PHYLUM ECHINODERMATA
Introduction
The echinoderms have a long evolutionary history and were once some of the most abundant
animals in the ancient oceans; only 7,000 species are living today compared to 13,000 fossil
species. This makes the Echinodermata another one of those animal groups that have a body
plan that was more successful in the past than now.
The ancestral echinoderms were sessile organisms attached to the substrate with their mouth
pointed up and feeding on the organic material that fell through the water of the ancient oceans.
Like other early deuterostomes, the feeding appendages surrounding the mouth were hollow
extensions of the tripartite coelom. To trap even more food the ancestor of the group had arms
that extended out from the central disc, and on the arms were tube feet that passed the captured
food to a central disc where the mouth was located.
Echinoderms are marine animals; the phylum has no freshwater or terrestrial representatives.
They come in a variety of different shapes and sizes including: sea stars, sand dollars, sea
urchins, brittle stars, sea cucumbers, and basket stars to name a few. They're usually found in
shallow waters but sea lilies and sea cucumbers are also found in the abyssal depths of the
oceans. One of the distinctive characteristics of the phylum is their pentaradiate symmetry. At
one time biologists combined them with the radially symmetric cnidarians and named them the
Radiata. But, once the bilaterally symmetric planktonic larval stage of echinoderms was
identified, it became clear that radial symmetry in the adult was a derived trait associated with
sessile suspension feeding. The first echinoderms lived attached to the ocean bottom with their
mouths facing up. The tube feet on their arms used mucus to collect food floating to the ocean
bottom or suspended in the ocean currents.
Asteroidea
Commonly called sea stars, Asteroidea display the typical pentaradiate symmetry of the phylum
and have arms that gradually increase in diameter as they fuse with the central disc. There are
approximately 1,600 species living in the shallower coastal waters of the world's oceans; like
all echinoderms, they're only found in the marine environment. The size and number of arms
that a sea star has varies. Some have the usual five, others more. If there are more, it's usually
based on some multiple of five. Sea stars are usually between 12 and 24 centimeters in diameter,
although some may become more than a meter across.
Fig. 1. External anatomy of the aboral and oral surface
of the sea star. © BIODIDAC
Asteris
External Anatomy
Phylum Echinodermata
-1-
Digital Zoology LabManual © Houseman
Examine the external features of your starfish identifying the aboral and oral surfaces with the
mouth located in the centre of the peristomial membrane. Does a starfish have a dorsal and
ventral surface? Five arms radiate from a central disc. The groove on the oral surface of each
arm is the ambulacral groove and contains four rows of tube feet with sucker like discs located
at their ends. The delicate tube feet are protected by the ambulacral groove and the spines found
along its edge. In your specimen, the water vascular system has been injected with a blue or
red dye and tube feet are easy to see in the ambulacral groove. At the end of each ambulacral
groove is a minute eye spot formed from a pigmented epidermal invagination. This may or may
not be visible depending on how the preservative and the injected dye have affected the
pigments.
Fig. 2.
Aboral surface of the sea star. © BIODIDAC
The aboral surface is composed of a central disc with the madreporite on one side of the disc,
rather in the centre. Examine the madreporite under the dissecting microscope. How is its
appearance related to its role in controlling the movement of water into the water vascular
system? The anal opening is located in the approximate middle of the central disc. Why is the
anal opening so small and how is this related to the way that starfish feed?
Fig. 3. Structure of a sea star pedicellarium.
© BIODIDAC
The position of the madreporite is used to name and number the starfish regions and arms. The
arms on each side of the madreporite are the bivium and the remaining three the trivium. Arm
A is immediately opposite the madreporite and in the middle of the trivium and when viewed
from above, each arm is lettered consecutively, counterclockwise, from A to E. Using this
nomenclature the madreporite is located between what arms?
Phylum Echinodermata
-2-
Digital Zoology Lab Manual © Houseman
The whole aboral surface is covered with spines. Take a close look under the dissecting
microscope to see the finger like sacs at the base of the spines. These are dermal branchia and
are outpockets of the underlying coelom involved in excretion and respiration. They are lined
internally with cilia that insure that the coelomic fluids are continually mixed so that the
exchange can occur. What kind of material is being exchanged across this surface? The
pedicellaria are small pincer-like structures located around the spines in a ring of tissue. Scrape
away a portion of the epidermis to expose the calcareous ossicles that form the skeleton. Place
your scrapings on a microscope slide and observe the pedicellaria. Compare yours to the
prepared microscope slide of pedicellaria.
Internal Anatomy
If there is time you are more than welcome to take a preserved starfish and observe the internal
anatomy. It’s really straightforward and you can just as easily see the main internal features on
the preserved museum mount. The protocol that follows is for if you have the time.
Fig. 4.
Internal antomy of the sea star. © BIODIDAC
Starting at the tip of each ray carefully lift the aboral body wall off the underlying tissue and as
you do this gently free the tissue from the body wall. Be particularly careful when you remove
the body wall over the central disk, especially near the madreporite, so that connections with the
underlying water vascular system are not broken. Examine the internal surface of a piece of the
body wall that you have removed so that you can see the fused calcareous ossicles that form the
endoskeleton of the starfish.
The part of the coelom that you have exposed is the pervisceral cavity and one of the three parts
of the tripartite coelom. Which part is it? The pervisceral cavity is filled with fluid that
circulates throughout the cavity by the cilia that line it. It was extensions of this cavity that
formed the dermal branchia that you saw on the external surface. The movement of fluid in the
pervisceral cavity, combined with the same in the water vascular system essentially defines the
circulatory system of the animal. The water vascular system is also one of the three parts of
the coelom; which is it?
Digestive system
Starfish feed on just about any organic material that they can find. The
mouth, and an inverted stomach, are used to cover the surface being fed on. This creates an
enclosed space into which the digestive enzymes are released. The organic material on the
covered surface is broken down before being swept into the alimentary tract for final digestion
by the cilia that line the system.
The major structures of the digestive system are best seen by submersing your specimen under
water. That way the different regions can float free of each other. A short esophagus leads from
the mouth, and opens into a stomach divided into two regions, the pyloric and cardiac
stomachs. A large cardiac portion everts during feeding and subsequently retracts back into the
body cavity. The cardiac region connects to the smaller pyloric region that is in turn connected
to the five paired pyloric ceca (digestive glands) contained in each arm. The pyloric ceca are
the sites of final digestion and their position throughout the body insures that there is an
Phylum Echinodermata
-3-
Digital Zoology LabManual © Houseman
adequate supply of nutrients to the whole organism by nutrient diffusion in the coelomic fluid
circulated by cilia. A rectal caecum is connected to the tube leading from the pyloric part of the
stomach and the anus on the aboral surface. Why is the anal opening so small?
Carefully remove the pyloric ceca to expose the paired gonads found in each of the arms. Their
size will depend on the reproductive state of the animal when it was collected. You will not be
able to see the external openings for the gonads in the interambulacral region. The sexes are
difficult to distinguish using physical features. Can you suggest a way to determine the sex
of your specimen?
Fig. 5. Organization of the water ascular system inn the
sea star. © BIODIDAC
Water vascular system
Remove the gonads and the remainder of the digestive system to
expose the underlying water vascular system. Be careful when you remove this in the area near
the madreporite. You don’t want to damage the madreporite’s connection to the rest of the water
vascular system. The principle elements of the system include the madreporite, stone canal,
and ring canal which are connected to the five radial canals and their associated short
transverse canals. The tube feet are connected to the transverse canals. The ring canal has nine
Tiedemann bodies and the position of the tenth is occupied by the connection of the stone
canal. Polian vesicles that are present on some species of starfish are absent in the one that you
are looking at today. The bulb-like ampulla of each tube foot is visible from the inner surface
of the ambulacral groove as it passes between the ambulacral plates. How many ampullae are
there on each transverse canal? The remaining parts of the tube feet are only visible when
viewed from the oral surface. You will notice how the ambulacral spines protect them. Cut a
section through the arm to see the relationship between the podia, ampulla of the tube feet and
the ambulacral skeleton.
Nervous and Circulatory systems The nervous system is composed primarily of a nerve
net over the body surface. If you remove the tube feet from one of the ambulacral grooves a
minute yellowish brown ridge running along the entire length of the groove may be apparent if
you fold the ambulacral groove back. This is the radial nerve cord that connects with a circumoral nerve ring. The circulatory system is all but absent and its remnants form a hemal system
closely associated with the first part of the tripartite coelom. You will not be able to see the
system.
Echinoidea
There are approximately 900 species of echinoids, which come in two shapes. They are either
spherical, regular echinoids, such as the sea urchins, or disk-shaped, irregular echinoids, such
as the sand dollars. Like all echinoderms, the echinoids are exclusively marine animals and can
be found anywhere from the intertidal zone to five kilometers deep. The pentaradiate symmetry
of the phylum is still visible externally if you look closely at the surface of both. Sea urchins
have spherical symmetry and the ambulacral zones alternate around the surface of their circular
test, and the aboral region has been reduced to a set of plates that surround the anus. Sand dollars
Phylum Echinodermata
-4-
Digital Zoology Lab Manual © Houseman
have become secondarily bilaterally symmetric. The ambulacral zones are on the aboral side,
and the mouth and anus are on what was the oral side. As you would expect with the solid shell,
the body is not part of a hydrostatic skeleton, and there is no musculature associated with the
body wall.
Sea Urchin
Sea urchins are either herbivores or detritivores grazing on large marine plants or the substrate,
using the unique feeding structure of Aristotle’s lantern.
External anatomy
The outer body surface of a sea urchin is covered with a large variety of different spines, some
of which articulate through a ball and socket type of joint. Submerse your specimen under water
and use the dissecting microscope to examine the spines, pedicellaria, and the long tube feet
that extend beyond the spines.
Fig. 6. External anatomy of the aboral surface of a sea urchin.
© BIODIDAC
Use one of the dried sea urchin tests to see the interlocking hexagonal plates that make up the
test. The plates are arranged into rows organized into alternating double columns of which five
are ambulacral and five are interambulacral. The ambulacral region is easily identified by the
perforations through which the tube feet extend. On the aboral surface a small distinct plate
surrounds the anal opening on the aboral surface and the opening of the gonopore perforates
Phylum Echinodermata
-5-
Digital Zoology LabManual © Houseman
the five genital plates. The madreporite is found on the largest of the five genital plates. Use
the dissecting microscope and look at one of the plates that make up the test. You should be able
to see the tubercles; and the ball and socket joint that articulates the large surface spines.
Fig. 7. External anatomy of the oral surface of the sea
urchin © BIODIDAC
Return to your preserved specimen in the dissecting dish and examine the oral surface. In the
centre you can see the five white protruding teeth of Aristotle’s lantern. The membranous
peristome surrounds the lantern. The feeding structure in a sea urchin is Aristotle’s lantern and
is an intricate series of interlocking plates. Observe the specimen available on demonstration.
We will remove Aristotle’s lantern for a closer look after we have looked at the internal
anatomy.
Fig. 8. Detail of the anal plates (left) and the spines and pedicellaria on the surface of
the sea urchin. © BIODIDAC
Internal Anatomy
Scrape the surface of the your urchin to remove the spines from the sea urchin test and submerse
your specimen under water in a finger bowl. With scissors carefully cut through the equatorial
region of the sea urchin and separate the two halves in much the same way that you would crack
an egg. As you separate the two halves be sure to keep them submersed so that none of the
internal tissues are damaged. Orient the two halves as shown in the museum mount taking care
Phylum Echinodermata
-6-
Digital Zoology Lab Manual © Houseman
to not rip the organs that extend between the two hemispheres of the animal. The complex
framework supporting the teeth is Aristotle’s lantern and is composed of muscles and calcareous
plates.
Fig. 9.
Internal anatomy of the sea urchin. © BIODIDAC
The esophagus extends from the centre of Aristotle’s lantern and connects with the flat
stomach which runs around the edge of the body cavity followed by the intestine. The digestive
system winds back on itself before moving into the aboral hemisphere containing the rectum
and anus. Because of the way that they feed, sea urchins consume a large volume of water along
with the food and a second part of the alimentary tract, the siphon running along the inner
margin of the looping stomach. It bypasses the stomach and allows water to be removed from
the ingested food so that it becomes concentrated. There are no digestive ceca and the looping
passage of the intestine through the test insures the distribution of nutrients.
Phylum Echinodermata
-7-
Digital Zoology LabManual © Houseman
You may not be able to see it, but the water vascular ring extends around the oral surface and
spongy Polian vesicles are attached to it. Identify the ampullae of the tube feet. Gonads are also
apparent in the upper hemisphere.
Fig. 10. Ossicles and muscles of Aristotle’s lantern in the sea
urchin. © BIODIDAC
Carefully remove Aristotle’s lantern and examine the arrangement of the musculature and the
plates and teeth. How does this work?
Sand dollar
The sand dollar is an example of another echinoid type and compared to the sea urchin the
modifications are dramatic. The echinoid body plan has been compressed in the oral aboral
plane and these animals are specialized for burrowing.
Fig. 11. External anatomy of the aboral surface of the
sand dollar. © BIODIDAC
The surface is covered with short spines that give a velvety appearance to the body. Look at the
oral and aboral surface through the dissecting microscope. On the aboral surface the ambulacral
area are visible as petaloids and in these animals the tube feet that extend through here are
involved in gas exchange. The five openings to the gonopore surround the madreporite. The
mouth and the anus are both on the oral surface and the feeding mechanism is not fully
Phylum Echinodermata
-8-
Digital Zoology Lab Manual © Houseman
understood. The openings in the sand dollar shell are lunules and it is not known whether they
are involved in feeding allowing food to pass to the aboral surface or whether they help stabilize
the animals where there are strong currents.
Holothuroidea
The sea cucumber is an example of the Holothuroidea and is an interesting class because many
of the ancestral echinoderm characters have been modified with a return to a bilateral type of
symmetry.
External Anatomy
The endoskeleton has been reduced to spicules buried in the leathery body of the animal. The
tube feet and the ambulacral grooves are still visible on the outside and you can still tell the
ambulacral from the interambulacral parts of the body. But, because these animals always
orient one side of their body against the substrate the tube feet on this side are much more visible
as they are still involved in locomotion, just like other Echinoderms. The tube feet on the
opposite side, which are not in contact with the substrate are much smaller or may have
disappeared completely depending on the species. The tube feet aren’t the only way that these
animals move and contractions of the muscles embedded in the body wall allows sea cucumbers
to wriggle across the substrate.
The next most obvious external feature are the tentacles surrounding the mouth and used for
deposit feeding typical of the class. Food is gathered on the tentacles and the whole tentacle is
pushed into the mouth and wiped clean of food before being extended back outside the animal.
Look at the opposite end of the animal and you’ll see another opening commonly referred to as
a vent. That’s because water is pumped in and out of the opening to aerate the unique respiratory
trees that these animals use for gas exchange.
Fig. 12.
Internal anatomy of the sea cucumber. © BIODIDAC
Internal Anatomy
Make a cut from the base of the tentacles to the anal opening (vent or cloaca) and pin back the
body wall. Inside is a mess of organs and structures and to see them you’ll need to flood the
specimen and gently tease the material apart. The cavity that you opened is the enterocoelom of
the animal and the walls are lined with cilia that constantly mix the fluid content.
The mouth leads to a pharyngeal area that can be seen as an enlargement of the digestive tract
at the base of the mouth. There are a number of structures that can be identified in this area. The
five retractor muscles are attached to a calciferous ring. Take a good look at this structure
under your dissecting microscope and you’ll see the ring canal behind the calciferous ring. This
is part of the water vascular system and the stone canal, and the madreporite attached to it.
The polian vesicles are also connected to the ring canal and they vary in size depending on the
species. In ours they are large. The rest of the water vascular system may be hard to see, but if
Phylum Echinodermata
-9-
Digital Zoology LabManual © Houseman
you look closely, five radial canals are connected to the ring and follow the large longitudinal
muscles embedded in the body wall. Don’t confuse these muscles with the tentacle retractor
muscles that you looked at earlier. The ring canal branches as it passes to the posterior of the
animal and ultimately connecting to the tube feet. As we mentioned earlier, tube feet aren’t the
only means of locomotion that these animals have and in addition to the longitudinal muscles
in the body there are circular muscles as well.
Fig. 13. Detail of the main internal systems of the sea
cucumber. © BIODIDAC
These animals are particle or suspension feeders and food collected by the tentacles is passed
into the mouth down a short esophagus and into the stomach. From here the food is passed to
the intestine that loops backwards and forwards through the body cavity. The intestine ends at
the vent. Two large respiratory trees are connected at the vent and water is pumped into the
respiratory tree by the musculature associated with the vent. At the base of the respiratory tree
are the Cuvierian organs which are filled with sticky mucous threads. These are used in
defence and when sea cucumbers are disturbed they release the threads. If a severe defensive
reaction is required then sea cucumbers eviscerate expelling part, or all of the digestive tract and
the respiratory trees.
A single gonad is present and gametes are released through a genital pore positioned at the base
of the tentacle. Internally the gonad looks like an old mop and the bigger the gonad is the more
mature the specimen is. Just like all echinoderms its hard to tell the sex of your specimen.
Echinoderm Diversity
Identify the major features of animals representing the other classes in Echinodermata.
The feather stars and sea lilies, Class Crinoidea, are the most primitive of the Echinoderms.
Distinguish between the aboral and oral surfaces. The oral surface has both the mouth and anal
openings, the latter projects away from the mouth by being positioned at the tip of an anal
papillae. Cilia in the ambulacral grooves are used to move food towards the mouth. Dermal
vesicles which may appear as yellowish brown dots along the sides of the ambulacral grooves
Phylum Echinodermata
- 10 -
Digital Zoology Lab Manual © Houseman
have an unknown function. The opening to the water vascular system is through a single central
aboral ossicle. Members of this class often possess a stem composed of a stack of disc shaped
ossicles containing a central axial canal which terminates in a holdfast.
Fig. 14. External anatomy of a crinoid.
© BIODIDAC
The class Ophiuroidea includes the brittle stars that are superficially similar to the Asteroidea,
and get their name from the way they drop their arms off when disturbed. They differ from the
starfish because of the distinct separation of the central disc and the radial arms. These animals
lack ambulacral grooves and the arms are completely flexible along their length. Calcareous
vertebrae like structures running the length of the arm facilitate this movement. Examine an arm
that has been broken off and attempt to locate vertebrae and four sets of muscles contained
inside.
Fig. 15. External anatomy of the oral surface of a brittle
star. © BIODIDAC
Some interesting things that you might want to make note of as you work with your specimen
is the location of the madreporite on the oral rather than aboral surface of the animal. While
you’re looking at this surface make a note of the bursal slits that open into an enclosed space
called the bursa. Seawater moves in and out for gas exchange and can also be used to protect
developing larval young.
Phylum Echinodermata
- 11 -
Digital Zoology LabManual © Houseman