08/Marine Worms

MARINE INVERTEBRATES
8
Marine Worms
When you have completed this chapter, you should be able to:
IDENTIFY the basic characteristics of the various worm phylums.
DESCRIBE the life functions of the different groups of worms.
DISCUSS some adaptations of worms to the marine environment.
In their general shape, worms tend to resemble one another. You
are probably most familiar with earthworms—the soft, moist, slowmoving creatures that live in the soil. Most worms move through substrates by wriggling their bodies from side to side. Appendages are
missing or greatly reduced in size. But not all worms are alike in how
they look and behave.
The delicate-looking feather duster worms shown above actually
live in stony tubes, which offer them some protection against other
animals. The worms’ colorful featherlike structures are thrust out of
their tubes to catch plankton and to take in oxygen.
Worms vary in size from microscopic to over several meters in
length. In fact, there are enough differences among the groups of
worm species to justify their being placed into several phylums. In this
chapter, you will learn about some similarities and some differences
among the worms that live in marine environments.
8.1
Flatworms and
Ribbon Worms
8.2
Roundworms
and Segmented
Worms
8.3
Giant Tube Worms
and Arrow Worms
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8.1 FLATWORMS AND RIBBON WORMS
The vertical zone of water that extends from the top of the ocean to
its bottom is called the water column. There are few places in the
water column where worms cannot be found. They inhabit the seas
from the surface down to the seafloor. There are significant differences among the various groups of worms that live at the different
depths. Most of the worms you will learn about in this chapter live
at the bottom of the water column, near or in the ocean’s sediments. Some are found closer to the surface. Yet others are parasitic,
living within the bodies of different marine organisms.
Flatworms
Look at the group of worms shown in Figure 8-1. As you can see,
the bodies of these worms are flat. The flat-body form is the distinguishing trait used to classify these worms in the phylum Platyhelminthes (meaning “flatworms”). There are both freshwater and
saltwater species of flatworms. Although some flatworms are microscopic, many others can be seen without a microscope. In fact, some
species can reach nearly 20 meters in length! There are more than
10,000 species of flatworms.
Figure 8-1 The three
types of flatworms.
Sucker
Head
region
Hooks
Head region
Eyespot
Sucker
Section
Mouth
Tail
region
Planaria
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Tapeworm
Liver fluke
The Planarian
Look at a macroscopic flatworm, the planarian (Planaria), shown
in Figure 8-1. Planarians, which feed on small organisms and
organic debris, are found in freshwater and marine habitats. These
worms have a mouth on their lower, or ventral, surface through
which they take in food as they move along rocks and other substrates. The mouth connects to a branched digestive cavity. Unlike
other worms, planarians have a two-way digestive tract. Food enters
the mouth and is digested in the intestine; the nutrients diffuse
throughout the body (there are no circulatory or respiratory systems). The undigested materials are discharged from the mouth
(there is no anus).
The planarian glides along the bottom, moving its head from
side to side. Locomotion results from the contraction of body muscles and the action of cilia, which are attached to cells on the
worm’s ventral side. When the cilia beat backward, the planarian
moves forward along the surface of a substrate. Planaria have two
eyespots in their head region, which are surrounded by the ganglia
(nerve cell clusters). The ganglia act like a simple brain, sending
nerve impulses along two ventral nerve cords to the rest of the body.
This lets the flatworm respond to stimuli (such as light) from its
environment.
Since the structures on the right side of its body are the same as
the structures on the left side of its body, the planarian is said to
have bilateral symmetry. In fact, bilateral symmetry, which first
appears in the flatworm, is a characteristic feature of all other worms
and more structurally complex animals. This type of body plan is
associated with the development of a head region, such as that seen
in the flatworm. Another feature first seen in the flatworm is the
possession of three cell layers: ectoderm, endoderm, and mesoderm
(the middle layer). This is important for the development of organ
systems, also first seen in the flatworms.
Planaria are capable of both asexual and sexual reproduction.
During asexual reproduction, a planarian can attach to a substrate,
stretch its body, and break in two. Each half can then regenerate the
parts needed to form a whole new organism.
The planarian, like most flatworms, is a hermaphrodite—it
contains both male and female reproductive organs. However,
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self-fertilization does not occur. During mating, two flatworms
exchange sperm, so that sperm from each of the flatworms fertilizes
the eggs of the other flatworm internally. Development of fertilized
eggs occurs externally in tiny capsules or cocoons. Offspring hatch
from the cocoons in two to three weeks.
In the same class as the planaria are other marine flatworms.
These worms are also bilaterally symmetrical, although the head
region is not obvious as in the planaria. They are characterized by
very beautiful colors and patterns, and frilly edges that seem to flutter as they swim.
Tapeworms and Trematodes
Some flatworm species live as parasites. A parasite is an organism
that obtains its food by living in or on the body of another organism. The organism that is fed on is called the host. The tapeworm,
shown in Figure 8-1, is sometimes found in the intestines of fish
and other animals, including humans. The tapeworm attaches itself
to the intestinal lining of its host and absorbs nutrients directly
through its thin body wall. As a result, it has no need for a digestive system. This feeding method works well for the tapeworm—
some can grow to more than 18 meters in length!
Another flatworm that is a parasite is the trematode. The
trematode, or fluke, lives in the bodies of mollusks, fish, birds, and
other animals. The liver fluke, which lives as a parasite in mammals, is shown in Figure 8-1. The blood fluke is a parasite that
attaches to a fish’s skin and then forms a cyst in which it lives
within the fish’s tissues. Trematode parasites that are accidentally
eaten in raw fish may then reproduce in the digestive tracts of people. Human wastes that are discharged into bodies of water may
contain the trematode’s eggs. These eggs develop into swimming
larvae, which penetrate the soft tissue of their first host—a snail.
The larvae develop inside the snail and, in time, are discharged
into the water, where they seek out their second host—a fish or
aquatic bird—thus completing their life cycle. If you happen to be
in the water when the trematode larvae are looking for a host,
they may attach to your skin, causing what is called swimmer’s
itch.
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Marine Invertebrates
Ribbon Worms
You have just learned that some flatworms (the tapeworms) can
grow nearly 20 meters in length. Interestingly, there are other types
of worms that grow very large. The largest free-living worm in the
sea is the ribbon worm (Cerebratulus lacteus), shown in Figure 8-2.
The nearly 1000 species of ribbon worms are classified in the phylum Nemertea. Although some are relatively small, the average
length of a ribbon worm, or nemertean, is about 1 meter. However,
ribbon worms 10 to 12 meters long have been observed. All ribbon
worms are unsegmented and ciliated, have a milky color, and are
thin and flat like ribbons, hence their common name. Although
they resemble flatworms in their general shape, ribbon worms have
more “highly” developed circulatory and digestive systems.
Ribbon worms live in the intertidal zone. Most are free-living,
although some live inside the shells of clams and oysters. Ribbon
worms burrow in the sand and also swim with a gently undulating
motion among animals that live encrusted on rocks. Prey such as
small fish and polychaete worms (see Section 8.2) are speared by a
sharp, sticky extension called a proboscis, which the ribbon worm
extends from a recessed area in its head region. Food is digested in a
one-way digestive tract (that has a stomach and intestine) and
wastes exit through the anus. Ribbon worms have a simple, closed
circulatory system for the distribution of nutrients and oxygen.
Ribbon worms can reproduce asexually by breaking into pieces;
each piece is capable of regenerating into a whole new worm. They
can also reproduce sexually, having external fertilization and development. Like many other worms, the ribbon worm has a nervous
system that consists of ganglia (more concentrated than those of
the flatworm), which send impulses to its muscles by way of two
nerve cords.
Proboscis
Figure 8-2 The ribbon
worm.
8.1 SECTION REVIEW
1. What two features are first seen in the planarians?
2. Describe asexual and sexual reproduction in the planarian.
3. How do the planarian’s and the ribbon worm’s digestive and
circulatory systems differ?
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8.2 ROUNDWORMS AND
SEGMENTED WORMS
Roundworms
Mouth
Esophagus
Female
genital pore
Larvae
in uterus
End of
esophagus
Midgut
Eggs in
uterus
Ovary
Anus
The most numerous of all worms in the sea—in numbers of individuals and of species—are the roundworms. There are more than
10,000 species of roundworms. Very tiny roundworms live in the
sand and mud at the bottom of the water column. When the tide
goes out, roundworms can be found in the moist substrate of the
intertidal zone. Gravel samples taken from an aquarium tank and
examined under a microscope will also reveal a number of roundworms. (See Figure 8-3.)
Typical whipping movements of the roundworm’s body propel
it through the spaces between sand grains. Some roundworms can
also swim through the water. In the roundworm’s head region are
the ganglia, which connect to nerves that run through the body.
These nerves control the muscles that enable the worm to move and
actively respond.
Roundworms are classified in the phylum Nematoda; they are
often referred to as nematodes. As their name implies, roundworms
are characterized by a cylindrical body shape, which is tapered at
both ends. Most roundworms are small, but some reach up to a
meter in length. The sexes are usually separate (a few nematode
species are hermaphrodites), and reproduction is sexual. Fertilization is internal; development of eggs is external. Males and females
tend to differ in their size and shape.
Most marine nematodes live freely in bottom sediments and
feed on organic debris. Like the ribbon worm, roundworms have a
one-way digestive tract. Food enters the mouth, is digested in the
stomach and intestine, and undigested wastes pass out through the
anus, located at the opposite end. Like the flatworm, the roundworm
has no circulatory or respiratory systems. Nutrients diffuse into its
cells; respiratory gases and cellular wastes pass through its skin.
Figure 8-3 A marine
roundworm.
Segmented Worms
Of all the worms, the earthworm is probably the most familiar
to you. Earthworms are typically found in soil and other moist
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DISCOVERY
The Cells of C. Elegans
Dozens of scientists around the world are focusing their efforts, and their microscopes, on
Caenorhabditis elegans, a transparent roundworm just 1 millimeter in length. This common
worm—found scavenging on debris in moist
sediments everywhere—offers scientists an
uncommon chance to investigate biological
processes such as cell division, embryology, and
disease.
Many scientists choose to study C. elegans
because the adult worm contains a total of only
959 cells—compared with millions of cells in
other multicellular animals. The worm’s transparent skin enables scientists to observe its muscle cells and nerve fibers at work. And it allows
them to trace the pathway that the worm’s cells
take during cell division, from the fertilized egg
cell to the 959 cells of the adult just three days
later. That means that the researchers have
observed differentiation, the process by which
embryonic cells develop into adult muscle,
nerve, and skin cells.
Scientists also found that some cells in the
embryo are predestined to die rather than differentiate. It is not known why the cells die off
during development. However, researchers think
that the study of these cells may help them find
out which genes control cell death. This, in turn,
may lead to a better understanding of what
happens during diseases such as Alzheimer’s,
in which a person’s nerve cells inexplicably die,
resulting in severe memory loss.
One of the most spectacular achievements
in the last few years was the deciphering of the
complete genome of C. elegans—the first multicellular organism for which the entire DNA was
mapped! Scientists discovered that C. elegans
has more than 19,000 genes, which code for
over 19,000 different proteins.
Deciphering an organism’s genome is the
first step in learning which genes are responsible
for a cell’s fate. It is hoped that, in the years to
come, further research on C. elegans will continue
to throw more light on our understanding of cell
processes and diseases in humans.
QUESTIONS
1. State three advantages of using C. elegans over other animals for cell research.
2. How might studying C. elegans help us understand Alzheimer’s disease in humans?
3. Why was deciphering of the roundworm’s genome a significant achievement?
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Figure 8-4 An earthworm
(partly cutaway view).
Anus
“Brain”
Mouth
“Hearts”
Nerve cord
Clitellum
Artery
Vein
Food tube
Segments
sediments. Some of the internal and external features of an earthworm are shown in Figure 8-4. Notice that the worm’s body is
divided into compartments, or segments. Referred to as the segmented worms, or annelids, earthworms and all other annelids are
classified in the phylum Annelida (meaning “little rings”). There are
three classes of annelids, comprising more than 10,000 species.
Earthworms, and their aquatic relatives that feed on organic matter
in sediments, belong to the class Oligochaeta (meaning “few
bristles”).
The Sandworm
A common marine annelid is the clamworm, or sandworm (Nereis).
(See Figure 8-5.) Sandworms live in muddy sands in the intertidal
and subtidal zones of inland bays and marshes. When the tide
comes in, sandworms crawl around on the bottom and prey on tiny
invertebrates. When the tide moves out, sandworms burrow in the
sand and scavenge on organic debris. Some live within tubes.
How do sandworms carry out their life functions? Nereis captures its prey with two sharp hooks located in its mouth. The sandworm also has a proboscis, which it can extend to catch prey or bits
of seaweed. Food is digested in a one-way digestive tube; waste products of digestion are eliminated through the anus. The digestive
tract is separated from the skin by a fluid-filled space called the
coelom. (All annelids have a coelom.)
Sandworms belong to the class Polychaeta (meaning “many
bristles”). They wriggle through the wet sand by using their paddlelike appendages called parapodia (singular, parapodium). The parapodia are located on each segment. Each parapodium also has
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Marine Invertebrates
Figure 8-5 The sandworm
Nereis.
Mouth
Hooks
Bristles
Parapodium
Segment
Anus
hairlike bristles called setae sticking out from it. Movement results
when longitudinal and circular muscles located under the worm’s
skin are contracted.
The sandworm has a nervous system to coordinate the movements of its body. Eyelike receptors that receive light stimuli, and
touch receptors in the skin that pick up other external stimuli, send
impulses to a ventral nerve cord. The impulses travel to the brainlike ganglia. Movement occurs when the “brain” sends the impulses
via nerve cords to muscles, causing them to contract.
Like earthworms, sandworms breathe through their skin—in
this case, through their parapodia. Because the worm’s skin is thin
and moist, oxygen diffuses from the surrounding water into the
body, and carbon dioxide diffuses from inside the body to the outer
environment. Nutrients and respiratory gases are transported
around the body in a closed circulatory system. This consists of a
dorsal blood vessel, arteries, veins, and capillaries. The dorsal blood
vessel contracts and pumps the blood. Arteries and veins carry the
blood back and forth; capillaries connect arteries with veins.
Sandworms have a well-developed excretory system. Waste
products of metabolism are excreted from a pair of coiled tubes
located in each body segment. These organs, which carry out excretion for the annelids as our kidneys do for us, are called the
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Mouth
Bristles
Anus
Figure 8-6 The bloodworm.
nephridia. The sandworm reproduces sexually; the sexes are separate (unlike the earthworm, which is a hermaphrodite).
The Bloodworm and Other Polychaetes
Feathery gills
Stony tube
Figure 8-7 The fan worm.
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Marine Invertebrates
Another segmented worm found in marine sediments is the bloodworm (Glycera). (See Figure 8-6.) Like the sandworm, the bloodworm
is a polychaete. However, unlike the sandworm, the bloodworm has
an open circulatory system in which the blood circulates through
the tissue spaces rather than through blood vessels. You can see a
bloodworm’s blood through its skin, hence its common name. Polychaete worms also include a variety of other “bristle worms,” such as
the plumeworm and the paddle worm.
Sandworms and bloodworms usually burrow in sand and hide
in seaweed to avoid detection by predators. Some polychaete worms
have developed a different strategy to avoid being eaten—they live
inside a tube, which they make themselves. Most of these tubedwelling worms are small and threadlike in appearance. The fan
worm (Sabella) constructs its hard tube by mixing sand grains with
mucus, which it secretes from special sacs. (See Figure 8-7.) The
parchment worm (Chaetopterus variopedatus) secretes a tube formed
from a tough fibrous material, in which it lies buried in sediments
below low tide. Both of these worms thrust their feathery gills out of
the tube into the water to obtain oxygen, give off carbon dioxide,
and capture plankton. The tube of the trumpet worm (Pectinaria
gouldii) looks like an ice-cream cone fashioned from sand grains and
bits of shell cemented together. This worm extends its ciliated tentacles into the sediment to obtain food. Another polychaete tube
worm, called the Atlantic tube worm (Hydroides), secretes a hard
tube composed of calcium carbonate, which is cemented to the surfaces of mollusks, rocks, and corals.
The Leech
An example of a segmented worm without bristles is the leech,
which is placed in class Hirudinea. (See Figure 8-8.) Some leeches are
free-living, while others are parasites that can be found attached to
the gills and mouths of fish. The leech attaches to its host by means
of two suckers, located at the anterior (front) and posterior (back)
ends of its body. Sharp teeth in the sucker at the anterior end pierce
the host’s skin. The leech then draws blood from its host. At the same
time, the leech secretes a chemical anticoagulant, called hirudin,
into the wound. This prevents the host’s blood from clotting, thus
enabling the blood to flow freely into the leech. While feeding,
leeches can increase their body weight threefold. After feeding, the
leech drops off the fish and fasts while it digests its blood meal.
Posterior
sucker
Digestive
tract
Anterior sucker
Figure 8-8 The leech.
8.2 SECTION REVIEW
1. In what part of the marine environment are roundworms usually found? What do they feed on there?
2. Describe two adaptations polychaetes have to avoid predators.
3. How do polychaete tube worms feed and protect themselves?
8.3 GIANT TUBE WORMS AND
ARROW WORMS
Giant Tube Worms
You have just read about the segmented worms that live in tubes.
There are other types of worms that live in protective tubes, and
some of these grow quite large. An interesting group of deep-sea gutless worms belongs to the phylum Pogonophora. In 1977, a new
species of worm belonging to this phylum was discovered living
near hot-water vents on the deep seafloor, thousands of meters
below the ocean’s surface. First photographed by the submersible
Alvin, these giant tube worms (Riftia pachyptila) measure up to 1
meter long and live clustered in water that is rich in hydrogen
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Feathery
plume
Segments
Ventral blood
vessel
Feeding body
Bacteria live
inside
Tube
Deep seafloor
Figure 8-9 The giant tube
worm.
sulfide. The tubes in which they live (which are made of proteins
and minerals) may measure up to 2 meters in length. (See Figure
8-9.)
Marine biologists have discovered unique bacteria that live
inside the giant tube worms. These bacteria are able to use the
hydrogen from the hydrogen sulfide in the water and combine it
with carbon dioxide from seawater to produce sugars. The energyrich sugars are also used by the worms. This way of obtaining energy
makes the giant tube worms different from most other animals that
live on Earth. Remember, most animal life depends on the energy of
the sun, energy that is captured by plants and algae during the
process of photosynthesis. Yet sunlight cannot reach the great
depths at which the giant tube worms live. So the worms live by
utilizing carbohydrates made by the bacteria, which get their energy
from the oxidation of hydrogen sulfide. (The worms excrete the sulfur from the hydrogen sulfide compound.) The process by which
the bacteria produce energy-rich compounds from inorganic chemicals is called chemosynthesis.
Arrow Worms
At the other end of the water column, near the surface of the ocean,
lives the tiny, transparent arrow worm (Sagitta), shown in Figure
8-10. The arrow worm is classified in the phylum Chaetognatha
(meaning “bristlejaw”). Arrow worms are just a few centimeters
long. They have tiny fins that enable swimming, but they mostly
drift as part of the plankton community. An active hunter, the
arrow worm uses its mouth bristles, which are modified as hooks,
to prey on other animal plankton such as copepods, fish eggs, and
fish larvae.
Arrow worms have a one-way digestive tract. Food is digested
in the worm’s narrow intestine, and undigested wastes are eliminated through the anus. Like the flatworms and roundworms, the
arrow worm has no circulatory or respiratory systems. Digested
nutrients diffuse into its cells. Respiratory gases and cellular wastes
are exchanged between the arrow worm and the outer environment
through the worm’s thin skin. Arrow worms have a simple nervous
system that lets them respond to stimuli. Two eyes in the head
region are sensitive to light. Sensory projections (called papillae)
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Marine Invertebrates
Hooks
(mouth
bristles)
Eyes
Sensory
papillae
Ventral
nerve
ganglion
Intestine
Anterior
lateral fin
Eggs
in ovary
Anus
Posterior
lateral fin
Testis
Tail fin
Anterior
teeth
Posterior
teeth
Head
Trunk
Tail
Figure 8-10 The arrow
worm.
located along the surface of the worm’s body are sensitive to touch.
Ganglia are located in the head and trunk regions.
Arrow worms are hermaphrodites, and reproduction is sexual.
Each worm produces both sperm and eggs, but self-fertilization does
not occur. The sperm and eggs are shed directly into the water,
where the sperm from one arrow worm fertilizes the eggs of another.
8.3 SECTION REVIEW
1. How are giant tube worms able to feed at such great depths?
2. In what part of the water column are arrow worms found?
What do they eat?
3. How are the arrow worms’ means of locomotion and feeding
adaptive in their particular habitat?
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Laboratory Investigation 8
Adaptations of the Sandworm
PROBLEM: How is the sandworm adapted for carrying out life functions?
SKILL: Observing features of the sandworm by means of dissection.
MATERIALS: Preserved sandworm, dissecting pan, water, hand lens, forceps,
scissors, pins.
PROCEDURE
1. Place a sandworm in a dissecting pan and cover it with water so that you
can see the tissues more clearly. Notice that the body is divided into segments. Count the number of segments and record the number.
2. Which end of the sandworm is the front, or anterior, end? Use your hand
lens to locate the mouth, which is located in the first two segments at the
anterior end. With the forceps, gently squeeze the area. Two sharp hooks
(jaws) should stick out from the mouth, showing that this is the worm’s front.
3. Which side of the sandworm is the upper surface and which is the lower?
Notice that the lower surface is lighter in color than the upper surface. The
lower surface is the ventral side, and the upper surface is the dorsal side.
4. Notice the many tiny fiberlike appendages projecting from each segment.
Examine one of the appendages with your hand lens. The appendage, called
a parapodium, has a fleshy paddle shape. The parapodia contain bristles
called setae. These appendages are used for swimming and for burrowing.
5. Sandworms breathe through the thin, moist skin of the parapodia. Use your
hand lens to examine the parapodia.
6. Now place the sandworm in a dry dissecting pan with its dorsal surface up.
Pin the sandworm to the tray at the worm’s anterior and posterior ends.
Beginning at the posterior end, cut through the skin with the scissors. Make
the cut just slightly to the right of the midline (center of worm, lengthwise).
Carefully cut the skin without cutting the underlying tissues. As you cut, pin
the skin on both sides to the tray. Cut all the way up to the anterior end.
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Marine Invertebrates
Pharynx
Crop
Intestine
Dorsal
blood vessel
“Hearts”
Mouth
Esophagus
Gizzard
Ventral
blood vessel
Figure 8-11 The internal anatomy of a sandworm.
7. To see the internal structures more clearly, pour just enough water over the
sandworm to cover it. (See Figure 8-11.) Look at the inside body wall. The
wall is composed of longitudinal and circular muscles for movement.
8. Use your hand lens to locate the “brain,” or cerebral ganglia, at the anterior
end near the dorsal surface. The brain is a white structure with two lobes.
Locate the connection of the brain to a double ventral nerve cord. Trace the
ventral cord and notice that it extends the entire length of the sandworm.
Locate the tiny lateral nerve branches going to the muscles in each segment.
The ventral nerve cord relays messages to and from the brain.
9. Food enters the worm’s mouth; then it is digested in a food tube, the digestive tract. The mouth is connected to a wider part of the digestive tract called
the pharynx.
10. The pharynx connects with a narrow esophagus, located in segments 6
through 14. Food moves from the esophagus into the crop. The crop, in
segments 15 and 16, temporarily stores the food. Then the food passes
into the gizzard, where it is ground up with sand that is ingested with it.
11. Next, the food passes into the intestine, where it is further digested and
then absorbed into the blood. Finally, solid wastes are eliminated through
the anus, located in the last two segments. Note that the digestive tract is
separated from the skin by the fluid-filled space called the coelom.
12. Nutrients and oxygen are transported in the blood. Use your hand lens to
locate the aortic arches (“hearts”) and the dorsal blood vessel (on top of
the food tube), which connects with the arteries, veins, and capillaries.
13. Locate the ventral blood vessel under the digestive tract. The smaller blood
vessels connect the dorsal and ventral blood vessels. The blood vessels are
the circulatory system of the sandworm. Blood flows only inside this network of blood vessels; thus the sandworm has a closed circulatory system.
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14. Wastes are removed by paired, coiled tubes called nephridia, located in
most segments. Use your hand lens to look for tiny white tubes attached to
the inside body wall. (You may have to push the digestive tract aside.)
15. Two sandworms are required for sexual reproduction to occur; the sexes
are separate. With your hand lens, locate a pair of testes in segments 10
and 11, or the ovaries in segments 12 and 13. Both fertilization and development are external; the larvae are planktonic.
OBSERVATIONS AND ANALYSES
1. What adaptations does the sandworm have for breathing?
2. Describe the structures of the sandworm that enable locomotion.
3. How is the sandworm adapted for carrying out ingestion and digestion?
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Chapter 8 Review
Answer the following questions on a separate sheet of paper.
Vocabulary
The following list contains all the boldface terms in this chapter.
annelids, arrow worm, bilateral symmetry, bloodworm, coelom,
flatworms, ganglia, giant tube worms, hirudin, host, leech,
nematodes, nephridia, parapodia, parasite, planarian, proboscis,
ribbon worm, roundworms, sandworm, segmented worms, setae,
tapeworm, trematode, water column
Fill In
Use one of the vocabulary terms listed above to complete each sentence.
1. A worm’s nerve cell clusters that act like a brain are the
____________________.
2. Sandworms wriggle through the sand by means of ____________________.
3. The segmented worms are also referred to as the ____________________.
4. A sharp organ used by ribbon worms to spear prey is the
____________________.
5. The chemical secreted by a leech to make its victim’s blood
flow is ____________________.
Think and Write
Use the information in this chapter to respond to these items.
6. Diagram and discuss the life cycle of the parasitic trematode.
7. Describe the structures that enable sandworms to move.
8. Why is chemosynthesis important for giant tube worms?
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Inquiry
Base your answers to questions 9 through 12 on the diagram below,
which shows the anatomy of an arrow worm, and on your knowledge of
marine biology.
9. Does the arrow worm have a oneway or a two-way digestive system?
Explain your answer.
10. Which is an accurate statement
regarding reproduction in
the arrow worm? a. Arrow worms
are separate males and females that
reproduce sexually. b. Arrow
worms are separate males and
females that reproduce asexually.
c. Arrow worms are hermaphrodites
that reproduce sexually. d. Arrow
worms are hermaphrodites that
reproduce asexually.
11. Based on the diagram, you could
conclude that arrow worms
a. have no adaptations for foodgetting b. have a closed
circulatory system c. have a
simple nervous system d. are
normally tube-dwelling worms.
Anterior teeth
Hooks
(mouth bristles)
Eyes
Sensory
papillae
Ventral
nerve ganglion
Intestine
Anterior
lateral fin
Eggs in ovary
Posterior
lateral fin
Anus
Testis
Tail fin
12. What kind of locomotion is the arrow worm capable of?
a. ameboid b. free-swimming c. deep-diving
d. crawling
Multiple Choice
Choose the response that best completes the sentence or answers the
question.
13. Two examples of parasitic flatworms are the a. tapeworm
and leech b. tapeworm and fluke c. sandworm and
tapeworm d. sandworm and leech.
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Marine Invertebrates
14. The worms are not all classified into a single phylum because
a. there are too many worms b. they live in such varied
habitats c. they are too difficult to classify d. they are too
varied in structure to be in one group.
15. Members of phylum Annelida are all characterized by
a. having a thick skin b. the absence of appendages
c. living in the same habitat d. having a segmented body.
16. What part of a worm’s anatomy is shown in the diagram
below? a. the anterior end b. the posterior end
c. the dorsal side d. the ventral side
Pharynx
Mouth
Crop
Esophagus
Intestine
Gizzard
17. Segmented worms such as the sandworm have a fluid-filled
space called the a. water column b. proboscis
c. coelom d. nephridia.
18. Which of the following worms always lives as a parasite?
a. trematode b. bloodworm c. sandworm
d. ribbon worm
19. A worm that does not live inside a substrate is the a. arrow
worm b. sandworm c. tube worm d. roundworm.
20. The largest free-living marine worm is the a. arrow worm
b. giant tube worm c. ribbon worm d. sandworm.
21. The most numerous worm species in the sea are the a. tube
worms b. roundworms c. bloodworms d. sandworms.
22. The pairs of coiled tubes that excrete wastes in annelids are
the a. setae b. parapodia c. ganglia d. nephridia.
23. Another term used to refer to roundworms is
a. platyhelminthes b. nematodes c. annelids
worms.
d. tube
Marine Worms
211
24. What worms would you most likely see under the microscope
in a sample of aquarium gravel? a. ribbon worms
b. roundworms c. arrow worms d. sandworms
25. Which of the following marine worms depend on
chemosynthesis by bacteria to obtain their food?
a. roundworms b. flatworms c. sandworms
d. giant tube worms
Research/Activity
Roundworms are commonly found in wet beach sand from the
intertidal zone. If available, obtain a sample and prepare a wet
mount slide. Examine the slide under a microscope’s low power.
Look for the typical whipping action by which the roundworm
propels itself. Design an investigation to determine if changes in
temperature, pH, or salinity affect its motion.
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Marine Invertebrates