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Sea Star Dissection
Phylum: Echinodermata (means spiky skin in Latin)
Class: Asteroidea
Anatomy:
Sea stars are radially symmetrical, and their arms are arranged around a central
disk. Most species have five arms; however sun stars can have as many as 40 arms
or more! Most sea stars are small usually measuring 12-24 cm across, but there are
some species that can be very small or very large. For example, Pycnopodia, the
many-rayed star of the Pacific Northwest can be as big as 1 meter across! The
mouth of a sea star is found in the centre of the underside of the sea star (called
the oral surface). Several rows of tube feet run from the mouth down each arm of
the sea star. These rows of tube feet are guarded by movable spines that line the
edges of the arms for protection. The back side of sea stars can be smooth, spiny,
or slimy.
Movement:
Sea stars walk using their tube feet to move themselves along a surface. Their
tube feet have suckers on the ends, which they use to attach themselves to rocks
and to trap prey items. Sea stars that live on soft surfaces (such as sand) do not
have suckers. These sea stars use their longer tube feet instead, to penetrate into
the sand and other soft surfaces.
Feeding Habits:
Most sea stars are carnivores, (eat other animals), they feed on gastropods,
barnacles, sea anemones, sea snails, sea urchins, bivalves (shellfish) and some times
even crabs, dead fish, and other sea stars. Pretty much any animal they can get
their feet on! Instead of their food into their mouth as we do, sea stars instead
flip their stomachs out through their mouth and digest their prey from the inside
out. When the animal is completely digested the stomach is pulled back into the sea
star's body.
Reproduction:
Sea stars are dioecious, meaning that the males and females are in separate
individuals, and have as many as ten gonads (two at the end of each arm)!! Sea stars
reproduce by free spawning which means the eggs and sperm are released into the
water from the male and female at the same time. The eggs and sperm then drift
off until they meet up with each other and fertilization occurs. Hundreds of
thousands of larvae are released into the ocean during one breeding season,
however a large portion of these do not survive. As sea star larvae float along
through the plankton they are food to fish and other sea creatures.
Regeneration:
Sea stars have the remarkable ability to regrow their arms if they are damaged or
eaten by predators. In fact, in some cases an entire sea star can be regenerated
from just a single arm! However, this process is slow and it may take up to a year
for a sea star's arm to grow back to its original size.
Procedure:
1. Rinse off the sea star and place on the dissection tray.
2. Place the sea star dorsal side up and locate the Sieve plate (Madreporite). It is
a perforated structure which regulates the movement of water in and out of the
Water Vascular System (locomotor). This system is involved in the functioning of
the tube feet that line the ambulacral grooves on the undersurface of the sea
star. Turn the sea star over, ventral side up, and locate the mouth, ambulacral
grooves and tube feet.
3. Cut the tip of one of the rays with a scissor. Place the sea star dorsal side up
and insert scissor through opening cutting toward the central disk. Very carefully,
cut around the central disk and remove the portion of the spiny skin. NOTE: DO
NOTcut the Sieve tube! Pin the sides of the sea star ray open with the dissecting
pins. Note that the most of the space is taken up by two highly branched digestive
glands.
4. Examine the glands with a hand lens and note the numerous lobed structures
which secrete digestive juices. Gently lift the digestive glands and locate their
connection to the pyloric duct. Follow the duct back to its connection with the
stomach which consists of a lobed lower cardiac region and an upper pyloric region.
During feeding the cardiac region of the stomach is everted through the mouth.
The food is partially digested and then passed into the pyloric region which
empties into the anus where waste products of digestion are discharged to the
outside. Two small rectal pouches can be found near the anus. These are thought to
function as temporary storage areas for wastes products.
5. Cut the pyloric duct where it enters the stomach and remove the digestive
glands. If the sea star was caught during the breading season, the rays will almost
be filled with the gonads reproductive organs. At other times the gonads are very
small. The male and female gonads look alike. To determine the sex of the sea star,
the gonads must be examined microscopically. To do this remove a small piece of
the reproductive organ and mince it in a drop of water on a slide. Add a cover slip
and examine under the low and high power of a microscope. The testes of the male
have flagellated sperm. The ovaries of the female produce spherical egg cells that
are considerably larger than sperm. Eggs and sperm are discharged into the water
where fertilization occurs.
6. The water vascular system of the sea star consists of a series of sea water
filled ducts that function in locomotion and feeding. To study this system, it will be
necessary to carefully remove the reproductive organs and the remaining parts of
the digestive system, the stomach and anus. Be careful not to damage the sieve
plate. By careful dissection, locate the following parts of the water vascular
system: stone canal, ring canal, sieve plate, radial canal and tube foot. Water
enters this system through the sieve plate which is connected to a circular ring
canal by the stone canal. The water is then distributed to the radial canals that
pass into each of the rays. Lining the ridge through which each radial canal passes
is a double row of bulb-like structures called ampullae. These are connected to the
tube feet which project from the ambulacral groove on the undersurface of each
ray. Water form the radial canal collects in the ampullae. Contraction of the
ampullae causes the tube feet to elongate as water is forced into them. Expansion
of the ampullae results in shortening the tube feet. Thus, through the use of small
suction discs at the end of each tube foot and the alternate expansion and
contraction of the ampullae, the sea star is able to move.
7. When finished, discard of the sea star as instructed by the teacher. Clean
materials and lab area.
Starfish Dissection Questions:
1. What phylum does the starfish belong to?
2. Is the starfish a fish?
3. Why are they referred to as benthic?
4. What does the water vascular system consist of? What is it used for?
5. How does regeneration occur in starfish?
6. What is the eyespot? Where is it located?
7. How can the sex of a starfish be determined?
8. Describe the feeding and digestive process in starfish.
9. Explain respiration and gas exchange in starfish.