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Standardized Test Prep
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Chapter 35
Mollusks and Annelids
Table of Contents
Section 1 Mollusca
Section 2 Annelida
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Chapter 35
Section 1 Mollusca
Objectives
• Describe the key characteristics of mollusks.
• Describe the body plan of mollusks.
• Name the characteristics of three major classes of
mollusks.
• Compare the body plans of gastropods, bivalves,
and cephalopods.
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Chapter 35
Section 1 Mollusca
Characteristics of Mollusks
• Phylum Mollusca is a diverse group of invertebrates
such as clams, snails, slugs, squids, and octopuses.
• They are called mollusks, from the Latin molluscus,
which means “soft.” Many mollusks have soft bodies
and some have a hard shell.
• Some mollusks are sedentary filter feeders, while
others are fast-moving predators.
• Mollusks are coelomates.
• Most aquatic mollusks and annelids have a larval
stage called a trochophore.
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Chapter 35
Section 1 Mollusca
Body Plan of Mollusks
• The body of a mollusk is generally divided into:
– the visceral mass, which contains the heart and the
organs of digestion, excretion, and reproduction
– the head-foot, which consists of:
• the head, which contains the mouth and sensory
structures
• the foot, a large, muscular organ for locomotion
• The coelom is limited to a space around the heart.
• A layer of epidermis called the mantle covers the
visceral mass.
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Chapter 35
Section 1 Mollusca
Body Plan of Mollusks, continued
• In most mollusks, the mantle secretes one or more
hard shells containing calcium carbonate.
• This disadvantage of a shell is offset by having gills.
• The gills are protected within the mantle cavity.
• Most mollusks are bilaterally symmetrical.
• The nervous system consists of paired clusters of
nerve cells called ganglia.
• The main feeding adaptation of many mollusks is the
radula.
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Chapter 35
Section 1 Mollusca
Class Gastropoda
• The largest and most diverse class of mollusks is
Gastropoda, whose members are called
gastropods.
• Most gastropods, including snails, abalones, and
conchs, have a single shell. Others, such as slugs
and nudibranchs, have no shell.
• Gastropods undergo the process of torsion during
larval development.
• Gastropods have an open circulatory system in which
a heart pumps hemolymph from gills or lungs into
the hemocoel.
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Chapter 35
Section 1 Mollusca
Class Gastropoda, continued
Snails
• Snails live in moist and aquatic environments.
• Most snails locate food using eyes at the end of
tentacles.
• Snails survive dry periods by retreating into their
shells and sealing the opening with a mucous plug.
Other Gastropods
• Slugs are terrestrial and lack shells.
• Nudibranchs are marine and lack shells.
• Pteropods, or “sea butterflies,” have a foot that is
modified into a winglike flap and used for swimming
rather than crawling.
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Chapter 35
Section 1 Mollusca
Class Bivalvia
• The class Bivalvia includes aquatic mollusks such as
clams, oysters, and scallops.
• They are called bivalves because their shell is
divided into two valves connected by a hinge.
• Each valve consists of three layers.
• Most bivalves are sessile filter feeders.
• Bivalves lack a distinct head and have three pairs of
nerve ganglia.
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Chapter 35
Section 1 Mollusca
Class Bivalvia, continued
Clams
• Clams are bivalves that live buried in mud or sand.
• The mantle cavity of a clam is sealed except for a pair of
hollow, fleshy tubes called siphons.
– Water enters through the incurrent siphon.
– Water leaves through the excurrent siphon.
Other Bivalves
• Oysters become attached to a hard surface early in their
development.
• Scallops can move through the water by repeatedly
opening their valves and snapping them shut.
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Chapter 35
Section 1 Mollusca
Class Cephalopoda
• The class Cephalopoda includes octopuses, squids,
cuttlefishes, and chambered nautiluses.
• They are called cephalopods, which means “headfoot.”
• They are specialized as free-swimming predators.
• They have tentacles and beaklike jaws on the head.
• Their nervous system is very advanced.
• They have a closed circulatory system.
• Many use pigments to hide and disguise
themselves.
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Chapter 35
Section 1 Mollusca
Class Cephalopoda, continued
Squids
• Squids are cephalopods with ten tentacles.
• They propels themselves by pumping jets of water with the
mantle through an excurrent siphon.
Octopuses
• Octopuses have eight tentacles and are similar to squids.
• They often crawl along the ocean bottom or lie in wait for prey.
Chambered Nautiluses
• The chambered nautilus is the only existing cephalopod that has
retained its external shell.
• The shell is coiled and divided into a series of chambers. The
body is confined to the outermost chamber.
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Chapter 35
Section 1 Mollusca
Mollusk Body Plan
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Chapter 35
Section 1 Mollusca
Characteristics of Mollusks
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Chapter 35
Section 1 Mollusca
Anatomy of a Trochophore
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Chapter 35
Section 1 Mollusca
Anatomy of a Mollusk
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Chapter 35
Section 1 Mollusca
Radula
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Chapter 35
Section 1 Mollusca
Types of Mollusks
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Chapter 35
Section 1 Mollusca
Characteristics of Gastropods
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Chapter 35
Section 1 Mollusca
Anatomy of a Snail
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Chapter 35
Section 1 Mollusca
Characteristics of Bivalves
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Chapter 35
Section 1 Mollusca
Anatomy of a Clam
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Chapter 35
Section 1 Mollusca
Characteristics of Cephalopods
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Chapter 35
Section 2 Annelida
Objectives
• Identify the structures that provide the basis for
dividing annelids into three classes.
• List the advantages of body segmentation.
• Describe the structural adaptations of earthworms.
• Compare the three classes of annelids.
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Chapter 35
Section 2 Annelida
Characteristics of Annelids
• The phylum Annelida is made up of bilaterally
symmetrical, segmented worms.
• This phylum includes common earthworms, featherduster worms, and bloodsucking leeches.
• They are called annelids, which means “little rings” and
refers to the many body segments.
• Annelids have a true coelom that is divided into
separate compartments by partitions.
• Most annelids have external bristles called setae
(singular, seta), and some have fleshy protrusions
called parapodia (singular, parapodium).
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Chapter 35
Section 2 Annelida
Class Oligochaeta
• Annelids of the class Oligochaeta generally live in the soil or
in fresh water and have no parapodia.
• Oligochaeta means “few bristles”; these annelids have a few
setae on each segment.
• The most familiar member is the earthworm.
Structure and Movement
• An earthworm’s body has over 100 nearly-identical
segments.
• Circular and longitudinal muscles line the interior body wall.
• Locomotion is made possible by segmentation.
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Chapter 35
Section 2 Annelida
Class Oligochaeta, continued
Feeding and Digestion
• Earthworms ingest soil as they burrow through it.
• Soil is moved through these structures:
– mouth
– pharynx
– esophagus
– crop
– gizzard
– intestine
• includes the typhlosole
– anus
• Earthworms play an important role in the condition of soil.
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Chapter 35
Section 2 Annelida
Class Oligochaeta, continued
Circulation
• Contractions of the aortic arches and the dorsal blood vessel force
blood through the closed circulatory system.
Respiration and Excretion
• Oxygen and carbon dioxide diffuse through the skin, which contains
many small blood vessels and must be kept moist.
• Cellular wastes and excess water are excreted through nephridia.
Neural Control
• The nervous system of an earthworm consists of a chain of ganglia
connected by a ventral nerve cord.
• Sensory structures are found in all segments but are
concentrated at the anterior end.
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Chapter 35
Section 2 Annelida
Class Oligochaeta, continued
Reproduction
• Earthworms are hermaphrodites, but an individual worm
cannot fertilize its own eggs.
• During mating, earthworms press their ventral surfaces
together.
• They are held together by their setae and by a film of mucus
secreted by each worm’s clitellum.
• The sperm from each worm move through the mucus to the
seminal receptacle of the other.
• The clitellum secretes a tube of mucus and chitin.
• Fertilization occurs inside the tube, which forms a
protective case for the young worms.
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Chapter 35
Section 2 Annelida
Class Polychaeta and Hirudinea
• Most annelids are members of the class Polychaeta, which
means “many bristles.”
• Polychaetes differ from other annelids in that they have
antennae and specialized mouthparts.
• They are the only annelids that have a trochophore stage.
• Hirudinea is the smallest class of annelids, consisting of about
500 species of leeches.
• Leeches have no setae or parapodia.
• Many leeches are carnivores but some are parasites that suck
blood from other animals.
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Chapter 35
Section 2 Annelida
Anatomy of the Earthworm
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Chapter 35
Section 2 Annelida
Characteristics of Annelids
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Chapter 35
Section 2 Annelida
Segmentation
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Chapter 35
Section 2 Annelida
Types of Annelids
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Chapter 35
Section 2 Annelida
Anatomy of an Earthworm
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Chapter 35
Section 2 Annelida
Movement of an Earthworm
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Chapter 35
Section 2 Annelida
Feeding Habits of an Earthworm
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Chapter 35
Standardized Test Prep
Multiple Choice
1. Why do terrestrial snails and slugs need an
environment with a high moisture content?
A. to avoid drying out
B. to see and hear better
C. to swim more efficiently
D. to avoid being eaten by birds
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
1. Why do terrestrial snails and slugs need an
environment with a high moisture content?
A. to avoid drying out
B. to see and hear better
C. to swim more efficiently
D. to avoid being eaten by birds
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
2. Which of the following are the only mollusks with a
closed circulatory system?
F. snails
G. bivalves
H. gastropods
J. cephalopods
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
2. Which of the following are the only mollusks with a
closed circulatory system?
F. snails
G. bivalves
H. gastropods
J. cephalopods
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
3. Annelids are divided into three classes based partly
on the number of which of the following?
A. setae
B. segments
C. nephridia
D. aortic arches
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
3. Annelids are divided into three classes based partly
on the number of which of the following?
A. setae
B. segments
C. nephridia
D. aortic arches
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
4. Parapodia are distinguishing characteristics of which
class of annelids?
F. Bivalvia
G. Hirudinea
H. Polychaeta
J. Oligochaeta
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
4. Parapodia are distinguishing characteristics of which
class of annelids?
F. Bivalvia
G. Hirudinea
H. Polychaeta
J. Oligochaeta
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
The diagram below shows the internal structure of a
bivalve. Use the diagram to answer the questions
that follow.
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
5. Which of the structures is involved in respiration and
feeding?
A. A
B. B
C. C
D. D
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
5. Which of the structures is involved in respiration and
feeding?
A. A
B. B
C. C
D. D
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
6. What is structure B?
F. a gill
G. a siphon
H. the mantle
J. an adductor muscle
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
6. What is structure B?
F. a gill
G. a siphon
H. the mantle
J. an adductor muscle
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
7. Nephridium : excretion :: clitellum :
A. digestion
B. circulation
C. respiration
D. reproduction
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
7. Nephridium : excretion :: clitellum :
A. digestion
B. circulation
C. respiration
D. reproduction
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
The diagram below shows the mouth structure of a
class of organisms. Use the diagram to answer the
question that follows.
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
8. In which class of organisms is the mouth structure
shown above most likely to be found?
F. bivalves
G. hirudines
H. gastropods
J. polychaetes
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Chapter 35
Standardized Test Prep
Multiple Choice, continued
8. In which class of organisms is the mouth structure
shown above most likely to be found?
F. bivalves
G. hirudines
H. gastropods
J. polychaetes
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Chapter 35
Standardized Test Prep
Short Response
During larval development, gastropods undergo a
process called torsion.
Describe the outcome of torsion.
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Chapter 35
Standardized Test Prep
Short Response, continued
During larval development, gastropods undergo a
process called torsion.
Describe the outcome of torsion.
Answer: Torsion is a developmental process in
which the visceral mass twists around 180
degrees in relation to the head, bringing the
mantle cavity to the anterior end of the snail.
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Chapter 35
Standardized Test Prep
Extended Response
Base your answers to parts A & B on the
information below.
Earthworms have three main structural features that
enable movement.
Part A Describe these features.
Part B Explain how they work together to enable
movement.
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Chapter 35
Standardized Test Prep
Extended Response, continued
Answer:
Part A Circular muscles and longitudinal muscles line the
interior body wall of an earthworm. Setae, or external
bristles, line the ventral surface of the earthworm.
Coelomic fluid is found in individual segments.
Part B: Contractions of the circular muscles increase the
pressure of the coelomic fluid. This increased pressure
causes the animal to elongate and pushes the anterior
end forward. The animal then uses its setae to grip the
ground as the longitudinal muscle contracts, pulling the
posterior end forward.
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