the pedicellariae and dermal branchiae.
5. Turn the organism over so the oral surface is facing upwards. Locate the eyespots on
the ends of each arm. They are small and pigmented. How many eyes does your sea
star have?
6. Notice the two or four rows of tube feet inside each ambulacral groove (Fig.1 #4)
which runs along the middle of each arm from the mouth in the center of the disk to
the end of each arm. Ambulacral spines or ossicles line the edge of each groove.
ACTION: Make a sketch of the oral surface of the sea star. Label the mouth, tube feet,
ambulacral groove and ambulacral spines.
Internal Anatomy
1. Turn the sea star over again so the oral surface is once again down. Using the
scissors cut the tip of each arm of the trivium. (The two arms that are on either side of
the madreporite make up the bivium. The remaining three arms make up the trivium.)
Carefully cut along the sides of these three arms so as not to damage the internal
organs.
2. Carefully lift and remove the aboral surface of each arm by loosening the delicate
mesentery tissue beneath the skin. This tissue also attaches to the soft organs so
you may need to delicately tease it from the skin using your probe.
3. Cut around the central disk (but not the bivium) and remove the aboral surface of the
central disk. Leave the madreporite in place.
4. Finally, cut transversely, at mid-length, one arm of the bivium to provide a cross
section of one arm.
5. Locate the coelom (Fig.1 #7) or body cavity. Find the stomach – a thin disk-shaped
sac that has five lobes. The
5-lobed, cardiac portion
(Fig. 1 #10) is larger with
Figure 1
pleated walls and retractor
muscles. This is the portion
most associated with being
ejected from the sea star
into the shell of a mollusk
for external digestion. The
pyloric stomach (Fig.1 #9)
is smaller, aboral, 5-sided
and smooth.
6. Find the short, slender
intestine which extends
from the pyloric stomach to
the anus.
7. Notice the hepatic caeca (Fig.1 #11). A pair of these appear in each arm as a long,
greenish extension with finger-like lobes that connect to the pyloric stomach by
narrow ducts. These are also termed digestive glands, the liver, or the pyloric caeca.
8. Finally, locate the gonads (Fig.1 #12). These are located in each arm below the
hepatic caeca. Each gonad is attached by a duct which opens aborally.
ACTION: Make a sketch of the internal anatomy of the sea star. Label the cardiac
stomach, pyloric stomach, intestine, hepatic caeca, and gonads.
9. Remove the side of the stomach near the madreporite. Starting with the madreporite,
trace the flow of water through the various parts of the water vascular system. Locate
the stone canal (Fig.1 #13). It is a hard tube which descends at an angle to the
bivium from the madreporite to the ring canal.
10. Examine the ring canal. Nine, small swellings called Tiedemann bodies appear on
this canal. Along each arm a radial canal branches to each arm. Many ampullae
(Fig.1 #6), small, spherical organs connect the radial canal to the tube feet.
ACTION: Make a sketch of the water vascular system. Label the madreporite, stone
canal, ring canal, Tiedemann bodies, radial canal, ampullae, and tube feet.
Observations:
Aboral Surface of Asterias frobesi
Pedicellarie
Oral Surface of Asterias forbesi
Dermal Branchiae
Internal Anatomy of Asterias forbesi
Water Vascular System of Asterias forbesi
Questions for Review:
1. Can you determine a pattern in the spines of the sea star? If so, what does this
pattern resemble?
2. What is the function of the water vascular system? How does it work?
3. Describe the tube feet of a sea star. For what purposes does the sea star use these
organs?
4. Based on its anatomy, for which activity do you think the sea star is most adapted?
References:
Common Sea Star. Chesapeake Bay Program. 10 Feb. 2004.
<http://www.chesapeakebay.net/seastar.htm>
Star Fish Dissection. BjBarton.com Lesson Planswith Fun Labs for High School
Science. 10 Feb. 2004. <http://www.bjbarton.com/lessons/stards.doc>