MARINE INVERTEBRATES
11
Echinoderms
When you have completed this chapter, you should be able to:
IDENTIFY the important characteristics of all echinoderms.
DISTINGUISH among the five main groups of echinoderms.
DISCUSS some important life functions of the echinoderms.
While slowly moving across the surface of a coral reef, the crown-ofthorns sea star (Acanthaster planci) devours the coral animals in its
path. A voracious predator, the crown-of-thorns is responsible for the
destruction of coral reefs around Hawaii and other tropical islands in
the South Pacific.
Sea stars, or starfish, are invertebrates that have a spiny skin covering, among other unique features. Such spiny-skinned animals are
classified in the phylum Echinodermata (meaning “spiny skinned”).
This group also includes such animals as the sea urchin, brittle star,
and sea cucumber. In this chapter, you will learn how these exclusively
marine animals are adapted to the ocean environment.
11.1
Stars in the Sea
11.2
Adaptations in the
Sea Star
11.3
Sea Urchins and
Sand Dollars
11.4
Eccentric
Echinoderms
259
11.1 STARS IN THE SEA
Stroll along a beach and you might see a “starfish” clinging to rocks
at the water’s edge. These bottom-dwelling invertebrates are not fish
at all; they have neither scales nor a backbone. In fact, starfish, or
sea stars, as they are now more appropriately called, are types of
echinoderms—spiny-skinned animals that lack body segmentation
but have radial symmetry (usually five-part) and an internal skeleton. In radial symmetry, all similar body parts are regularly arranged
around the central point of an animal’s body.
There are more than 5000 species of echinoderms, which are
placed in five main classes: sea stars; sea urchins and sand dollars;
brittle stars; sea lilies and feather stars; and sea cucumbers. This first
section describes the familiar sea stars, members of the class Asteroidea, as representative of this phylum.
Types of Sea Stars
Figure 11-1 The common
Atlantic sea star Asterias.
260
Marine Invertebrates
Sea stars are found from the subtidal zone to the deepest parts of
the ocean. These echinoderms usually have five (or multiples of five)
appendages, or arms, radiating out from a central body—hence the
“star” in their name. However, there is great variety among the sea
stars.
The common Atlantic sea star (Asterias), which looks typical, is
found in mussel and clam beds along the East Coast. (See Figure
11-1.) Likewise, the West Coast sea star (Pisaster) is found in beds of
California mussels. The seafood industry regards sea stars as pests,
because they can eat large numbers of commercially important
bivalves. The bat star (Patiria), whose five arms are connected in a
weblike structure like the wings of a bat, is commonly found in
kelp beds along the West Coast, from Alaska to California. Another
echinoderm from the Pacific, the sun star (Solaster), has 10 to 15
arms. The sun star lives on a variety of ocean bottoms, from low
tide to depths of more than 400 meters. Sun stars are atypical in
that they prey on other sea stars and even eat members of their
own species. (See Figure 11-2.)
Figure 11-2 Two Pacific
sea stars—the bat star
Patiria, which has a weblike structure, and the sun
star Solaster, which has up
to 15 arms.
Bat star (Patiria)
Sun star (Solaster )
11.1 SECTION REVIEW
1. Why is it more accurate to say “sea star” than “starfish”?
2. List some important characteristics of the sea stars.
3. Why do some people consider sea stars to be pests?
11.2 ADAPTATIONS IN THE SEA STAR
Sea stars often lose an appendage in struggles with other marine animals. When a sea star loses an arm, it can grow another one back, or
regenerate it, as evidenced by the fact that one arm will be noticeably shorter than the others.
The spines that give sea stars their characteristic rough skin are
composed of calcium carbonate (CaCO3). The spines are connected
to an internal skeleton, or endoskeleton, within the skin, also composed of CaCO3. The spiny covering helps support and protect the
echinoderm.
Sea stars breathe through their skin and through their tube feet.
On the dorsal surface of the skin are small, ciliated fingerlike projections called skin gills. Oxygen from the water diffuses through
the thin membrane of the tube feet and skin gills into a fluid-filled
space under the skin called a coelom. The coelom is lined with ciliated cells that beat back and forth to circulate oxygenated fluid
Echinoderms
261
around the body. Cell wastes and carbon dioxide diffuse from the
coelom through the skin gills and tube feet to the outside. In effect,
the sea star has an open circulatory system.
Feeding and Locomotion in the Sea Star
Sea stars use their arms for locomotion and for food-getting. The
underside, or ventral surface, of each arm contains numerous little
tube feet located in a groove. At the end of each tube foot is a suction disk. When the suction disk comes into contact with a hard
surface, it clings to that surface. Muscles in the tube feet control the
clinging and pulling actions that enable the sea star to move. This
“walking” motion helps the sea star find its food. In addition, as discussed above, for most echinoderms the thin walls of the tube feet
serve as an important respiratory surface for the exchange of gases.
(See Figure 11-3.)
Bivalve mollusks are a favorite food of the sea star. How does a
sea star open up a clam? The sea star uses its hundreds of tube feet
to grasp the clam and cling onto each of its shells. The tube feet
exert a force that pulls the two shells in opposite directions. When
that force is applied for several hours, the adductor muscles inside
the clam become tired, and the clam opens.
Figure 11-3 External
anatomy of a sea star
(ventral view).
Ventral surface
Arms
Grooves
Mouth
262
Marine Invertebrates
Tube
feet
How does the sea star consume the clam? Since clams are usually too big to fit through a sea star’s mouth (located in the center of
its underside), the sea star pushes its thin, membranous stomach
out through its mouth to engulf the food. (In some cases, the sea
star’s stomach can be pushed into a shell that is not tightly shut,
without the tube feet first prying the shell open.) Digestive enzymes
secreted by the sea star’s stomach digest the food externally. The sea
star then pulls back its stomach, which contains the digested food
particles. Nutrients are absorbed and transported to its body cells in
the fluid-filled coelom. Wastes are eliminated through the anus.
(Undigested wastes, such as shell fragments, are eliminated through
the mouth.)
Locomotion is necessary for food-getting by sea stars. How is
movement accomplished? A network of water-filled canals and
tubes, called the water vascular system, enables movement in sea
stars. Tracing the pathway of water through this system will help
you to understand how it works. (See Figure 11-4.) Water enters the
sea star (when there is a loss of internal liquid) through a small filter
called the sieve plate, also called the madreporite. The sieve plate is
found on the topside, or dorsal surface, of the sea star near its cenDorsal surface
Figure 11-4 External and
internal (cut-away)
anatomy of a sea star
(dorsal view).
Radial
canal
Ampullae
Digestive
gland
Ring
canal
Gonad
Anus
Coelom
Mouth
Groove
Central
disk
Upper part
of stomach
Arm
Tube
feet
Stone
canal
Spines
Sieve
plate
Eyespot
Echinoderms
263
ter, an area referred to as the central disk. After entering, the water
passes down through a short stone canal, then into a circular ring
canal within the central disk. From the ring canal, the water flows
through the radial canals. There is one radial canal in each arm.
Many tube feet are connected to each of the radial canals.
Movement occurs when water enters the tube feet. At the top
of each tube foot is an ampulla, a structure that resembles the rubber bulb on a medicine dropper. After the ampulla fills with water
from the radial canal, it contracts. This contraction of the ampullae (by ampullar muscles) forces water into the tube feet, causing
them to extend. Then, when the tube feet make contact with a substrate, circular and longitudinal muscle fibers within them contract,
forcing water back into the ampullae. This exit of water from the
tube feet creates the suction that holds the sea star to a substrate or
clamshell. The sea star uses this suction force to push and pull itself
along or to open a bivalve shell.
Sea Star Response, Reproduction,
and Regeneration
Sea stars are sluggish creatures and slow to respond to stimuli
because they have a simple nervous system. However, they can
respond to stimuli such as changes in the amount of light. Tiny
light receptors, called eyespots, are located at the end of each arm.
The eyespots convert light into electrical impulses, which are carried by nerves to a central nerve ring that encircles the mouth. The
nerve ring coordinates the movements of the arms by sending messages to and from radial nerves located in the arms.
Sea stars have separate sexes, but the sexes look identical so you
cannot know the sex by looking at them. You have to examine the
sea star internally. Look at the cross section of part of a sea star’s
arm. (Refer to Figure 11-4). Gonads are located inside each arm, near
the central disk. Ovaries and testes shed the eggs and sperm, respectively, into the water through openings found between the
appendages. Both fertilization and development occur externally.
Sea stars can also increase their numbers through regeneration. If
an arm is torn off during a struggle (for example, with a predator), a
new arm can be regenerated; and a whole new sea star can grow from
the severed appendage, provided part of the central disk is present.
264
Marine Invertebrates
The lab investigation at the end of this chapter will give you a
better understanding of the sea star’s external anatomy.
11.2 SECTION REVIEW
1. How does a sea star open a bivalve such as a clam?
2. Explain how a sea star uses its tube feet to move.
3. Describe ingestion and digestion in a sea star.
11.3 SEA URCHINS AND SAND DOLLARS
The echinoderm with the most impressive spines is definitely the
sea urchin, a member of the class Echinoidea. The sea urchin’s
movable spines are attached to its internal skeleton, which is
formed by bony plates that are fused. (As in the sea star, both the
spines and endoskeleton are made of CaCO3.) This endoskeleton,
which remains when a sea urchin dies, is sometimes found washed
up on a beach. It has an attractive pattern of raised bumps, evidence
of the former attachment points for the spines.
The animals in this class, which also includes sand dollars and
sea biscuits, are characterized by oval or round bodies that lack arms.
They are the only echinoderms that use both their spines and tube
feet to move. Sea urchins inhabit the intertidal and subtidal zones
along rocky coasts. They move very slowly along the rock surfaces,
scraping off algae with their unique five-toothed mouth structure,
called an Aristotle’s lantern (because of its resemblance to an ancient
Greek lantern). Along the rocky coasts of Maine, California, the
Pacific Northwest, and elsewhere in the world, sea urchins do such a
good job of grazing that they often scrape the rocks bare of seaweeds.
Predation and Protection Among Sea Urchins
In shallow tropical waters, be careful where you walk—you could
step on the long-spined sea urchin (Diadema). The sharp spines can
inflict a very painful puncture wound. In some species, the spines
may be hollow and contain toxins as well. Other species of sea
urchin, such as the purple sea urchin (Arbacia) and the green sea
Echinoderms
265
Figure 11-5 Three representative sea urchins; these
echinoderms use both
their tube feet and spines
for locomotion.
Green sea urchin
Hatpin sea urchin
Long-spined
sea urchin
urchin (Strongylocentrotus), which graze on seaweeds along the
Pacific Coast, have shorter, thicker spines. For protection from
predators and strong wave action, sea urchins often use their spines
to wedge themselves in the spaces between rocks. (See Figure 11-5.)
The rock-boring urchin (Echinometra) that inhabits the Caribbean
takes this a step further—it uses its teeth to bore into the rock, forming a cup to hide in.
The spines of the sea urchin are a natural protection against
most predators, except the California sea otter. The sea otter (see
Chapter 14) is a marine mammal that dives to the ocean floor to
hunt for sea urchins. After picking up a sea urchin, the sea otter
swims to the surface, rolls over on its back, then places the sea
urchin on its chest. Using a rock that it also picks up from the
seafloor, the sea otter cracks open the sea urchin and eats the contents. Humans also eat sea urchins. In many countries, sea urchins
are considered a delicacy because of the eggs they contain.
Life Cycle of the Sea Urchin
There are male and female sea urchins but, as with the sea star, you
cannot tell the animal’s sex just by looking at it. During the breeding season, the female sea urchin releases a great number of large
eggs into the water. Sperm released from the male sea urchin fertilizes the eggs externally.
The processes of fertilization and development in the sea urchin
can be easily observed under the microscope. For this reason, biologists use the sea urchin in embryological studies. A useful feature of
their development is that up to the blastula stage, all the cells of the
embryo are identical—if separated from the embryo, each cell can
266
Marine Invertebrates
Figure 11-6 Development of the sea urchin,
from zygote to adult
stage.
Blastula
Gastrula
Larva
(early)
Larva
(late)
Adult
sea urchin
develop into a separate, identical animal. Much of what we know
today about embryology has come from studies done on the sea
urchin. Like those of the other echinoderms, embryos of the sea
urchin go through a free-swimming larval phase. The larvae, which
are bilaterally symmetrical, live as part of the plankton community
until they settle on the seafloor and develop into adult sea urchins.
(See Figure 11-6.)
Sand Dollars and Sea Biscuits
The sand dollar (Echinarachnius) looks like a large coin (hence its
name), and has short spines covering its skin. Sand dollars use their
spines to burrow in the sand, where they feed by catching plankton and organic debris in sticky strings beneath their spines. The
food is then pushed toward the mouth. Members of this class have
a well-developed intestine and anus, through which the food is
digested and eliminated, respectively. When a sand dollar dies and
its soft parts decay, the flat internal skeleton of calcium carbonate
remains. People often collect these attractive “shells,” which have a
distinctive star-shaped pattern on them. (See Figure 11-7.)
Closely related to the sand dollar is the sea biscuit (Plagiobrissus). However, this echinoderm is more rounded (like a biscuit), has
longer spines, and inhabits the sandy seafloor around coral reefs.
Sea biscuits feed on organic debris and algae.
11.3 SECTION REVIEW
1. Compare food-getting in sea stars and sea urchins.
2. By what method do sand dollars feed? What do they eat?
3. Why is the sea urchin considered a good organism for embryological studies?
Sand dollar
Figure 11-7 The sand dollar uses its short spines for
burrowing in the sand; its
“shell,” or internal skeleton, has a unique pattern.
Echinoderms
267
11.4 ECCENTRIC ECHINODERMS
The sea urchin and the sea star are probably the most commonly
encountered echinoderms. Species of echinoderms that may be less
familiar to you are described below.
More “Stars” in the Sea
One of the most curious of the echinoderms is the brittle star, which
is placed in its own class, Ophiuroidea. Although they are actually
the most abundant of the echinoderms (in terms of both numbers
of species and individuals), brittle stars (such as Ophiopholis, Ophiocoma, and Ophioderma) are not very obvious because they are
nocturnal, bottom-dwelling animals that hide under rocks during
the day. Brittle stars live in the
intertidal zone, from the arctic to
the tropics. A subgroup of brittle
Flexible arms
stars, called basket stars (Gorgonocephalus), have coiled, branching
arms and live on the deep ocean
Central disk
floor, thousands of meters below
the surface.
Unlike the sea stars, brittle
stars have a distinct, flattened central disk; and they do not use their
tube feet for movement. Rather,
they have muscles in their long,
narrow flexible arms that enable
Figure 11-8 The brittle
them to scurry rapidly about on
star uses its long flexithe seafloor, looking for morsels of
ble arms to move and
food. (See Figure 11-8.) The brittle
to catch food.
Brittle star
star is so named because of its delicate appearance and its ability to
detach its arms when attacked, thus evading predators. Like the sea
stars, brittle stars can regenerate their missing arms.
Brittle stars have more than one feeding method. They can use
their arms to gather organic debris from the seafloor, to capture live
invertebrates, to filter-feed by trapping bits of food in sticky strands,
268
Marine Invertebrates
CONSERVATION
In a “Pickle” over the Sea Cucumber
It doesn’t look very appetizing, this spinyskinned, oblong-shaped animal. Yet in Southeast Asia, a single cooked and dried sea
cucumber is considered a delicacy and sells for
$80. With interest in, and profits from, the sea
cucumber so high, the demand for these echinoderms has far outstripped their numbers in
local South East Asian waters. So, the sea
cucumber fishing federation turned to the Galápagos Islands, located over 900 km off the coast
of Ecuador, as a potential source of this item.
Although much of the area has been declared a
national park by Ecuador, the islands are home
to 15,000 people, most of whom make their living from the sea.
By 1992, about 30 million sea cucumbers
had been collected from the waters around the
Galápagos Islands. Scientists were concerned
that the echinoderm was in danger of being
over-harvested. So, the government of Ecuador
imposed a one-year ban on the harvest, followed by a partial ban. Then, in the mid-1990s,
Ecuador established a fishing season and quotas
to reduce over-harvesting. Unfortunately, these
conservation measures were not successful. By
the late 1990s, more than 6 million sea cucumbers were being harvested each year. In 1999, a
complete ban on commercial fishing of sea
cucumbers was enacted. This led to strikes and
protests by the local fishermen, and to an
increase in the illegal harvest of sea cucumbers.
The ban was lifted again in 2002, based
on the outcome of a scientific study of the sea
cucumber population and on a meeting that
included local fishermen, government officials,
and the scientific community. Stricter guidelines
and new quotas for the harvest were established. Now, all fishermen will be licensed; the
harvest will be permitted in designated areas
only; and monitors will be hired to check for
compliance.
Hopefully, a compromise has been reached
that will allow the development of a sustainable
harvest of Galápagos sea cucumbers (one that
does not threaten their survival). Then, all parties concerned will no longer be in a “pickle”
over these unlikely objects of desire.
QUESTIONS
1. Why are some people concerned about the harvest of Galápagos sea cucumbers?
2. What groups of people are involved in this controversy? Defend the position of one group.
3. Describe a possible compromise (solution) that might satisfy all the parties involved.
4. How is sustainability of the harvest related to survival of the sea cucumber?
Echinoderms
269
or to capture suspended food bits with their tube feet—all of which
is brought into their jawed mouth.
“Lilies” and “Feathers” in the Sea
Sea lily
Figure 11-9 The sea lily
is a sessile crinoid with
feathery arms, used for
filter feeding.
The sea lilies and feather stars—members of the class Crinoidea—
look much more like flowers than like animals. Known as crinoids,
they are the most ancient group of echinoderms, having originated
hundreds of millions of years ago. The body of a crinoid is composed of dozens of feathery arms, usually perched atop a jointed
stalk. Crinoids generally have just a limited ability to move. The sea
lilies are sessile; they live attached by a stalk to the ocean bottom.
(See Figure 11-9.) The feather stars mostly crawl along coral reefs,
but some swim by flapping their arms. Using a type of feeding similar to that of the brittle stars, crinoids filter feed by waving their
arms, thereby capturing bits of zooplankton in their tube feet
(which then pass the food to the mouth). Like the brittle stars,
crinoids do not use their tube feet for locomotion.
“Cucumbers” on the Seafloor
Sea cucumber
Figure 11-10 The sea
cucumber has five rows of
tube feet, used for feeding
and movement.
270
Marine Invertebrates
At first glance, members of this last group of echinoderms do not
look much like echinoderms; in fact, they do not even look like animals! However, on closer examination you can see that the sea
cucumber—whose soft, oblong body lacks arms—has tube feet that
are arranged in five rows, similar to the five-part radial pattern seen
in the sea star. The sea cucumbers, which are placed in the class
Holothuroidea, have lost the endoskeleton and spines typical of
their phylum, retaining only small bony pieces in the skin. They
live on sandy and rocky seafloors in intertidal and subtidal zones
and are most abundant at great depths. (See Figure 11-10.)
Sea cucumbers such as Holothuria use their sticky, branching tentacles—which are actually enlarged tube feet—to trap microscopic
organisms. The tentacles, which are located around the mouth, are
extended during feeding and retracted when the animal is disturbed. Members of the genus Cucumaria that live on the East and
West coasts have five rows of tube feet along their bodies, which are
used for slowly moving along the substrate and for trapping food
particles in the sand. Sea cucumbers have a one-way digestive tract;
wastes are excreted through the anus. Whereas most echinoderms
exchange gases through their tube feet and skin gills, sea cucumbers take in and release water through their anus. Gas exchange
then occurs inside the coelom across the membranes of a structure
called the “respiratory tree.” Another unusual feature of the sea
cucumber is that it can release its digestive organs when disturbed
by a predator, thus leaving a meal for the predator while it escapes.
It later regenerates the lost organs.
11.4 SECTION REVIEW
1. Describe some feeding methods of the brittle stars.
2. What is the basic structure of a crinoid? How does it feed?
3. What features of the sea cucumber show it is an echinoderm?
Echinoderms
271
Laboratory Investigation 11
Adaptations of Sea Stars
PROBLEM: How is the sea star adapted for carrying out its life functions?
SKILL: Identifying relationships between body structures and life functions.
MATERIALS: Living sea star, pan of seawater, hand lens, fresh clam or mussel.
PROCEDURE
1. Put a sea star, dorsal side up, in a shallow pan and cover it with seawater.
Use the sea star diagrams in Figures 11-3 and 11-4 as a guide. How many
arms or appendages does the sea star have? Make a sketch of your sea star.
Label one of the arms in your drawing.
2. Feel the skin of the sea star. Then examine the skin with a hand lens. Notice
the short spines, which you were able to feel. The spines are connected to an
endoskeleton, which is composed of calcium carbonate (like the shells of
mollusks). Label the spines in your drawing.
3. How does the sea star breathe? Examine the skin with your hand lens. Look
for tiny fingerlike projections, called skin gills. Oxygen diffuses from the water
through the thin membrane of the skin gills and into the coelom.
4. Locate the sieve plate, or madreporite, which is a white or orange spot on
the dorsal surface. Water enters through the sieve plate, then passes through
a network of canals that ends in the tube feet.
5. Locate the tube feet by turning the sea star over. The many tube feet are in
grooves that run down the center of each arm. Touch the tube feet; you will
notice that they cling to your finger. Each tube foot looks like a tiny plunger.
Put the sea star back in the pan of water, with the tube feet facing down.
Notice the clinging and pulling action of the tube feet used in locomotion.
Make a sketch of a tube foot and describe its function.
6. Now place the sea star ventral side up in the pan of seawater. Make a sketch
of the sea star that shows its ventral side. Describe the motion of the tube
feet. Can the sea star turn itself over? Which arms does it use to turn over?
Record your observations in a copy of Table 11-1 in your notebook.
272
Marine Invertebrates
TABLE 11-1 SEA STAR STRUCTURES AND FUNCTIONS
Sea Star Observations
Structure
Function
Behavior
Dorsal Side
Ventral Side
7. How does the sea star feed? Look for the mouth in the center of the sea star
on its ventral side. The mouth is too small to ingest a whole clam. Instead,
the sea star pushes its thin, membranous stomach out through its mouth
and into the clam’s shell, where it digests the food externally. Open up a
mussel or clam shell and put it in a pan of seawater. Place a sea star that has
not been fed for a few days next to the clam. Record your observations.
8. How does a sea star open up a clam? Put your hand underwater and place a
sea star on top of it. Gently try to pull the sea star off your hand. Notice how
it clings to your skin. The tube feet, with their suction disks, generate a
pulling force. When the arms of a sea star are draped over the two shells of a
clam, hundreds of tube feet pull the shells in opposite directions. The adductor muscles in the clam become fatigued, causing the shells to open.
OBSERVATIONS AND ANALYSES
1. How does a sea star move?
2. How does the sea star ingest and digest food?
3. Compare the “skeleton” of a mollusk with that of an echinoderm.
Echinoderms
273
Chapter 11 Review
Answer the following questions on a separate sheet of paper.
Vocabulary
The following list contains all the boldface terms in this chapter.
ampulla, Aristotle’s lantern, brittle stars, crinoids, echinoderms,
endoskeleton, eyespots, feather stars, sand dollar, sea cucumbers,
sea lilies, sea stars, sea urchin, sieve plate, skin gills, tube feet,
water vascular system
Fill In
Use one of the vocabulary terms listed above to complete each sentence.
1. Delicate echinoderms found on the seafloor are the ____________________.
2. The ____________________ uses its short spines to burrow in the sand.
3. Water enters a sea star through its madreporite, or ____________________.
4. The ____________________ are the most ancient group of sessile echinoderms.
5. In sea stars, the clinging and pulling of muscles in ____________________
allows movement.
Think and Write
Use the information in this chapter to respond to these items.
6. Describe what happens if a sea star loses one of its arms.
7. What functions do spines serve in the sea urchins and sand
dollars?
8. Compare and contrast the lifestyles of sea lilies and feather
stars.
Inquiry
Base your answers to questions 9 through 12 on the results of the experiment described below and on your knowledge of marine science.
A marine biology student hypothesized that a brittle star would
have a slower turnover response than an Atlantic sea star. To test
this idea, he placed the two species of echinoderms upside down
274
Marine Invertebrates
in separate containers of seawater under the same experimental
conditions. The time it took for each animal to turn over (in each
of six trials) is shown in the table below.
Brittle Star
Atlantic Sea Star
Trial
Turnover Response Time
(minutes)
Trial
Turnover Response Time
(minutes)
1
0.15
1
6.0
2
0.17
2
10.0
3
0.33
3
2.0
4
0.25
4
1.75
5
0.23
5
2.50
6
0.15
6
2.0
Average
0.21
Average
4.04
9. Which part of the scientific method is represented by the data
in the table? a. hypothesis b. materials c. results
d. conclusion
10. Which is an accurate statement regarding the data in the
table? a. The data support the hypothesis. b. The
hypothesis is not supported by the data. c. The average
turnover response for the brittle star is 21 seconds. d. The
turnover response was recorded in seconds, not minutes.
11. A tentative conclusion that can be drawn from the data in the
table is that a. the brittle star moves more quickly than the
Atlantic sea star b. the Atlantic sea star moves more quickly
than the brittle star c. turnover response in echinoderms
cannot be measured in minutes d. there is no significant
difference in turnover response time between the sea star and
the brittle star.
12. Which of the following suggests the best way to verify the
results of this experiment? a. Perform the experiment again,
but with fewer trials. b. Perform the experiment again, but
with more trials. c. Add food to give each animal an
incentive for movement. d. Use brittle stars only in both
containers of seawater.
Echinoderms
275
Multiple Choice
Choose the response that best completes the sentence or answers the
question.
13. The small ciliated projections that
enable breathing in this animal are
called
a. spines
b. skin gills
c. ampullae
d. eyespots.
14. You notice that a sea star in an aquarium has one very short
arm. The best explanation for this is that a. its growth
hormones have been suppressed b. the appendage was lost
and is regenerating c. its tube feet are not functioning
d. the arm is not really needed.
15. The side of a sea star on which its sieve plate is found is the
a. dorsal b. ventral c. anterior d. posterior.
16. What prevents a sea star from falling off the side of an
aquarium tank? a. clinging action of its tube feet
b. suction by its mouth c. adhesive properties of its spines
d. water pressure
17. The symmetry of echinoderms is referred to as
b. radial c. spiral d. unilateral.
a. bilateral
18. A sea star can open up a clam because of the functioning of its
a. tube feet b. spines c. stomach d. madreporite.
19. The crown-of-thorns sea star is considered a pest because it
a. destroys coral reefs b. consumes bivalve mollusks
c. is harmful to humans d. is harmful to fish.
20. The function of the water vascular system in the sea star is to
enable a. locomotion b. digestion c. sensitivity
d. respiration.
21. Which of these echinoderms moves most rapidly on the
seafloor? a. sea star b. brittle star c. sea lily
d. sea urchin
276
Marine Invertebrates
22. The echinoderm that uses both its spines and its tube feet to
move is the a. sea urchin b. brittle star c. feather star
d. sea star.
23. Sea urchins scrape the algae from rock surfaces with a
specialized mouthpart called the a. sieve plate
b. madreporite c. Aristotle’s lantern d. skin gill.
24. The echinoderm that differs from all others in that it lacks
an endoskeleton, and only retains small bony pieces in its
skin, is the a. sand dollar b. sea lily c. brittle star
d. sea cucumber.
Research/Activity
■
Observe sea stars moving in an aquarium. Examine the underside (ventral surface) of the sea stars as they move along the
sides of the tank. Describe the motion of their tube feet and
explain how sea stars use their arms to grip surfaces and turn
over.
■
Use the Internet to get an update on the sea cucumber harvest
in the Galápagos Islands or to research the latest findings on the
damage done to coral reefs by the crown-of-thorns sea star.
Write a report and present your findings to the class.
Echinoderms
277