Section 1
Section 1
Mollusks
Focus
Overview
Before beginning this section
review with your students the
objectives listed in the Student
Edition. Tell students that the purpose of this lesson is to teach them
about the general structural and
physiological characteristics of
mollusks, including their body plan
and organ systems. They will also
learn about distinguishing characteristics of the three major groups
of mollusks: the gastropods,
bivalves, and cephalopods.
TAKS 2 Bio 8C
Bellringer
Ask students to think about a
snail, a clam, and an octopus. Tell
students that these are all mollusks.
Ask them to write down some
characteristics that these animals
have in common. (Students will
most likely note the prominent foot,
or its modification.) TAKS 2 Bio 8C
Motivate
Identifying
Preconceptions
Ask students to hypothesize why
some mollusks secrete a shell and
others do not. Guide their thinking
by asking how mollusks without
shells might protect themselves.
(Students should recall that squids
and octopuses spray ink and can
propel themselves away from danger.
Slugs secrete noxious substances that
discourage predators.) TAKS 3 Bio 7B
Objectives
A True Coelom
While most of the simple invertebrates you read about in the last
chapter may be unfamiliar to you, chances are good that you have
seen many mollusks and annelids. Snails, slugs, oysters, clams,
TAKS 2, TAKS 3
scallops, octopuses, and squids are all mollusks. If you have seen an
● Describe the key characteristics of mollusks.
8C TAKS 2 earthworm, then you know what an annelid is. While a snail may
not seem to have much in common with an earthworm, these two
● Describe excretion,
very different-looking animals are related.
circulation, respiration, and
Mollusks and annelids were probably the first major groups of
reproduction in mollusks.
10A
TAKS 2 organisms to develop a true coelom. (Recall that in animals that
● Compare the body plans
have a true coelom, or body cavity, the gut and other internal
and feeding adaptations of
organs are suspended from the body wall and cushioned by the
gastropods, bivalves, and
fluid within the coelom.) Another feature shared by mollusks and
8B 8C TAKS 2
cephalopods.
annelids is a larval stage called a trochophore (TRAHK oh fawr),
which develops from the fertilized egg. In some species, the trochoKey Terms
phore, shown in Figure 1, is free-swimming and propels itself
trochophore
through the water by movement of cilia on its surface. The presence
visceral mass
of a trochophore larva in mollusks and annelids suggests that they
mantle
share a common ancestor.
foot
Members of the phylum Mollusca make up the second largest
radula
nephridium
animal phyla, exceeded only by phylum Arthropoda. Mollusks
adductor muscle
are abundant in almost all
siphon
marine, freshwater, and terrestrial
Cilia
habitats. There are more species
of terrestrial mollusks than there
are of terrestrial vertebrates.
These mollusks often go unnoticed
because people are not accustomed
to looking for them. Seven classes
of mollusks make up the phylum
Figure 1 Trochophore larva
Mollusca. The three major classes
The microscopic trochophore
are Gastropoda (snails and slugs),
larva has a belt of cilia that
Mouth
Bivalvia (clams, oysters, and scalcircles its body. The beating
lops), and Cephalopoda (octopuses
Anus
of the cilia propels the trochophore through the water.
and squids).
● Summarize the evolutionary
relationship between mollusks
and annelids.
7B 8B 8C
Evolutionary Milestone
5 Coelom
A true coelom develops entirely within the mesoderm. Contact between the mesoderm and endoderm during the development of the embryo leads to the
development of complex organs.
642
Chapter Resource File
pp. 642–643
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8B, 8C, 10A
Teacher Edition
TAKS Obj 2 Bio 8C
TAKS Obj 3 Bio 7B
TAKS Obj 4 IPC 9B
TEKS Bio 5A, 7B, 8C
TEKS IPC 9B
642
• Lesson Plan GENERAL
• Directed Reading
• Active Reading GENERAL
• Data Sheet for Quick Lab GENERAL
• Data Sheet for Data Lab GENERAL
Chapter 29 • Mollusks and Annelids
Planner CD-ROM
• Reading Organizers
• Reading Strategies
Transparencies
TT Three Part Body Plan of Mollusks
TT Mollusk Organ Systems
Key Characteristics of Mollusks
Despite their varied appearance, the members of the different
groups of mollusks share a number of key characteristics.
Teach
www.scilinks.org
Topic: Mollusks
Keyword: HX4124
1. Body cavity. The body cavity in mollusks is a true coelom,
although in most species it is reduced to a small area immediately
surrounding the heart.
Activity
2. Symmetry. Most mollusks exhibit bilateral symmetry.
3. Three-part body plan. The body of every mollusk has three
distinct parts: the visceral mass, the mantle, and the muscular
foot, as shown in Figure 2. The visceral (VIS uhr uhl) mass is
a central section that contains the mollusk’s organs. The mantle is wrapped around the visceral mass like a cape. The mantle
is a heavy fold of tissue that forms the outer layer of the body.
Finally, every mollusk has a muscular region called a foot , which
is used primarily for locomotion.
4. Organ systems. Mollusks have organ systems for excretion,
circulation, respiration, digestion, and reproduction.
5. Shell. Many mollusks have either one or two shells that serve as
an exoskeleton, protecting their soft body. The shell is composed
of protein that is strengthened by calcium carbonate, an
extremely hard mineral.
6. Radula. All mollusks except bivalves have a radula (RAJ uh luh),
a tongue-like organ located in their mouth. The radula, shown in
Figure 2, has thousands of pointed, backward-curving teeth
arranged in rows. When a mollusk feeds, it pushes its radula out
of its mouth, and the teeth scrape fragments of food off rocks or
plant matter. Mollusks that are predators use their radula for
attacking their prey.
READING
SKILL
BUILDER
Figure 2 Three-part body plan
Interactive Reading Assign
Chapter 29 of the Holt Biology
Guided Audio CD Program to help
students achieve greater success in
reading the chapter.
All mollusk bodies are composed of a visceral mass, a
mantle, and a foot. Most mollusks also have a radula.
Mouth
Teeth
GENERAL
Mollusks on the Menu Provide
students with menus from several
seafood restaurants and seafood
cookbooks from the library. Ask
students to identify all the mollusks
they can find. (Students should be
able to identify oysters, clams, and
scallops. Depending on the menu or
cookbook, there may also be snails
[escargot], squids [calamari], octopuses, abalone, and mussels.) Ask
students what part of the body is
eaten for each mollusk they identify. (oysters, clams, snails, abalone,
mussels – the whole body, except the
shell; squids and octopuses – the
foot, which is modified into tentacles
and the mantle; scallops – the adductor muscle) TAKS 2 Bio 8C
Mantle
Visceral mass
Mantle
Foot
Mantle
cavity
Visceral
mass
Radula
Foot
643
did you know?
Banana Slugs Banana slugs live on damp
foggy forest floors on the west coast of North
America only. They like foggy summers and
rainy winters, and hide in damp places. The
yellow banana slug is only found in the small
region between the Salinas Valley and the San
Francisco peninsula of California. It is the
mascot for the University of California at
Santa Cruz. TAKS 3 Bio 7B
MISCONCEPTION
ALERT
Mantle versus Shell Students often
confuse the mantle with the shell or assume
that the mantle composes part of the shell.
Be sure students understand the distinction
between the two: the mantle, which is composed of tissue, secretes the shell, which is
composed of protein and calcium carbonate.
Bio 5A
Chapter 29 • Mollusks and Annelids
643
Figure 3 Mollusk body plan
Although mollusks vary greatly in body form, they all have complex organ systems.
Cilia
Teach, continued
continued
Nephridium
Coelom
Pore
Teaching Tip
Reproductive
organs
Ganglia
Wastes
Shell
Useful
molecules
Mouth
Radula
Foot
Mantle
cavity
Gill
Nephridium
Intestine
Nerve cords
Organ Systems
Mollusks are the only coelomates without segmented bodies. Like
the roundworms, mollusks have a one-way digestive system. Mollusks, however, are more complex than roundworms. As a group,
mollusks are quite diverse, and no one mollusk can represent the
phylum as a whole. Figure 3 shows the basic mollusk body with
organs that are characteristic of the phylum.
TAKS 2 Bio 4B
GENERAL
Use Figure 3 to point out that
mollusks have advanced organ
systems. Ask students to categorize
each of the organs as part of one of
the following systems: digestive, circulatory, respiratory, or excretory.
(digestive system—mouth, radula,
intestine, stomach; circulatory
system—heart; respiratory system—
gill; excretory system— nephridium,
coelom) LS Visual TAKS 2 Bio 10A
SKILL
Heart
Stomach
Excretory Strategies Relate the
excretion of wastes in nephridia to
cleaning up a table after a meal.
Nephridia get rid of wastes as well
as substances that the mollusk
needs; thus, some substances must
be reabsorbed before the wastes are
discharged. The process is similar
to throwing away everything on the
table after the meal, and then going
through the trash to recover the
plates, silverware, and leftovers.
Using the Figure
Mantle
Body
tissue
Excretion Mollusks are one of the earliest evolutionary lines to have
developed an efficient excretory system. A mollusk’s coelom is a
collecting place for waste-laden body fluids. The beating of cilia pulls
the fluid from the coelom into tiny tubular structures called
nephridia (nee FRIHD ee uh), also shown in Figure 3. The nephridia
recover useful molecules (sugars, salts, and water) from the coelomic
fluid. The recovered molecules are reabsorbed into the mollusk’s body
tissues. The remaining fluid waste leaves the mollusk’s body through
a pore that opens into the mantle cavity. Nephridia are found in all
coelomate animals except arthropods and chordates.
Circulation The digestive tube of mollusks and other coelomates is
surrounded by mesoderm, which acts as a barrier to the diffusion
of nutrients into the cells of the body. Mollusks have a circulatory
system. Recall that in a circulatory system, blood carries nutrients
and oxygen to tissues and removes waste and carbon dioxide. Most
mollusks have a three-chambered heart and an open circulatory
system. Octopuses and squids are exceptions because they each
have a closed circulatory system.
BUILDER
Vocabulary Nephros is the Greek
word for “kidney.” Nephritis is
inflammation of the kidneys, a
nephrectomy is the removal of a
kidney, a nephron is the structure
in a kidney that filters blood, and
nephrology is the field of medicine
dedicated to the study and treatment of the kidneys. TAKS 2 Bio 10A
Respiration Respiration among mollusks is carried out in a variety
of ways. Most mollusks respire with gills, which are located in the
mantle cavity. Mollusk gills extract 50 percent or more of the dissolved oxygen from the water that passes over them.
In freshwater snails, ciliated gills beat on the inner surface of the
mantle. This causes a continuous stream of water to pass over the
644
Trends in Marine Biology
pp. 644–645
Student Edition
TAKS Obj 1 Bio/IPC 2C, 2D
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 3E, 7B, 10A
TEKS Bio/IPC 2C, 2D
Teacher Edition
TAKS Obj 1 Bio/IPC 2C, 2D
TAKS Obj 2 Bio 4B, 10A
TAKS Obj 3 Bio 7B
TEKS Bio 3E, 4B, 7B, 8B, 10A
TEKS Bio/IPC 2C, 2D
644
Mollusk Classes The living mollusks are
divided into the following seven classes:
Aplacophora contains about 250 species of
marine, wormlike animals that have no shell.
Monoplacophora has about a dozen living
species; they are very small and covered with
a shell. They live at great ocean depths.
Polyplacophora has about 800 species of animals known as chitons. Chitons have a shell of
eight plates that usually overlap. Scaphopods
have elongate, tubular shells that are open at
both ends. All 350 or so species are marine.
Chapter 29 • Mollusks and Annelids
The cephalopods include the octopus, squid, cuttlefish, and nautilus. The foot of cephalopods is
modified into tentacles, and only a few have a
shell. All 600 to 650 species are marine predators or scavengers. Gastropods usually have a
single, coiled shell and a well-developed foot.
They are found in marine, freshwater, and terrestrial habitats. There are about 60,000 species
of gastropods. Bivalves have two valves connected by a flexible ligament. Most are filter
feeders. There are about 10,000 living species
of bivalves. TAKS 3 Bio 7B; Bio 8B
gills. Most terrestrial snails have no gills. Instead, the thin
membrane that lines their empty mantle cavity functions
like a primitive lung. This membrane must be kept moist
for oxygen to diffuse across it. Therefore, terrestrial
snails, shown in Figure 4, are most active at night or after
it rains when air has a high moisture content. During dry
weather, a terrestrial snail pulls back into its shell and
plugs the opening with a wad of mucus to keep water in.
Sea snails also lack gills, and gas exchange takes place
directly through their skin.
Teaching Tip
Reproduction Most species of mollusks have distinct
male and female individuals, although some snails and
slugs are hermaphrodites. Certain species of oysters and
sea slugs are able to change from one sex to the other and back
again. Many marine mollusks are moved from place to place as
their trochophore larvae drift in the ocean currents. Octopuses,
squids, freshwater snails, and some freshwater mussels have no
free-swimming larvae. In these mollusks, the larval stage occurs
within the egg, and a juvenile-stage mollusk hatches from the egg.
Figure 4 Terrestrial snails.
Terrestrial snails are most
active when the air is moist.
GENERAL
Oxygen Delivery Systems Point
out to students that it takes both
respiratory and circulatory systems
to deliver oxygen to the tissues of
mollusks. If either system stops
functioning, the mollusk will die.
Ask students to consider the various
ways a mollusk could suffocate.
(oxygen-deficient water, nonfunctioning cilia, nonfunctioning heart, loss of
blood, dried out gills or lungs)
TAKS 2 Bio 4B
Modeling an Open
Circulatory System
TAKS 1 Bio/IPC 2C, 2D; Bio 3E
Modeling an Open Circulatory
System 2C 2D 3E TAKS 1
Skills Acquired
Applying information,
modeling, observing,
evaluating
You can model an open circulatory system using simple
items to represent the heart, blood vessels, blood, and
body tissues of a living organism.
Teacher’s Notes
Make sure that students
remove any air from the tubing
once it is submerged. Use a
dark dye. If the dye diffuses too
quickly in the water, try using
an oil-based dye or small,
lightweight colored beads.
Materials
surgical tubing, 15 cm (about 6 in.) piece; clear plastic tubing,
15 cm (about 6 in.) and 7.5 cm (about 3 in.) pieces; shallow pan
filled with water; eyedropper; food coloring
Procedure
1. Connect the surgical tubing
to the two pieces of clear
plastic tubing, as shown
above.
2. Place the tubing into the tray
filled with water. Allow the
tubing to fill with water and
rest on the bottom.
3. With the tubing still submerged, use an eyedropper
to place two drops of food
coloring into the short piece
of clear plastic tubing.
4. With your thumb and index
finger, squeeze along the
piece of surgical tubing to
pump the food coloring
through the system.
3. Critical Thinking
Evaluating Results
Evaluate your model’s
efficiency at pumping blood
through the system.
5. As you continue to pump,
observe the movement of
food coloring.
4. Critical Thinking
Analyzing Methods How
does this model differ from a
real circulatory system?
Analysis
1. Describe what happened
when you squeezed along
the tubing.
Answers to Analysis
5. Critical Thinking
Analyzing Methods How
could you modify the model
to make it more accurate?
2. Identify the structures represented by the pan of water,
the surgical tubing, and the
clear plastic tubing.
645
did you know?
Reproduction in Oysters Oysters are broadcast spawners, ejecting sperm and eggs into the
water, where fertilization occurs. Oysters may
spawn multiple times during the season, with
each female releasing millions of eggs during
each spawning event. Oysters are “protandric”
because they develop first as males and then
become females due to regression of the testes
and subsequent development of the ovaries.
Fertilized eggs develop in a free-floating
environment, and a trochophore larva hatches
from the egg 6–9 hours after fertilization. After
about three weeks, the larvae progress through
a number of larval stages. The last larval stage
has a muscular foot that the larva uses to
crawl on the bottom, searching for a suitable
place to attach. Attachment sites include
almost any hard surface such as other living
oysters, oyster shell, rocks, docks, pilings, and
glass bottles. Larvae often settle on top of
other oyster shells, forming large beds or oyster reefs. Once attached, the newly settled
oysters stay in the same location throughout
their lives. TAKS 2 Bio 4B, 10A
1. The dye moves away from
the pumping action but is
also drawn back into the
open tubing.
2. The pan of water represents
the body and tissues of the
mollusk, surgical tubing represents the heart, and the clear
plastic tubing represents the
blood vessels.
3. Answers will vary.
4. In a real system, there would
be many more vessels carrying
the blood out to the tissues
and back toward the heart.
The oxygenated blood would
also be absorbed by the various tissues, and the used
blood would be reoxygenated
by the gills.
5. Answers will vary, but may
include adding models for
individual organs, adding
more “blood,” and adding
more blood vessels.
Chapter 29 • Mollusks and Annelids
645
Body Plans of Mollusks
Teach, continued
continued
Teaching Tip
Mollusk Shells The beauty, diversity, and hardness of mollusk shells
accounts for their wide use as decorations. Even the inside linings of
shells are used. Shells of freshwater
mussels are used to make pearl
buttons, and mother-of-pearl from
oysters is used to decorate ornamental boxes, jewelry, and musical
instruments.
Organizing Information
Make a table to organize
information about mollusks.
Across the top, write the
headings Gastropods,
Bivalves, and Cephalopods.
Along the side, write
Foot, Shell, Feeding,
Reproduction. Add information (and perhaps more
terms along the side) to
the table as you read.
Activity
Snail Behavior Allow students to
explore snail behavior. Obtain several land snails from gardens,
wooded areas, or a biological supply house. Have students devise
ways to test the snails’ responses to
touch, light, moisture, and gravity.
Caution students to use extreme
care in handling snails, making
sure not to let them dry out. When
not in use, keep the snails in a
cool, moist terrarium with pieces
of lettuce. LS Kinesthetic Bio 11B
The basic mollusk three-part body plan differs in each class of mollusks. As you read about the different classes of mollusks, you will
see how the mollusk shell and foot are adapted for many different
living conditions.
Gastropods
Gastropods—snails and slugs—are primarily a marine group that
has successfully invaded freshwater and terrestrial habitats. They
range in size from microscopic forms to the sea hare Aplysia, which
reaches 1 m (almost 40 in.) in length. Most gastropods have
a single shell. During the evolution of slugs and nudibranchs
(NOO dih branks), the shell was lost completely. Figure 5 shows
three terrestrial tree snails and a nudibranch (sea slug). The foot of
gastropods is adapted for locomotion. Terrestrial species secrete
mucus from the base of their foot, forming a slimy path that they
can glide along. Most gastropods have a pair of tentacles on their
head with eyes often located at the tips.
Gastropods display varied feeding habits. Many are herbivores
that scrape algae off rocks using their radula. Some terrestrial
snails can be serious garden and agricultural pests, using their
radula to saw off leaves. Sea slugs and many other gastropods are
active predators. Whelks and oyster drills, for example, use their
radula to bore holes in the shells of other mollusks. Then
they suck out the soft tissues of their prey. In cone shells, such as
the one shown in Figure 6, the radula is modified into a kind of
poison-tipped harpoon that is shot into prey. The poison paralyzes
the prey, which is then swallowed whole.
Throughout human history, snails have been a source of food for
humans. Land snails belonging to the genus Helix are raised on
snail farms and are consumed in great quantities. While freshwater
snails are rarely eaten, a few marine species, such as conchs
(KAHNGKS), are considered delicacies.
Tree snails
READING
SKILL
BUILDER
Paired Summarizing Encourage
students to pause after each paragraph they read to summarize the
main points to a partner. You
might model the technique by
having a student read a paragraph
out loud and summarize it. Then
have another student read the
next paragraph and
summarize it.
English Language
LS Interpersonal
Figure 5 Snails and slugs.
These Florida tree snails live
where the air is moist enough
to keep them from drying out.
Sea slugs are often brilliantly
colored, and most are under
15 cm (6 in.) in length.
Learners
646
Cultural
Awareness
pp. 646–647
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8C, 10A
Teacher Edition
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7A, 7B, 12B
TAKS Obj 4 IPC 9B
TEKS Bio 7A, 7B, 10A, 11B, 12B
TEKS IPC 9B
646
Uses of Shells Mollusk shells have served
many different functions in various cultures,
especially in making jewelry and crafts. In
addition, shells have been used for several
practical purposes. Phoenicians and
Romans used Murex sea snails to make a
purple dye for coloring fabric. Native
North Americans carved large clam shells
into wampum beads and used other shells
as money. Filipinos cut thin Placuna oyster
shells to fit into wooden frames as
windowpanes.
Chapter 29 • Mollusks and Annelids
Sea slug
Figure 6 Cone shell.
This cone shell searches the
ocean bottom for prey. Once
located, the prey is secured by
the cone shell’s radula and
swallowed whole.
Bivalves
Most bivalves are marine, but some also live in fresh water. Many
species of freshwater mussels are found throughout the rivers and
lakes of North America and are important links in aquatic food
chains. Oysters and mussels are important sources of food for
humans. All bivalves have a two-part hinged shell. The valves, or
shells, of a bivalve are secreted by the mantle. Two thick muscles,
the adductor muscles , connect the valves. When these muscles are
contracted, they cause the valves to close tightly. While most
species of mollusks are sessile, some can move from place to place
quite fast if necessary. For example, a swimming scallop opens and
closes its valves rapidly. This pushes it along with the jets of water
released when its valves snap shut.
Bivalves are unique among the mollusks because they do not have
a distinct head region or a radula. A nerve ganglion above their foot
serves as a simple brain. Most bivalves have some type of simple
sense organs. For example, some bivalves have sensory cells located
along the edge of their mantle that respond to light and touch.
Most bivalves are either male or female, but a few species are
hermaphroditic. Bivalves reproduce sexually by releasing sperm
and eggs into the water, where fertilization occurs. The fertilized
eggs develop into free-swimming trochophore larvae. The larvae of
a few freshwater mussels are brooded in a pouch within the mollusk’s gills. The larvae are then released into the water, and they
complete their larval stage as parasites on fish. This is a very
unusual life cycle for a mollusk.
Most bivalves are filter feeders. Many, such as
the clam illustrated in Figure 7, use their muscular
foot to dig down into the sand. Once there, the
cilia on their gills draw in sea water through
Siphons
hollow tubes called siphons (SIE fuhns). The
water moves down one siphon tube, over the
gills, and out the other siphon tube. The gills are
used for feeding as well as respiration. A sticky
mucus covers the gills, and as water moves
over the gills, small marine organisms and
organic material become trapped in the mucus.
The cilia then direct the food-laden mucus to the
bivalve’s mouth.
Demonstration
GENERAL
Show students a dissected fresh
or preserved mollusk such as
a clam or oyster. Ask them to
sketch the mollusk and label
the following parts: shell, mantle, coelom, foot, gut, heart,
and gills. English Language
LS Visual
Learners
TAKS 2 Bio 10A
The class name Bivalvia
comes from the Latin bi,
meaning “two,” and valva,
meaning “part of a door.”
Activity
Identifying Mollusks Obtain
several fossil mollusks, and set
them out for students to view.
Fossil mollusks are some of the
most common and inexpensive
fossils you can obtain, and they are
easily identifiable as bivalves, gastropods, or shelled cephalopods.
Have students match modern
varieties of mollusks to their fossils.
LS Visual TAKS 3 Bio 7A, 7B
Figure 7 Clam. Many
bivalves, like this clam, burrow
into sand or mud and feed
by drawing sea water in one
siphon and expelling it out
the other.
Foot
647
IPC Benchmark Mini-Lesson
GEOGRAPHY
CONNECTION
Biology/IPC Skills TAKS 4 IPC 9B
Relate the concentration of ions in a solution to
physical and chemical properties.
Activity Explain to students that osmosis is a process
in which water molecules move from one solution into
another solution that has fewer free water molecules
(or has more molecules of substance in solution).
Snails on land are killed by salt because the salt mixes
with the moisture on their surface, and becomes a
highly concentrated solution drawing free water molecules out of their bodies. Ask students why they think
this does not happen to sea slugs. (The concentration
of ions inside sea slugs is equal to the concentration of
ions in the surrounding sea water.)
The zebra mussel is a small, freshwater
bivalve native to Europe. At some point,
zebra mussels hitchhiked across the Atlantic
Ocean in the ballast water of ships. Although
they were not here in detectable numbers
before 1985, they have infested waters in the
Great Lakes, where they have no natural
predators. Although small, the mussels attach
themselves to any hard surface, including
ship hulls, docks, and even other mussels.
TAKS 3 Bio 12B
Chapter 29 • Mollusks and Annelids
647
Teach, continued
continued
Teaching Tip
A Smashing Success Tell students that some bivalves have
evolved to survive the crushing
pressure of waves. Muscles and
other bivalves that often cling to
exposed rocks must be able to
withstand daily battering by waves.
They survive the poundings by
having shells that are very hard
and are shaped so as to deflect
onrushing water. TAKS 3 Bio 7B
Figure 8 Pearl oyster.
The best pearls come from
oysters belonging to the
genus Pinctada.
Activity
Internet Research Have students
research mollusk consumption
using the Internet or library
resources. Have students write a
report summarizing their findings.
Eating Mollusks Safely
S
Eating Mollusks Safely
TAKS 3 Bio 4C, 4D, 7B; TAKS 2 Bio 10A
Teaching Strategies
Point out that there are risks
associated with eating any
animal product raw, but the
often-polluted environment of
many mollusks makes them
especially risky. Some coastal
areas are so polluted that
bivalves have completely
died off.
Discussion
Ask students what can be done
to clean up the coastal areas
where bivalves are found? (eliminate oceans as a place to dump
waste, control agricultural runoff,
increase coastal wetland acreage
to improve water quality and provide more habitat for mollusks)
Like clams, oysters and scallops use their gills to
filter food from the water. Oysters are permanently
attached to rocks in the open water, where they feed.
Scallops swim, and water passes over their gills as
they move.
Many species of bivalves, such as the oyster in
Figure 8, produce pearls. Pearls form when a tiny foreign object, such as a grain of sand, becomes lodged
between the mollusk’s mantle and shell. Bivalves
respond to these irritants by coating them with thin
sheets of nacre (NAY kuhr), also called mother-ofpearl. Nacre is the same hard, shiny substance that composes the
inner shell surface. Successive layers of nacre are added until the
foreign body is completely enclosed in the newly formed pearl.
While many bivalves form pearls, only a few species produce the
beautifully colored nacre essential for gem-quality pearls. In fine
pearls, the nacre contains tiny, overlapping mineral crystals. These
crystals act like prisms, breaking up any light that falls on them into
rainbows of color. This is what gives these pearls their iridescence.
The mineral crystals found in the nacre of ordinary pearls are larger
and do not reflect light as beautifully.
ince prehistoric times, mollusks have served as a
human food resource. Today the
annual worldwide harvest for
bivalve mollusks alone amounts
to an astounding 3 million metric
tons (6,615,000,000 lb). But
severe illness has been associated with eating mollusks. What
causes these illnesses, and are
mollusks really safe to eat?
Bivalves—oysters, mussels,
clams, and scallops—are filter
feeders. Any contaminant in a
bivalve’s environment circulates
through and accumulates in its
body. One source of contamination is water that is polluted with
sewage, an ideal breeding ground
for some bacteria and viruses.
Because of this, harvesting is
prohibited in certain areas. The
primary danger is from eating
infected bivalves raw. Contrary
TAKS 2, TAKS 3
to popular opinion, hot sauce will
not kill dangerous organisms that
infect them. But it is now also
evident that brief cooking, such
as lightly steaming clams, does
not destroy all pathogens either.
Follow the Guidelines
An estimated 20 million Americans eat oysters, and the yearly
consumption of mollusks is
increasing. To reduce the risk of
illness from eating mollusks, the
Food and Drug Administration
(FDA) has developed guidelines
for preparing them. If you observe
these guidelines, the health risk
from eating mollusks is very low.
Avoid eating raw bivalves. Make
your purchases from reputable
sources to assure that the mollusks were not illegally harvested.
Test for freshness by trying to
move the shells sideways. If the
shells move, the mollusk is not
fresh and should be discarded.
Follow FDA recommendations for
proper cooking times and temperatures. By following these
guidelines, you can safely enjoy
this delicious and healthful food
choice.
648
OCEANOGRAPHY
CONNECTION
pp. 648–649
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8C, 10A
Teacher Edition
TAKS Obj 2 Bio 8C, 10A
TAKS Obj 3 Bio 4C, 4D, 7B
TEKS Bio 4C, 4D, 7B, 8C, 10A
648
Between 150 to 200 species of octopus inhabit
the world’s oceans. The common octopus, the
species best known to scientists, thrives in
warm rocky shallows off the coasts of the
southeastern United States, western Central
America, and Japan, as well as in the
Mediterranean and the Caribbean. It can
weigh up to 50 pounds and have a 10-foot
arm span. The species that comes closest in
size to the monsters of science fiction is the
Chapter 29 • Mollusks and Annelids
giant Pacific octopus, found off the western
coast of North America and across the northern Pacific to Japan. The biggest ever captured
weighed more than 600 pounds and measured
31 feet from arm tip to arm tip. Despite their
impressive growth, Pacific giants rarely live
longer than three years. At the other extreme,
adults of some pygmy species weigh less than a
penny, grow to an arm span of less than 2 inches
and are lucky to live six months. TAKS 3 Bio 7B
Cephalopods
Squids, octopuses, cuttlefish, and nautiluses are all cephalopods.
Most of their body is made up of a large head attached to tentacles
(a foot divided into numerous parts), as shown in Figure 9. The tentacles are equipped with either suction cups or hooks for seizing
prey. Squids have 10 tentacles, while octopuses have eight. The nautilus has 80–90 tentacles, although they are not nearly as long as
those of the other cephalopods. Although cephalopods evolved
from shelled ancestors, most modern cephalopods lack an external
shell. The nautilus is the only living cephalopod species that still
has an outer shell. Squids as well as the cuttlefish have a small
internal shell. Cuttlefish “bones” are often attached to bird cages to
provide calcium for canaries and other pet birds.
Cephalopods are the most intelligent of all invertebrates. They
have a complex nervous system that includes a well-developed
brain. Cephalopods are capable of exhibiting complex behaviors.
Octopuses can easily be trained to distinguish between classes of
objects, such as between a square and a cross, and they are the only
invertebrates with this ability.
The structure of a cephalopod eye is similar in many ways to that
of a vertebrate eye, and some species have color vision. The eyes of
a squid can be very large. A giant squid that washed up on a beach
in New Zealand in 1933 had eyes that were 40 cm (almost 15 in.)
across. At over 20 m (65 ft) in length, the giant squid is the largest
of all invertebrates and has the largest eyes known in any animal.
Like most aquatic mollusks, cephalopods draw water into their
mantle cavity and expel it through a siphon. In squids and octopuses, this system functions as a means of jet propulsion. When
threatened, they quickly close their mantle cavity, causing water to
shoot forcefully out of the siphon. Squids and octopuses can also
The name cephalopod
comes from the Greek
kephalicos, meaning “head,”
and pous, meaning “foot.”
Activity
GENERAL
Major Characteristics of
Mollusks Ask students to develop
a table similar to the Graphic
Organizer at the bottom of this
page, to compare the major characteristics of bivalves, gastropods,
and cephalopods. LS Logical
TAKS 2 Bio 10A
Demonstration
Figure 9 Cephalopods.
Like all cephalopods, the squid
is an active predator. Cuttlefish
are agile swimmers that hunt
at night, seeking small fishes
and crustaceans. The nautilus
swims with its coiled shell
positioned over its head.
Nautilus
Have students test the suction of
dry and wet suction cups. Ask them
whether suction cups work better
when dry or wet. (They work better
when wet.) Cephalopods have suction cups on their tentacles that
have tremendous gripping power.
Even sperm whales can receive suction disk scars during fights with
giant squids. The suction disks hold
so tight that they can damage the
flesh. LS Kinesthetic
0100010110
011101010
0010010001001
1100100100010
0000101001001
1101010100100
0101010010010
Analyzing the
Molluscan Body
Plan TAKS 2 Bio 8C; TAKS 3 Bio 7B
Skills Acquired
Observing, applying
information, predicting
Teacher’s Notes
If possible, display examples of
shells from a cephalopod, gastropod, and bivalve.
Squid
Answers to Analysis
Cuttlefish
649
Graphic Organizer
Use this graphic organizer with the Activity
on this page.
Characteristics
Have head, foot,
and visceral mass
Have true coelom
Have shells
Bivalves
Yes; head
undeveloped
Yes
Two shells
Use locomotion
Most do not
Gastropods
Yes; torsion of
visceral mass
Yes
Most have
one shell
Yes
Cephalopods
Yes; foot divided
into tentacles
Yes
No, except
chambered nautilus
Yes
1. A. cephalopod; B. gastropod;
C. bivalve
2. The shells of the different
classes have evolved to support
different ways of life.
3. In cephalopods, the foot is
modified into tentacles and
aids in movement and in capturing food. The flat-bottomed
foot of gastropods secretes a
substance that allows the animal to glide over surfaces. The
foot of a bivalve allows it to
bury itself in the sand.
4. Slugs would be more abundant
in areas with acidic soils
because the environment
cannot support many shellbuilding organisms.
Chapter 29 • Mollusks and Annelids
649
release a dark fluid that clouds the water and conceals the direction
of their escape. The ink of the cuttlefish contains a reddish brown
pigment called sepia. For centuries this ink was used by artists as a
pigment and is found in many famous paintings.
All cephalopods are active marine predators. They feed on fish,
mollusks, crustaceans, and worms. Once the prey has been snared
by the tentacles, it is pulled to the mouth, where it is torn apart by
strong, beaklike jaws. The cephalopod’s radula then pulls the pieces
into the mouth.
Close
Reteaching
Ask students to list the different
functions of the mantle with examples of mollusks that exhibit such
functions. (primitive lung – terrestrial snails; shell formation –
bivalves; jet propulsion – squids and
octopuses) TAKS 2 Bio 10A
Quiz
GENERAL
1. What is the most prominent
characteristic of a mollusk?
(its foot)
2. What body systems do
mollusks share with mammals?
(excretory, circulatory, respiratory,
reproductive)
3. How are cephalopods different
from gastropods and bivalves?
(Cephalopods have a foot that is
modified into tentacles, and they
usually lack a shell.)
Alternative
Assessment
GENERAL
Have students choose a mollusk
that affects humans. Then have
them write a report on how the
mollusk has been affected by
humans and how humans have
been affected by the mollusk.
LS Verbal
Analyzing the Molluscan
Body Plan TAKS 2, TAKS 3
0100010110
011101010
0010010001001
1100100100010
0000101001001
1101010100100
0101010010010
Background
Mollusks share many common characteristics, yet there
is great variety among the classes. The drawings on the
right show how the shell (brown) and foot (green) vary in
three classes of mollusks. Use the drawings to answer
the analysis questions.
Analysis
A
B
1. Determine the class of
mollusk A, mollusk B, and
mollusk C.
2. Compare the shell
modifications. Why might a
shell suited to one mollusk be
inappropriate for another?
3. Critical Thinking
Identifying Functions
For each class shown, explain
how the foot is useful for the
animal’s environment or kind
of movement.
4. Critical Thinking
Predicting Outcomes
Terrestrial snails and slugs
are nearly identical except
that slugs do not have a shell.
Acidic forest soils are often
poor in minerals, including calcium. Alkaline or neutral soils
are rich in minerals. In which
kind of soil would you be more
likely to find a slug? Explain
your answer.
C
Section 1 Review
Identify two characteristics that mollusks and
Critical Thinking Forming Hypotheses
annelids have in common.
A chemical pollutant accidentally spills into a bay.
One of the effects of this chemical is that it paralyzes cilia. The next day almost all of the oysters
in the bay are dead. Develop a hypothesis that
2C
explains why the oysters died.
7B 8B 8C
Summarize six characteristics common to most
groups of mollusks.
8C
Describe how a nephridium functions in waste
removal.
TAKS Test Prep
10A
Compare the distinguishing features of each of
the three major classes of mollusks.
8B 8C
what organ system?
A respiratory
B circulatory
A gastropod’s radula is part of
10A
C digestive
D excretory
650
Answers to Section Review
pp. 650–651
Student Edition
TAKS Obj 1 Bio/IPC 2C
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8B, 8C, 10A
TEKS Bio/IPC 2C
Teacher Edition
TAKS Obj 1 Bio/IPC 2C
TAKS Obj 2 Bio 8C, 10A
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8B, 8C, 10A
TEKS Bio/IPC 2C
650
1. Mollusks and annelids both exhibit true
coeloms, and most members of both groups
develop from larvae called trochophores.
TAKS 2 Bio 8C; Bio 8B TAKS 3 Bio 7B
shell and lack a head and radula. Most
bivalves possess one or more siphons used for
filter feeding. Cephalopods have large eyes on
a large head attached to numerous tentacles.
2. Most mollusks share the following key characTAKS 2 Bio 8C; Bio 8B
teristics: coelom, bilateral symmetry, a three5. The beating of cilia moves water over the gills.
part body plan (visceral mass, mantle, and foot),
Therefore, the oysters suffocated when the
organ systems, a shell, and a radula. TAKS 2 Bio 8C
chemical paralyzed their cilia. TAKS 1 Bio/IPC 2C
3. A nephridium funnels coelomic fluid, which
6. A. Incorrect. The gill is part of the respiratory
includes wastes, into the mantle cavity, where it
system. B. Incorrect. The heart is part of the
is excreted to the outside environment. TAKS 2 Bio 10A
circulatory system. C. Correct. The radula is
part of the digestive system. D. Incorrect. The
4. Most gastropods have a single shell, a single
nephridium is part of the excretory system.
large foot, and, on their head, tentacles tipped
TAKS
2 Bio 10A
with eyes. Bivalves have a two-part hinged
Chapter 29 • Mollusks and Annelids
Annelids
Section 2
Section 2
Focus
The First Segmented Animals
Objectives
You have probably heard the expression “a can of worms,” which
calls up an image of a lot of wiggly, wriggly creatures. An earthworm may come to mind, but there are many different species
of segmented worms. Worms might not look like much, but this
group of coelomates belongs to an ancient group, phylum Annelida.
Annelid fossils can be found in rock that is 530 million years old.
Scientists think that annelids evolved in the sea, where two-thirds
of today’s annelid species live. Most other annelid species are terrestrial earthworms. Annelids range in size from less than 1 mm
(0.04 in.) long to more than 3 m (10 ft) long.
Annelids, such as the earthworm and fireworm shown in Figure 10,
are easily recognized by their segments, which are visible as a series
of ringlike structures along the length of their body. Each segment
contains digestive, excretory, circulatory, and locomotor (movement)
organs. Some of the segments are modified for specific functions,
such as reproduction, feeding, or sensation. A well-developed
cerebral ganglion , or primitive brain, is located in one anterior
segment. The brain is connected to a nerve cord that runs along the
underside of the worm’s body.
Overview
● Identify the major
change in body plan that
distinguishes annelids
from mollusks.
8B 8C TAKS 2
● Describe the basic annelid
body plan.
8C TAKS 2
● Describe the annelid
digestive system.
10A TAKS 2
● Compare the three classes
8B 8C TAKS 2
of annelids.
Key Terms
cerebral ganglion
septa
seta
parapodium
Before beginning this section
review with your students the
objectives listed in the Student
Edition. Tell students that the purpose of this lesson is to teach them
about the distinguishing structural
and functional characteristics of the
annelids, the segmented worms.
They will also learn about the three
major groups of annelids: the
marine or Polychaete worms, the
Oligochaetes, and the leeches or
Hirudinea. TAKS 2 Bio 10A
Bellringer
Ask students to write a short
description of the evolutionary
advantages of body segmentation.
(It allowed specialization of the
body segments.) TAKS 3 Bio 7B
Figure 10 Annelids
The ringlike segments of this earthworm and marine fireworm identify them
as annelids.
Motivate
Demonstration
Earthworm
Obtain earthworms from garden
soil or a bait shop, and have students observe their behavior. Ask
students to describe how they can
tell which end of the earthworm is
the head. If students say that the
head is the leading end, remind
them that squid sometimes move
tail first. Direct their attention to
the earthworm’s mouth. Caution
students to keep their worms
moist.
Fireworm
Evolutionary Milestone
6 Segmentation
Annelids were the first organisms to have a body plan based
on repeated body segments. Segmentation underlies the body
organization of all coelomate animals except mollusks.
651
Chapter Resource File
• Lesson Plan GENERAL
• Directed Reading
• Active Reading GENERAL
• Data Sheet for Quick Lab
Transparencies
TT Bellringer
TT Nereis
TT Anatomy of the Earthworm
GENERAL
Planner CD-ROM
• Reading Organizers
• Reading Strategies
Chapter 29 • Mollusks and Annelids
651
www.scilinks.org
Topic: Annelids
Keyword: HX4012
Teach
Internal body walls, called septa , separate the segments of most
annelids. Nutrients and other materials pass between the segments
through the circulatory system. Sensory information is delivered by
a nerve cord that connects nerve centers in the segments to the brain.
Characteristics of Annelids
Modeling a Closed
Circulatory System
In addition to segmentation, annelids share a number of other
characteristics.
TAKS 1 Bio/IPC 2C, 2D; Bio 3E
1. Coelom. The fluid-filled coelom is large and is located entirely
within the mesoderm.
Skills Acquired
Observing, modeling,
interpreting results,
applying information,
evaluating
2. Organ systems. The organ systems of annelids show a high
degree of specialization and include a closed circulatory system
and excretory structures called nephridia. The gut has different
regions that perform different functions in digestion.
Teacher’s Notes
Have students conduct Quick
Lab: Modeling an Open
Circulatory System before
conducting this lab. Ask
students how oxygen and nutrients get to the tissues that need
them if the blood never touches
the body tissues directly. (The
oxygen and nutrients pass
through the vessel walls.) Use a
dark dye. If the dye diffuses too
quickly in the water, try using an
oil-based dye or small, lightweight colored beads.
3. Bristles. Most annelids have external bristles called setae
(SEET ee). The paired setae located on each segment provide
traction as the annelid crawls along. Some annelids, like the fireworm shown in Figure 10 on the previous page, also have fleshy
appendages called parapodia (par uh POH dee uh).
Modeling a Closed Circulatory
System 2C 2D 3E TAKS 1
You can model a closed circulatory system using simple
items to represent the heart, blood vessels, blood, and
body tissues of a living organism.
Materials
clear plastic tubing, 30 cm (about 12 in.) piece; surgical
tubing, 15 cm (about 6 in.) piece; shallow pan filled with water;
eyedropper, food coloring
Answers to Analysis
1. The “blood” moved through
the blood vessels.
2. The pan of water represents
the body and tissues, surgical
tubing represents the heart,
and clear plastic tubing represents the blood vessels.
3. Answers will vary.
4. Answers will vary, but may
include adding model tissues
and increasing the number of
model blood vessels.
5. Students should note that the
closed system exerted a greater
pressure on the fluid than the
open system.
Procedure
1. Connect one end of the clear
tubing to the surgical tubing.
Submerge in the pan.
4. Observe the food coloring as
it moves through the tubing.
2. Use the eyedropper to insert
several drops of food coloring
into the surgical tubing. Close
the tubing by attaching the
two free ends together, as
shown above.
1. Describe what happened
when you pumped the food
coloring through the system.
3. With your thumb and index
finger, squeeze along the
piece of surgical tubing to
pump the food coloring
through the system.
Analysis
2. Identify what structures the
pan of water, the surgical
tubing, and the clear plastic
tubing represent.
3. Evaluate your model’s efficiency at pumping blood
through the system.
652
MISCONCEPTION
pp. 652–653
Student Edition
TAKS Obj 1 Bio/IPC 2C, 2D
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TEKS Bio 3E, 8C, 10A
TEKS Bio/IPC 2C, 2D
Teacher Edition
TAKS Obj 1 Bio/IPC 2C, 2D
TAKS Obj 2 Bio 12E
TAKS Obj 3 Bio 7B
TAKS Obj 4 IPC 8A
TEKS Bio 3D, 3E, 7B, 8B, 12E
TEKS Bio/IPC 2C, 2D; IPC 8A
652
ALERT
Insect Larvae Many people mistake
grubs, maggots, and caterpillars for worms.
These animals are arthropods, not annelids,
and are the larval form of certain insects
(beetles, flies, and moths or butterflies,
respectively). Have students list the annelid
characteristics that are shared by these
unrelated insects. (Answers may vary, but
arguments could be made for each of the four
characteristics of annelids.) Bio 8B
Chapter 29 • Mollusks and Annelids
4. Critical Thinking
Analyzing Methods How
could you modify the model
to make it more accurate?
5. Critical Thinking
Inferring Relationships
If you did the Quick Lab in
the previous section that
models an open circulatory
system, recall what happened
in that system. Which model
do you think exerted a greater
pressure on the fluid in
the tube?
Annelid Groups
Annelids differ in the number of setae (bristles) they have on
each segment, and not all annelids have parapodia. These
two external characteristics are used to classify annelids.
Teaching Tip
Marine Worms
Marine segmented worms are members of class Polychaeta
(PAHL ih keet uh), the largest group of annelids. Polychaetes
live in virtually all ocean habitats. They are often beautiful,
showing unusual forms and iridescent colors. A distinctive characteristic of polychaetes is the pair of fleshy,
paddle-like parapodia that occur on most of their segments. The
parapodia, which usually have setae, are used to swim, burrow,
or crawl. Parapodia also greatly increase the surface area of the
polychaete’s body, making gas exchange between the animal and
the water more efficient.
Many polychaetes are burrowing species, but others live in
protective tubes formed by the hardened secretions of glands
located on their segments. Grains of sand or other foreign material
may be cemented into the tube. Such tubeworms, like the feather
duster shown in Figure 11, live with only their head stuck out of the
tube. Featherlike head structures trap food particles from the water
that passes over them. Other species of polychaetes feed by pumping
water through their body. Free-swimming polychaetes, such as
Nereis shown in Figure 12, are predators that use their strong jaws
to feed on small animals.
Figure 11 Feather duster.
Feather dusters filter-feed by
trapping food particles in their
featherlike head structures.
Using the Figure
GENERAL
Ask a volunteer to examine
Figure 11 and offer an explanation
for how feather duster worms
acquire food. (Bristle-like appendages filter small organisms and
organic matter out of water that
flows over them.) Have the class
name an advantage and a disadvantage of this feeding strategy.
(Advantage: Allows for a sessile
lifestyle. Disadvantage: Potential
food must pass close by to be
captured.) LS Visual TAKS 3 Bio 7B
Figure 12 Nereis
Nereis, a polychaete worm, grasps its prey in its jaws, which open
when it thrusts out its pharynx.
GENERAL
The Ultimate Worm Ask students
to list the anatomical features that
make earthworms more advanced
than flatworms or roundworms.
Help students recall that earthworms have a segmented body, a
true coelom, a highly specialized
gut, a closed circulatory system,
and external bristles.
Tentacles
IPC Benchmark
Review
Jaw
To prepare students for the TAKS
and accompany the discussion of
earthworms, have students review
Water Produces Physical Change,
TAKS 4 IPC 8A, on p. 1052 of the IPC
Refresher in the Texas Assessment
Appendix of this book.
Pharynx
Eyes
Setae
Parapodia
653
did you know?
Importance of Polychaetes Polychaetes are
some of the most important animals in marine
benthic (bottom-dwelling) communities.
Though few people notice polychaetes or even
know of their existence, these worms exert a
strong influence in marine benthic food webs.
Some of the 10,000 species so far described eat
animals, others eat plants, and many more
provide food for commercially important fish.
TAKS 3 Bio 12E
Career
Oceanographer Oceanography is a diverse
field with branches in the biological chemical,
physical, geological, and engineering fields. A
graduate degree is usually required, along with
an undergraduate degree in biology, chemistry,
geology, or engineering. Biological oceanographers study the interrelationships of marine
life, energy transfer, and the impact of human
activities on marine communities. Encourage
students to learn more about this field at the
library or on the Internet. Bio 3D
Chapter 29 • Mollusks and Annelids
653
Earthworms
Real Life
Teach, continued
continued
Group Activity
GENERAL
Comparing Mollusks and
Annelids Have students make a
table similar to the one in the
Graphic Organizer on the bottom
of this page. Students’ tables
should compare body cavity, body
plan, organ systems, and larvae.
Ask students to write a paragraph
explaining the differences.
“The early bird catches
the worm.”
Because earthworms
respire through their skin,
they must stay moist. As
a result, earthworms come
to the surface of the soil
only at night, when the air
is more moist. The “early
bird” catches
worms that
have not
yet burrowed in for
the day.
LS Logical TAKS 2 Bio 10A
Group Activity
GENERAL
Worm Observations Obtain samples of the three different classes of
segmented worms. Try to obtain a
predatory marine worm (such as
Nereis), an earthworm, and a leech.
Have students observe the worms
and provide answers to the following questions: How is each worm’s
method of movement and the
medium in which it lives reflected in
its body structures? (All types of segmented worms have flexible, muscular
bodies; Nereis and the earthworm
have body appendages or extensions
that enable them to move across a
surface while the leech has a smooth
body with a sucker that it can use for
attachment.) How is each worm’s
method of obtaining food reflected
in its body structures? (Nereis has
jaws used for preying on other organisms; the leech has piercing structures;
the earthworm does not have a welldeveloped head region and ingests soil
particles as it moves along.) How is
each worm’s method of respiration
reflected in its body structures? (All
of these worms respire by diffusing
oxygen across their body surfaces;
their bodies are long and narrow,
which results in a high surface-tovolume area for diffusion.)
Co-op Learning
Earthworms and some related freshwater worms are members of
the class Oligochaeta (AHL ih goh KEET uh). Oligochaetes have no
parapodia and only a few setae on each segment. Earthworms lack
the distinctive head region of polychaetes and have no eyes. They
do, however, have light-sensitive and touch-sensitive organs located
at each end of their body. They have other sensory cells that detect
moisture.
Earthworms, such as the ones in Figure 13, are highly specialized
scavengers. They literally eat their way through the soil, consuming
their own weight in soil every day. As they tunnel, earthworms take
in organic matter and other materials using their muscular pharynx.
The ingested soil moves through their one-way gut, down the
esophagus and into a storage chamber called the crop. From here,
the soil moves to an area called the gizzard. The grinding action of
the gizzard crushes the soil particles together, breaking them down.
The crushed material moves to the intestine, which extends to the
posterior end of the earthworm’s body. Digested food molecules are
absorbed into the intestinal wall, and the remaining material passes
out through the anus in a form called castings. The tunneling activity
of earthworms allows air to penetrate the soil, and their castings
fertilize it. Rich, organic soil may contain thousands of earthworms
per acre.
Hydrostatic Skeleton
The fluid within the coelom of each body segment creates a hydrostatic skeleton that supports the segment. Each segment contains
muscles that pull against this hydrostatic skeleton. Circular
muscles wrap around the segment, while longitudinal muscles span
its length. As shown in the Up Close: Earthworm, on the next page,
when the circular muscles contract, the segment becomes longer.
When the longitudinal muscles contract, the segment bunches up,
increasing in diameter. An earthworm crawls by alternately contracting the two sets of muscles in its segments. The brain coordinates the
muscular activity of each body segment, thus controlling movement.
Figure 13 Earthworms
burrowing. Earthworms come
to the surface only at night or
during heavy rains. During dry or
cold weather, they burrow deep
into the soil and become inactive.
654
Graphic Organizer
TAKS 2 Bio 4B; TAKS 3 Bio 7B Use this graphic organizer with the Group
Activity: Comparing Mollusks and Annelids on
this page.
pp. 654–655
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8C, 10A
Teacher Edition
TAKS Obj 2 Bio 4B, 8C, 10A
TAKS Obj 3 Bio 7B
TEKS Bio 3F, 4B, 7B, 8C, 10A
654
Chapter 29 • Mollusks and Annelids
Body cavity
Mollusks
True
coelom
Body plan
Muscular
foot, head,
visceral mass
Annelids
True
coelom
Segmented
with bristles
Organ systems
Larvae
Circulatory (usually open
system), respiratory,
Trochophore
digestive, excretory
Circulatory (closed),
respiratory, digestive,
Trochophore
excretory
Up Close
Earthworm TAKS 2
Up Close
Movement of an Earthworm
●
Scientific name: Lumbricus terrestris
●
Size: Grows up to 30 cm (12 in.) long
●
Range: Europe; eastern and northwestern
North America
●
Habitat: Damp soil
●
Diet: Organic matter contained in soil
Earthworm
1 Rear anchor
Teaching Strategies
Earthworms, which have no
teeth, eat their way through soil
and grind it up for nutrients.
Ask students what structure
earthworms use to grind up
soil. (gizzard) Ask them which
other toothless animals use gizzards to grind food. (birds)
Finally, ask why food needs to
be ground up. (Smaller particles
are digested more easily.)
2 Elongation
3 Front
anchor
Characteristics
4 Pull
Respiration Oxygen and carbon dioxide diffuse
through the earthworm’s skin. This exchange can
take place only if the worm’s
Movement As shown in the diagram,
1 first the earthworm anchors several of its rear
skin is kept moist.
segments by sinking their setae into the ground.
2 The worm then contracts the circular muscles in
front of the anchored segments. This causes the
anterior segments to elongate. 3 Then the setae in
front of the stretched region are anchored and the
Anus
rear setae are released. 4 The circular muscles
relax and the longitudinal muscles contract, pulling
the rear segments forward.
Clitellum
Digestion Earthworms
“eat” soil, which is ground
up in a thick, muscular
gizzard. Food molecules
pass across the walls of
the intestine and are
absorbed into the
bloodstream.
▲ Longitudinal muscles
Circular
muscles
Dorsal blood vessel
Esophagus
Gizzard
TAKS 3 Bio 7B, 8C, 10A
Discussion
• How do earthworms loosen
soil and help recycle soil
nutrients? (Earthworms make
holes in the soil by moving
through it, digesting soil matter into simpler components.)
• What is the role of the clitellum? (It secretes a protective
layer of chitin for the fertilization and incubation of eggs.)
Hearts
Crop
Pharynx
Mouth
▲ Intestine
Leeches Make a
Comeback TAKS 3
Nephridium
Ventral blood vessel
Ventral nerve cord
Segmental ganglion
Setae
▼
Reproduction Earthworms are hermaphrodites, each
Reproductive organs
▼ Cerebral ganglion (brain)
individual containing both sexes. Mating occurs when two
Brain The brain coordinates
earthworms join ventrally head to tail, exchanging sperm.
During egg laying, the clitellum (a thickened, glandular ring
of cells) of each worm secretes a mucous cocoon that
encloses the fertilized eggs. Young worms emerge from
the cocoon several weeks later.
the muscular activity of each body
segment. It also processes sensory
information from light-sensitive and
touch-sensitive organs located at
each end of the body.
655
HISTORY
CONNECTION
Charles Darwin (1809–1882), the British
naturalist famous for his theory of evolution
by natural selection, was fascinated with the
ways that earthworms aerate and enrich soil.
In his last botanical book, The Formation of
Vegetable Mould Through the Action of
Worms, Darwin noted the service of earthworms in recycling organic matter in soil. He
also observed that an earthworm could ingest
its weight in soil each day and that in one year,
the earthworms inhabiting one hectare could
digest 22 to 40 metric tons of soil. The work
was published only 6 months prior to Darwin’s
death and was called a pioneering study in
quantifying ecology. Bio 3F
Bio 7B
Teaching Strategies
Have students research past
and present uses of leeches in
medicine and write a brief
(one-page) paper that describes
their findings.
Discussion
Ask if any students have ever
had a leech attach while swimming or wading in a pond or
stream. Did it hurt as it
attached? (Usually there is no
sensation.) Did you bleed after
the leech was removed? (If the
leech was fully attached and
feeding, students should answer
yes because leeches use anticoagulants to facilitate feeding. The
bleeding stops soon after the
leech is removed.)
Chapter 29 • Mollusks and Annelids
655
Leeches
Close
Reteaching
Have students pick an annelid in
this chapter and pretend that the
animal is a pet. Ask them to
provide a picture of the pet and
prepare a report about it. In their
report, students should identify the
kind of habitat necessary to keep
the pet, describe how and what the
animal is fed, and provide any
necessary warnings for handling.
Quiz
GENERAL
1. Why is “poly-” an appropriate
prefix for polychaete worms?
(They have many body segments.)
2. Why are earthworms likely to be
found in richly organic soil but
not in a very sandy soil?
(Because they use the energy they
obtain from organic matter in the
soil as their source of energy for
survival.)
3. Why is it advantageous for parasitic leeches to be able to distend
their bodies by ingesting a huge
meal? (Because they have to search
for food, consuming a large meal at
one time would last them until they
can find another source of food.)
Alternative
Assessment
Leeches Make a Comeback
TAKS 3
F
or many centuries, it was
commonly believed that an
excess amount of blood was
the cause of a wide range of
illnesses, from a fever or headache to severe heart disease. A
standard treatment for these conditions was bloodletting using
leeches. Physicians applied
leeches to the patient’s body,
allowing the leeches to suck out
the patient’s “bad blood.”
Use in Microsurgery
Although doctors no longer
believe in “bad-blood,” leeches
are making a comeback in the
field of health care. One use of
leeches is during surgery to
reattach severed limbs, fingers,
or toes. In this type of operation,
called microsurgery, the surgeon
uses tiny instruments and a
microscope to reconnect tendons, blood vessels, and nerves.
It is not possible to reconnect
the smallest of the blood vessels,
so circulation in the reattached
part is usually poor. Often tissues
in the region die, and the
reattached part cannot heal. By
applying leeches to suck out
the accumulated blood, tissues
remain healthy until new blood
vessels grow and circulation is
restored to normal. As a result,
the success rate of surgery for
reattachments has increased.
Other Applications
Leeches possess other useful
qualities. Their saliva contains
anticoagulants — substances
that prevent blood from clotting—
and enzymes that can break up
blood clots. It is not necessary to
apply leeches to a patient to take
advantage of these chemicals.
Today these substances are
produced through genetic engineering and have proven useful
in the treatment of some heart
patients.
www.scilinks.org
Topic: Leeches
Keyword: HX4113
Section 2 Review
GENERAL
Have students combine their new
understanding of annelids with
their artistic skills and draw a
“new” species of annelid. Tell them
to label their annelids and to make
sure that the creatures include both
the fundamental annelid traits as
well as any reasonable adaptations
to their designated environment.
LS Visual
pp. 656–657
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A, 10B
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8B, 8C, 10A, 10B
Teacher Edition
TAKS Obj 2 Bio 8C, 10A, 10B
TEKS Bio 8B, 8C, 10A, 10B
656
Figure 14 Leech. Most
species of leeches are small,
2.5 to 5.0 cm (1 to 2 in.) long.
When people hear the word leech, they usually associate it with
bloodsucking, and for a good reason. A leech, shown in Figure 14,
has suckers at both ends of its body. Most species are predators or
scavengers, but some are parasites of vertebrates and crustaceans.
Leeches are the only members of class Hirudinea (hihr yoo DIHN
ee uh). Leeches lack both setae and parapodia. The body of a leech is
flattened, and unlike other annelids, its segments are not separated
internally.
Summarize how you can tell if a wormlike
8B 8C
organism is an annelid worm.
Compare the external appearance of marine
Relate an annelid’s septa to its overall
TAKS Test Prep What happens to a segment in
an earthworm when the circular muscles in that
10A
segment contract?
A It elongates.
C It increases in diameter.
B It shortens.
D It bends to one side.
body plan.
8C 10A 10B
Describe the major features of an earthworm’s
digestive system.
8C 10A
annelids, earthworms, and leeches.
8B 8C
656
Answers to Section Review
1. Annelid worms have segments, and most have
external bristles. TAKS 2 Bio 8C; Bio 8B
2. Septa are internal body walls that separate the
segments of most annelids. Blood vessels and a
nerve cord connect the segments to one another,
and to the annelid’s brain. TAKS 2 Bio 8C, 10A, 10B
3. The earthworm’s digestive system is a highly
modified gut consisting of the following
regions: mouth, pharynx, esophagus, crop,
gizzard, intestine, and anus. TAKS 2 Bio 8C, 10A
4. Marine annelids are often very colorful and
have a head region with eyes; their segments
usually have a pair of fleshy extensions called
Chapter 29 • Mollusks and Annelids
parapodia. Earthworms lack parapodia and
have no head region. Usually there are several
bristly setae on each segment of an earthworm. Leeches lack setae and parapodia, and
their bodies are generally flat. TAKS 2 Bio 8C; Bio 8B
5.
A. Correct. B. Incorrect.
Contraction of the longitudinal muscles results
in shortening. C. Incorrect. Contraction of the
circular muscles does not increase the diameter
of the worm. D. Incorrect. Contraction of the
circular muscles does not cause the worm to
bend to one side. TAKS 2 Bio 10A
Study
CHAPTER HIGHLIGHTS
ZONE
Alternative Assessment
Key Concepts
Key Terms
1 Mollusks
Section 1
●
trochophore (642)
visceral mass (643)
mantle (643)
foot (643)
radula (643)
nephridium (644)
adductor muscle (647)
siphon (647)
●
●
●
●
●
●
●
●
●
Many mollusks and annelids have a larval form called
a trochophore.
The mollusk body has three distinct parts: a visceral
mass, a mantle, and foot.
All mollusks except bivalves have a rasping tonguelike
radula.
Mollusks have a true coelom and well-developed
organs.
Most mollusks respire with gills but some respire with
a primitive lung.
Nephridia enable mollusks to recover the useful
substances from their bodily wastes.
Gastropods (snails and slugs) live in oceans, in fresh
water, and on land.
Bivalves (clams, oysters, and their kin) are aquatic
and have hard shells called valves that protect their
soft bodies.
Gastropods and bivalves have an open circulatory
system.
Cephalopods (octopuses, squids, and their kin) have
a well-developed head region, many tentacles, and a
closed circulatory system. Most cephalopods have no
external shell.
Chapter Resource File
• Science Skills Worksheet
• Critical Thinking
Worksheet
• Test Prep Pretest GENERAL
• Chapter Test GENERAL
2 Annelids
Section 2
●
cerebral ganglion (651)
septa (652)
seta (652)
parapodium (652)
●
●
●
●
●
Annelids are coelomate worms that have segmented
bodies and complex nervous systems.
Annelids are classified according to the presence or
absence of setae and parapodia.
Annelids respire through their skin, and they have a
closed circulatory system.
Earthworms burrow through the soil, ingesting it as
they crawl.
Marine polychaetes have parapodia and setae. Some
are active predators and others are filter feeders.
Leeches lack parapodia and setae, and their segments
are not separated internally. They may be aquatic or
terrestrial, and some are parasites.
Have students design a new species
of mollusk. Students might draw
and describe their animal or create
a model. They should give their
creation a scientific name based on
its features and describe its habitat,
reproductive habits, and feeding
habits. Designs must include the
key characteristics of a mollusk.
GENERAL
657
Answer to Concept Map
The following is one possible answer to
Performance Zone item 15.
Mollusks
include
bivalves
bodies consist of
mantle
foot
have
siphons
open circulatory
systems
gastropods
cephalopods
have
have a
radula
closed circulatory
system
visceral mass
Chapter 29 • Mollusks and Annelids
657
Performance
ZONE
CHAPTER 29
ANSWERS
Using Key Terms
1. c TAKS 2 Bio 10A
2. a TAKS 2 Bio 8C, 10A
3. d TAKS 2 Bio 10A
4. b TAKS 2 Bio 8C
5. a. Setae are bristlelike
appendages found on the
segments of earthworms.
Parapodia are fleshy
appendages found on some
annelids.
b. The mantle is the outer fleshy
layer of the mollusk, surrounding the visceral mass.
The visceral mass is the central, organ-containing
section of a mollusk.
Understanding Key Ideas
6. a TAKS 2 Bio 8C; Bio 8B
7. b TAKS 2 Bio 10A
8. a TAKS 2 Bio 8C
9. c TAKS 2 Bio 8C
10. d TAKS 2 Bio 8C; Bio 8B
11. d TAKS 2 Bio 10A
12. b TAKS 2 Bio 8C
13. Bivalves may harbor dangerous
bacteria if they are not fresh or
were harvested in contaminated
waters. Bio 11C
14. Leeches remove blood from damaged tissue, which helps keep the
tissues healthy until new blood
vessels can form.
15. A possible answer to the concept
map is found at the bottom of the
Study Zone page. Bio 3E
CHAPTER REVIEW
8. Which of the following is not true of
Using Key Terms
8C
bivalves?
a. They have a distinctive head region.
b. They have sense organs.
c. They have an open circulatory system.
d. Most are filter feeders.
1. In mollusks, ______ extract useful
molecules from coelomic fluid.
a. radula
b. ganglia
c. neprhidia
d. parapodia
10A
9. Cephalopods have all of the following
8C
characteristics except
a. bilateral symmetry.
b. a three-part body plan.
c. an open circulatory system.
d. a true coelom.
2. All of the following are a part of the
mollusk’s three-part body plan except a
a. radula.
8C 10A
b. mantle.
c. visceral mass.
d. foot.
10. Annelids are divided into three classes.
This classification is based on the number
of setae and the presence or absence of
8B 8C
a. segments.
b. hearts.
c. a gizzard.
d. parapodia.
3. The valves of bivalves are connected
by the
10A
a. siphons.
b. parapodia.
c. visceral mass.
d. adductor muscles.
4. Annelids are most easily recognizable
by their
8C
a. cephalization.
b. segmentation.
c. nephridia.
d. body cavity.
5. For each pair of terms, explain the
11. Blood in the circulatory system of
an annelid
10A
a. flows into its body cavity.
b. delivers carbon dioxide to its tissues.
c. passes through gills.
d. stays within its circulatory system.
12. Earthworm movement requires all of the
following except
8C
a. circular muscles.
b. secretion of mucus.
c. muscle contractions.
d. traction provided by setae.
difference in their meanings.
a. setae, parapodia
b. mantle, visceral mass
Understanding Key Ideas
8B 8C
6. Both mollusks and annelids
a. are coelomates.
b. have at least a remnant of a shell.
c. have no larval form.
d. have a visceral mass.
7. Terrestrial snails respire
a. with gills.
b. with a primitive lung.
c. through their skin.
d. through their siphon.
13.
What are some possible
risks associated with purchasing bivalves
from an unknown vendor?
11C
14.
How can leeches be beneficial following microsurgery?
15.
Concept Mapping Make a concept
map that shows the characteristics and
diversity of the mollusks. Include the following words in your map: foot, visceral
mass, mantle, radula, siphons, gastropod,
bivalve, cephalopod, open circulatory
system, and closed circulatory system.
3E
10A
658
Assignment Guide
Section
1
2
Review and Assess
TAKS Obj 1 Bio/IPC 2B, 2C, 2D
TAKS Obj 2 Bio 8C, 10A
TAKS Obj 3 Bio 7A, 7B
TEKS Bio 3D, 3E, 7A, 7B, 8B, 8C,
10A, 11C
TEKS Bio/IPC 2B, 2C, 2D
658
Chapter 29 • Mollusks and Annelids
Questions
1, 2, 3, 5b, 8, 9, 13, 17–21
4, 5a, 6, 7, 10, 11, 12, 14, 16, 22
Critical Thinking
Alternative Assessment
Critical Thinking
16. Recognizing Patterns The gizzards of
20. Finding and Communicating Information
16. The food of aquatic annelids is
not as coarse and does not require
as much grinding for digestion as
the food of terrestrial annelids.
annelids that have returned to an aquatic
environment are smaller and less muscular
than those of terrestrial annelids, such as
earthworms. How do the differences
between the gizzards represent adaptations
7A 7B
in aquatic and terrestrial feeding?
17. Relating Concepts Explain the significance
of the evolution of a coelom in mollusks.
7B 10A
18. Evaluating Results A particular bivalve
mollusk called a shipworm can do extensive damage to ocean pier pilings (supports) by burrowing into them with its
radula. As a project, a student decides to
determine if a new paint can reduce shipworm damage more than other paints do.
What variables must the student control in
order for the experimental results to be
considered valid?
2D
19. Justifying Conclusions Classification of
organisms has traditionally been based on
physical similarities. Given the physical
differences of gastropods, bivalves, and
cephalopods, how did they come to be
8C
grouped into the phylum Mollusca?
Use the library or Internet resources to
compile a list of mollusks used by humans
as food sources. For each mollusk listed,
tell where it is harvested and which part of
the mollusk is eaten. Prepare a brochure to
summarize your findings. Include a visual
that informs the reader of the geographic
location of the food source.
2B 2C 2D
21. Summarizing Information Throughout
history, people around the world have used
mollusk shells for adornment and other
purposes. Research some specific ways
humans have used shells, and prepare an
oral or visual report of your findings. If you
can obtain some mollusk shells, display
them to illustrate your report.
2B 2C 2D
22. Career Connection Worm Farmer Research
the field of growing segmented worms for
use in research, as fishing bait, and for soil
improvement. Your report should include
a job description, training required, kinds
of employers, growth prospects, and
3D
starting salary.
TAKS Test Prep
Use the diagram below and your knowledge of
science to answer questions 1–3.
C
B
D
2. Which structure is part of the digestive
system?
F A
G B
H C
J D
10A
TAKS 3 Bio 7A (grade 11 only), 7B
17. The coelom allowed for the evolution of complex organs composed of more than one tissue
type. A coelom also allows the
internal organs to be suspended
from the body wall and cushioned
by fluid. This allows mollusks to
move around without damaging
their organs or interfering with
their function.
TAKS 2 Bio 10A; TAKS 3 Bio 7B
18. Every variable other than type of
paint must be identical, including
the shape and size of the pier
pilings, the type of wood, the
thickness of paint, the method of
application, and the numbers
of mollusks used. TAKS 1 Bio/IPC 2D
19. Gastropods, bivalves, and
cephalopods are grouped together
as mollusks because they share a
common evolutionary ancestor
and because they share several
key characteristics, including the
presence of a coelom, a three-part
body plan, and a trocophore larval stage. TAKS 2 Bio 8C
3. One characteristic shown by this
A
1. Which structure is part of the respiratory
system?
A A
B B
C C
D D
8C
mollusk is
A cephalization.
B radial symmetry.
C body segmentation.
D a pseudocoelom.
Alternative Assessment
10A
Test
Focus on one question at a time unless you
are asked to refer to previous answers.
659
20. Answers will vary. Students should
list a variety of mollusks that are
used for human consumption.
They should also describe areas
where each mollusk can be
found. TAKS 1 Bio/IPC 2B, 2C, 2D
21. Answers will vary, but could
include trade currency, tools,
jewelry, and dishes.
TAKS 1 Bio/IPC 2B, 2C, 2D
Standardized Test Prep
1. A. Incorrect. The foot is used for locomotion.
B. Correct. The gill is used in respiration.
C. Incorrect. The heart is used in circulation.
D. Incorrect. The intestine is used in
digestion. TAKS 2 Bio 10A
2. F. Incorrect. The foot is used in locomotion.
G. Incorrect. The gill is used in respiration.
H. Incorrect. The heart is used in circulation.
J. Correct. The intestine is used in digestion.
TAKS 2 Bio 10A
3. A. Correct. This mollusk has a distinct head
region—cephalization. B. Incorrect. Mollusks
are bilaterally symmetrical. C. Incorrect.
Mollusks are not segmented. D. Incorrect.
Mollusks have a coelom. TAKS 2 Bio 8C
22. Answers will vary. Worm farmers
have varying degrees of education, from high school through
graduate school. They all share a
working knowledge of growing,
breeding, and harvesting worms
on a large scale. Worm farmers
usually own their own businesses
or work as part of a larger agricultural products business. They
may also be employed by waste
management agencies. The
growth potential for this field
is fair. Starting salary will vary
by region. Bio 3D
Chapter 29 • Mollusks and Annelids
659