LABORATORY
Phylum Echinodermata – Starfish, Sea urchins and Sea cucumbers
Kingdom Animalia
Subkingdom Eumetazoa
Branch Bilateria
Deuterostomia
Phylum Echinodermata
Class Asteroidea
Class Ophiuroidea
Class Holothuroidea
Class Echinoidea
Class Crinoidea
Class Concentricycloidea
Echinoderms are radially symmetrical animals that are only found in the sea (there are none on land or in fresh water).
Echinoderms mean "spiny skin" in Greek. Many, but not all, echinoderms have spiny skin. There are over 6,000 species.
The echinoderms originated approximately 525 million years ago in the first 20 million years of the Cambrian known as
the Cambrian explosion. Over 20,000 fossil species have been identified.
The word echinoderm means spiny skin. These are marine organisms with five-fold radial symmetry, commonly called
starfish (Class Asteroidea), sand dollars and sea urchins (Class Echinoidea), sea cucumbers (Class Holothuroidea), and sea
lilies (Class Crinoidea). They are coelomate.
Just under the skin, they have an endoskeleton that is made of calcium carbonate, the substance of corals and clam shells.
The skeleton arises from mesodermal tissue, an origin unlike that of bivalve shells, which are secreted by the epidermal
sheet of mantle tissue. If you look very closely at the skin of some echinoderms, like sea stars, you will notice that there
are different types of growths on the surface. Some bumps are used to absorb oxygen, they are called dermal
branchiae. Pedicellariae are pincher-like organs used to clean the surface of the skin. Since echinoderms are slow moving,
these organs are there to prevent other animals like barnacles from landing on them and start growing.
Sexes are separate in echinoderms and fertilization is external; there is a larval stage with bilateral symmetry. As they
mature, they become radially symmetrical. Radial symmetry means that the body is like a bub, like a bicycle wheel, and
the arms (or tentacles) are spokes coming out of it. They possess a top –down axis with the mouth on the side known as
the oral surface and the anus on the side known as the aboral surface. The adaptation to radial symmetry was the result of
their habitat on the sea floor.
A unique derived feature of the group is its water-vascular system in which seawater is pumped through a series of ducts
to work the tube feet (or podia) and the suction cups at their tips. Tube feet are used by echinoderms for locomotion and
prey capture. The tube feet and can be filled with sea water to extend them towards a substrate or prey. Muscles within
the tube feet are used to extend or retract them. The water vascular system is used to classify animals as echinoderms.
Water enters the water vascular system by way of a perforated plate known as the madreporite (in sea stars - on the
aboral surface; in sea cucumbers its internal). The madreporite leads to a short stone canal which connects to a ring canal.
Radial canals branch off the ring canal and travel into the arms. Numerous tube feet branch from each radial canal. In sea
stars, these canals are located in ambulacral grooves created by ossicles of the endoskeleton. The grooves are visible
internally and externally. In sea cucumbers, the radial canals are found along the body wall associated with retractor
muscles. Only the very tip of the tube feet are seen outside the cucumber.
By moving sea water from the radial canals of the water vascular system into the tube feet, the echinoderm can make the
tube foot move by expanding it. This is controlled by muscular contractions of the ampulla of the tube foot. Think of how
a turkey baster can squirt liquid through the tube and you have the idea of how a tube foot works.
You can also find fluid outside the water vascular system. This fluid is the internal fluid of the echinoderm and is derived
from the sea water you find in the water vascular system. It is found in the internal cavity of the echinoderm, known as
the coelom. By circulating this internal fluid around within the coelom, the echinoderm can move respiratory gases,
nutrients and wastes amongst its cells. Ciliated cells lining the inside of the echinoderm are responsible for this fluid
movement.
Echinoderms are predators, suspension feeders and grazers. The mouth in the oral surface leads to a stomach. The first
part of the stomach is large and is for storage of food. It is known as the cardiac stomach. The food eventually passes into
the pyloric stomach. If the echinoderm is hunting prey, the stomach may be everted and used to introduce digestive
enzymes into their prey. Connected to the pyloric stomach are tubular intestines that are found in groups of 5. Large, very
obvious pyloric caeca attach to each intestine. These caeca facilitate digestion through the production of digestive
enzymes (which go to the stomach) and by increasing the absorptive surface area for nutrients once they are digested by
the enzymes. Absorbed nutrients then pass into the internal fluid in the coelom for distribution. Undigested materials
pass out through the anus (if it is a functional anus). If the anus is non-function, the food passes out through the mouth.
Echinoderms are known as deuterostome animals (along with chordates), in which the blastopore becomes the
anus and the mouth opens elsewhere. Deuterostomes have radial cleavage and their embryonic cells are
indeterminate. Deuterostomes are enterocoels, their coelom forms as out-pockets along the gut.
Lab Materials
Models – sea star
Prepared slide – starfish development
Demo slide – starfish late bipinnaria
Dissection – sea stars & sea cucumbers
Preserved samples – sea urchin
-brittlestar
-sea cucumbers
Echinoderm dry mount
Class Asteroidea – the Sea Stars
Sea stars (group name Stelleroidea) are sometimes called starfish, though they are not real fish (they lack both vertebrae
and fins). There are two sub-types of sea stars:
Asteroideas are the true sea stars and sun stars.
Ophiuroideas are brittle stars and basket stars.
The differences between the two sub-types lies in how the arms connect to the central disk. Ophiuroids have arms that do
not connect with each other. There is a distinct boundary between arm and central disk. Asteroids have arms that are
connected to each other. Also, it is harder to tell with asteroids where the central disk ends and the arms begin.
Each ray (or arm) of a sea star has a light sensitive organ called an eyespot. Using their eyespots, the sea star can detect
light and its general direction.
Starfish Dissection
Materials:
Preserved starfish, dissecting pan, scissors, scalpel, forceps, T-pins, pencil, lab apron, safety glasses
Procedure (Aboral Surface):
1. Obtain a preserved starfish and rinse off any preservative with water.
2. Place the starfish in the dissecting pan with its dorsal or aboral (top) surface upward.
3. Observe the starfish and determine its symmetry.
4. Locate the central disc in the center of the starfish. Count and record the number of arms or rays the starfish
has.
5. Locate the small, round hard plate called the madreporite on top of the central disc. Water enters through
this into the water vascular system.
6. Feel the upper surface of the starfish for spines. These spines protect the starfish and are part of their
internal skeleton.
7. Look at the tip of each arm and find the eyespot. Though it cannot see nearly as well as we do, the eyespot of sea
stars can detect light and its general direction. They have some idea of where they are going.
Procedure (Oral Surface):
7. Turn the starfish over to its ventral or oral surface (underside).
8. Locate the mouth in the center of the central disc. Find the ring of oral spines surrounding the mouth.
9. Find the groove that extends down the underside of each arm. This is called the ambulacral groove.
10. Feel the numerous, soft tube feet inside each groove. These are part of the water vascular system & aid in
movement and feeding.
Procedure (Internal anatomy):
11.
With the starfish's aboral surface facing you, cut off the tip of a ray. Cut along lines a, b, and c (Figure 1)
and then remove this flap of skin.
Figure 1 - Cuts in Arm
12. Inside each
arm, locate two
long digestive
glands called the
pyloric caeca. These
make enzymes to
digest food in the
stomach.
13. Cut a circular
flap of skin from the
central disc. (You
will have to also cut
around the
madreporite in
order to remove
this flap.) Observe
14.
15.
16.
17.
18.
19.
20.
21.
the stomach under the central disc.
Remove the pyloric caeca from the dissected ray. Find the gonads (testes or ovaries) underneath. These
may be small if the starfish is NOT in breeding season. Remove these to see the rest of the water
vascular system.
Cut off the tip of a ray to observe the parts of the tube feet. Find the zipper-like ridge that extends the
length of the ray. The tube feet are attached to these.
Locate the bulb-like top of a tube foot called the ampulla. This sac works like the top of an eyedropper to
create suction. The bottom of the tube foot is a sucker.
Embedded in the soft body wall are skeletal plates called ossicles. Locate these
Running down the center of each arm is a radial or lateral canal to which tube feet are attached. Label
this in Figure 2.
In the central disc the five lateral canals connect to a circular canal called the ring canal. Find this canal &
label it on figure 2.
A short, canal called the stone canal leads
from the ring canal to the madreporite
where water enters. Find this canal &
label the stone canal & madreporite on
Figure 2.
Draw an arrow on Figure 2 tracing the
path that water takes when it enters &
moves through the starfish.
Figure 2 - Water Vascular System
Questions:
1. What type of symmetry did your starfish have?
2. What is the upper surface of the starfish called?
3. What is the lower surface of the starfish called?
4. On which surface are these parts of a starfish visible?
a. Mouth –
b. Madreporite c. Suckers d. Oral spines e. Eyespots d. Ambulcaral groove 5. In words, trace the path water takes through the water vascular system.
6. What part of the tube foot creates suction to open clams whenever the starfish feeds?
7. Why do the gonads sometimes appear larger?
8. What type of skeleton, endoskeleton or exoskeleton, does the starfish have?
9. What bony plates make up its skeleton?
10. What is the function of the pyloric caeca?
11. Where is the stomach of a starfish located? What can the starfish do with its stomach when feeding on
clams & oysters?
12. Name the kingdom, phylum, and class for the starfish you dissected.
Class Holothuroidea – the sea cucumbers
Sea cucumbers are rather bizarre-looking echinoderms. They are unlike other members of the Echinodermata
in a number of features. There are approximately 1,000 living species in Class Holothuroidea.
The body of the sea cucumber is elongated on its oral-aboral axis. The oral end contains the mouth surrounded
by a number of food-gathering tentacles. These mucus-covered structures are modified tube feet and can be
retracted within the body. The opposite end bears the anus. The body wall is leathery and spineless and
contains an embedded endoskeleton of microscopic, individual ossicles that remain unfused. The tube feet are
scattered over the body surface, or are grouped into 5 ambulacral areas or totally lacking. Sea cucumbers move
through muscular contractions of their body walls. The tube feet (if present) aid in their locomotion.
Sea cucumbers are typically dioecious and undergo external fertilization. They show amazing powers of
regeneration. When disturbed or under stress, it can eviscerate its internal organs. These organs are then eaten
by whatever predator was hunting them. Regeneration of the lost organs then occurs after the sea cucumber
crawls to safety. Some cucumbers can eject out a sticky mass of Cuvierian tubules that entangle their prey.
Sea Cucumber dissection - Cucumaria
1. Examine the external appearance of the sea cucumber. Determine the aboral and oral surfaces. Feel the
body surface and compare it to the sea star.
2. In Cucumaria, the tube feet are present in five distinct ambulacral areas: three on the ventral surface
and two on the dorsal surface. Can you see evidence of these tube feet on the external surface?
3. Beginning at the aboral end, make an incision through the body wall along the ventral surface toward
the oral end. Pin the animal open to the dissection pan
4. Find the pair of highly branched respiratory trees that extend off the rectum. The muscular rectum
pumps water into the trees and expels it back out to the sea. These trees are for gas exchange and are
unique to sea cucumbers
5. Surrounding the pharynx is a hard, calcerous ring for muscle attachment. This calcerous ring is analogous
to Aristotle’s Lantern in the sea urchins
6. The pharynx continues on into the stomach and the intestine. Locate the intestine. Don’t confuse the
tubular intestine with the multiple, smaller gonads found inside the cucumber.
7. Behind the calcerous ring is the ring canal. The madreporite is internal and attaches to the ring canal via
a stone canal. You may not be able to find the madreporite and stone canal, but if you are careful, you
should be able to find the ring canal.
8. Try to find the sac-like Polian vesicles that attach to the ring canal. These vesicles may maintain water
pressure in the water vascular system
9. Locate the 5 radial canals. These canals follow the large longitudinal muscles that are attached to the
inner surface of the body wall. These radial canals denote the position of the ambulacral areas.
10. One of the most obvious structures will be the gonads. They are very numerous and consist of a large
tuft of filaments. They increase in size as the cucumber matures. This gonad is a singular structure.
Microscopic examination is necessary to tell the sex of the cucumber. If you wish, cut a small section of
gonad and crush it slightly on a microscope slide. Add a coverslip and then a drop of methylene blue.
The round head and tail of the sperm may be visible under oil immersion. Eggs of the female are large
and ovoid.