Sea Cucumber 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 cucumber 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 sea
cucumber. These organisms belong to the class Holothuroidea which contains about
1,000 species. They are different from other echinoderms because their bodies are
elongated and have an oral end and an aboral end rather than oral bottom and aboral
top. They also have tentacles, spines embedded in their tough skin, and they have the
ability to self-eviscerate their digestive tubes should they be harassed by an attacker.
Materials:
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Preserved sea cucumber specimen
Dissecting pan
Scalpel
Probe
Scissors
Paper towels
Binocular dissecting microscope
Procedure:
External Anatomy
1. Place the sea cucumber in your dissecting pan. Determine the oral-aboral axis. How
is the body surface different from other echinoderms?
2. Feel the body surface. How does it compare to other echinoderms that you have
examined?
3. Locate the five rows of tube feet that run down the sides of the sea cucumber. The
ventral (bottom) podia will be more elongate and obvious than the dorsal (top) podia.
ACTION: Make a sketch of the external anatomy of the sea cucumber. Label the
tentacles, ventral podia, and dorsal podia. Indicate the oral and aboral ends of the
sea cucumber in your drawing.
4. If available, examine the endoskeletal ossicles of the sea cucumber prepared by your
teacher. These ossicles were obtained by boiling the skin of the sea cucumber in
bleach to dissolve the soft tissue. Do these ossicles resemble in any way the spines
of other echinoderms?
ACTION: Sketch the endoskeletal ossicles.
Internal Anatomy
1. Beginning in the aboral end, make a ventral incision through the body wall and
extend this incision through the anterior end. Open the body wall laterally and pin it to
the dissection pan. Cover the specimen with a little bit of water. The enterocoelom, or
body cavity is spacious and is usually filled with a coelomic fluid when the animal is
alive. The water will simulate this and make the internal organs appear more natural.
2. Locate the mouth, expanded pharynx, short esophagus, muscular stomach, and long
intestine that leads to the muscular rectum (See Fig. 1). Note the muscles that attach
the rectum to the body wall.
3. Find the respiratory trees that extend off the rectum. The rectum pumps water into
these trees and also releases this water back into the ocean. The sea cucumber
exchanges gases in these trees. Some burrowing sea cucumbers lack these trees
and instead respire directly through their skin.
ACTION: Make a sketch of the of the internal anatomy of the sea cucumber. Label the
mouth, pharynx, esophagus, stomach, intestine, rectum, and respiratory trees.
4. Locate the hard calcareous ring that surrounds the pharynx. Strong retractor muscles
attach this to the posterior body wall and retract the entire oral opening into the body
cavity.
5. Below the calcareous ring is the ring canal. Look for the madreporite and the stone
canal that connect to this structure. The madreporite will be on the dorsal surface.
6. Extending below the ring canal are one to several sac-like structures called Polian
vesicles. These may act in maintaining the water pressure in the water vascular
system of the sea cucumber.
7. Locate the radial canals off the ring canal that run toward the oral tentacles and then
turn to follow the interior body wall down to the aboral end of the body. The canals
follow large longitudinal muscles that are attached to the inner body wall for the
length of the sea cucumber. The radial canals connect with the podia that extend
through the body wall to the exterior.
8. Note the circular muscles that form rings within the body wall. These work in concert
with the longitiudinal muscles to allow the sea cucumber to expand and contract its
body.
9. Identify the gonad, if possible. It is a feather duster looking organ that extends from
the oral region into the body cavity. Sea cucumbers have separate sexes and unlike
other echinoderms, they only possess one gonad.
ACTION: Make a sketch of the water vascular system of the sea cucumber. Label the
madreporite, stone canal, ring canal, radial canals, and Polian vesicles.
Figure 1 – Anatomy of a the sea cucumber
Observations:
External Anatomy of the sea cucumber
Ossicles
Internal Anatomy of the sea cucumber
Water Vascular System of the sea cucumber
Questions for Review:
1. What parts does the sea cucumber have in common with the sea star and sea
urchin? What parts are unique to the sea cucumber?
2. The sea cucumber has an elaborate system of digestive organs and internal
musculature. How do you think each of these features benefits the sea cucumber?
3. Based on its anatomy, for which activity do you think the sea cucumber is most
adapted?