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Table of Contents
Chapter: Mollusks, Worms,
Arthropods, Echinoderms
Section 1: Mollusks
Section 2: Segmented Worms
Section 3: Arthropods
Section 4: Echinoderms
Mollusks
1
Characteristics of Mollusks
• Mollusks (MAH lusks) are soft-bodied
invertebrates with bilateral symmetry and
usually one or two shells.
• Their organs are in a fluid-filled cavity.
• Most mollusks live in water, but some live
on land.
• Snails, clams, and squid are examples of
mollusks.
Mollusks
1
Body Plan
• All mollusks have a thin layer of tissue called
a mantle, which covers the body organs.
These are
located in
the visceral
(VIH suh
rul) mass.
Mollusks
1
Body Plan
• Between the soft body and the mantle is a
space called the mantle cavity. It contains
gills—the
organs that
exchange
carbon
dioxide for
oxygen.
Mollusks
1
Body Plan
• The mantle also secretes the shell or protects
the body if the mollusk does not have a shell.
• The shell is made up of several layers.
• The inside layer is the smoothest. It is
usually the thickest layer because it’s added
to throughout the life of the mollusk.
• The inside layer also protects the soft body.
Mollusks
1
Body Plan
• Most mollusks have an open circulatory
system in which the heart moves blood out
into the open spaces around the body
organs.
• The blood, completely surrounds and
nourishes the body organs.
Mollusks
1
Body Plan
• Most mollusks have a well-developed head
with a mouth and some sensory organs.
• Some mollusks have tentacles.
• On the underside of a mollusk is the muscular
foot, which is used for movement.
Mollusks
1
Classification of Mollusks
• Mollusks that have shells are then classified
by the kind of shell and kind of foot that they
have.
• The three most common groups of mollusks
are gastropods, bivalves, and cephalopods.
Mollusks
1
Gastropods
• The largest group of mollusks, the gastropods,
includes snails, conchs, abalones, whelks, sea
slugs, and garden slugs.
• Gastropods use a
radula (RA juh
luh)—a tonguelike
organ with rows of
teeth—to obtain
food.
Mollusks
1
Gastropods
• Slugs and many snails are adapted to life
on land.
• They move by rhythmic contractions of the
muscular foot.
• Glands in the foot secrete a layer of mucus
on which they slide.
• Slugs do not have shells but are protected by
a layer of mucus instead, so they must live in
moist places.
Mollusks
1
Bivalves
• Mollusks that have a hinged, two-part
shell joined by strong muscles are called
bivalves.
• Clams, oysters,
and scallops are
bivalve
mollusks.
Mollusks
1
Bivalves
• These animals pull their shells closed by
contracting powerful muscles near the
hinge. To open their shells, they relax
these muscles.
Mollusks
1
Bivalves
• For protection, clams burrow deep into the
sand by contracting and relaxing their
muscular foot.
• Mussels and oysters attach themselves with
a strong thread or cement to a solid surface.
This keeps waves and currents from washing
them away.
Mollusks
1
Cephalopods
• The most specialized and complex mollusks
are the cephalopods (SE fuh luh pawdz),
which includes squid, octopuses, cuttlefish,
and chambered nautiluses.
• Cephalopods have a
large, well-developed
head.
Mollusks
1
Cephalopods
• Their foot is divided into many tentacles
with strong suction cups or hooks for
capturing prey.
• All cephalopods are predators. They feed on
fish, crustaceans, worms, and other mollusks.
Mollusks
1
Cephalopods
• Squid and octopuses have a well-developed
nervous system and large eyes similar to
human eyes.
• Unlike other mollusks, cephalopods have a
closed circulatory system, in which blood
containing food and oxygen moves through
the body in a series of closed vessels.
Mollusks
1
Cephalopod Propulsion
• Squid and other cephalopods have a waterfilled cavity between an outer muscular
covering and its internal organs.
• When the cephalopod tightens its muscular
covering, water is forced out through an
opening neat the head.
• According to Newton’s third law of motion,
when one object exerts a force on a second
object, the second object exerts a force on the
first that is equal and opposite in direction.
Mollusks
1
Value of Mollusks
• Many people make their living raising
or collecting mollusks to sell for food.
• Many mollusk shells are used for jewelry and
decoration.
• Pearls are produced
by several species
of mollusks.
Mollusks
1
Value of Mollusks
• Even though mollusks are beneficial in
many ways, they also can cause problems
for humans.
• Land slugs and snails damage plants.
• Certain species of snails are hosts of
parasites that infect humans.
• Eating infected mollusks can result in
sickness or even death.
Section Check
1
Question 1
Which structure is responsible for exchanging
carbon dioxide from the mollusk for oxygen in
the water?
Section Check
1
A. heart
B. gills
C. mantle
D. radula
Section Check
1
Answer
The answer is B. Gills are located in the mantle
cavity.
Section Check
1
Question 2
Which structure is responsible for helping this
organism move?
Section Check
1
A. foot
B. mantle
C. Mantle cavity
D. radula
Section Check
1
Answer
The answer is A. Slugs and snails move by
rhythmic contractions of the muscular foot.
Section Check
1
Question 3
A squid is what type of mollusk?
A. bivalve
B. cephalopod
C. gastropod
D. radula
Section Check
1
Answer
The answer is B. Cephalopods have large, well
developed heads and their foot is divided into
many tentacles.
Segmented Worms
2
Segmented Worm Characteristics
• Annelids (A nuh ludz) have tube-shaped
bodies that are divided into many segments.
• On the outside of each body segment are
bristlelike structures called setae (SEE
tee).
• Segmented worms
use their setae to
hold on to the soil
and to move.
Segmented Worms
2
Segmented Worm Characteristics
• Segmented worms also have bilateral
symmetry, a body cavity that holds the
organs, and two body openings—a mouth
and an anus.
• Earthworms, marine worms, and leeches
are examples of annelids.
Segmented Worms
2
Earthworm Body Systems
• The most well-known
annelids are earthworms.
They have a definite
anterior, or front end, and
a posterior, or back end.
• Earthworms have more than 100 body
segments. The segments can be seen on the
outside and the inside of the body cavity.
• Each body segment, except for the first and
last segments, has four pairs of setae.
Segmented Worms
2
Digestion and Excretion
• As an earthworm burrows through the soil,
it takes soil into its mouth.
• The soil ingested by an earthworm moves to
the crop, which is a sac used for storage.
• Behind the crop is a muscular structure called
the gizzard, which grinds the soil and the bits
of organic matter.
Segmented Worms
2
Digestion and Excretion
• This ground material passes to the intestine,
where the organic matter is broken down
and the nutrients are absorbed by the blood.
• Wastes leave the worm through the anus.
• Their wastes pole up at the openings to their
burrows.
• These piles are called castings which help
fertilize the soil.
Segmented Worms
2
Circulation and Respiration
• Earthworms have a closed circulatory system.
• Two blood vessels meet in the front end of the
earthworm and
connect to
heartlike
structures called
aortic arches,
which pump
blood through
the body.
Segmented Worms
2
Circulation and Respiration
• Oxygen and carbon dioxides are exchanged
through their skin, which is covered with a
thin film of
watery mucus.
Segmented Worms
2
Nerve Response and Reproduction
• Earthworms have a small brain in their
front segment.
• Nerves in each segment join to form a
main nerve cord that connects to the brain.
• Earthworms respond to light, temperature,
and moisture.
Segmented Worms
2
Nerve Response and Reproduction
• Earthworms are hermaphrodites (hur MA
fruh dites)—meaning they produce sperm
and eggs in the same body.
• Even though each worm has male and female
reproductive structures, an individual worm
can’t fertilize its own eggs.
• Instead, it has to receive sperm from another
earthworm in order to reproduce.
Segmented Worms
2
Marine Worms
• More than 8,000
species of marine
worms, or
polychaetes, (PAH
lee keets) exist.
• Polchaetes, like earthworms, have segments
with setae. However, the setae occur in
bundles on these worms.
Segmented Worms
2
Marine Worms
• Sessile, bottom-dwelling polychaetes,
have specialized tentacles that are used for
exchanging oxygen and carbon dioxide
and gathering food.
• Some marine worms build tubes around their
bodies and retreat into their tubes when
startled.
Segmented Worms
2
Marine Worms
• Free-swimming polychaetes have a head
with eyes, a tail, and parapodia (per uh
POH dee uh).
• Parapodia are paired, fleshy outgrowths
which aid in feeding and locomotion.
Segmented Worms
2
Leeches
• Leeches are segmented worms, but their
bodies are not as round or as long as
earthworms are, and they don’t have setae.
• They feed on the blood of other animals.
• A sucker at each end of a leech’s body is
used to attach itself to an animal.
Segmented Worms
2
Leeches
• Leeches produce many
chemicals, including an
anesthetic (a nus THEH
tihk) that numbs the
wound so you don’t feel
its bite.
• After the leech has attached itself, it cuts into
the animal and sucks out two to ten times its
own weight in blood.
Segmented Worms
2
Leeches and Medicine
• Sometimes, leeches are used after surgery
to keep blood flowing to the repaired area.
• Besides the anti-clotting chemical, leech
saliva also contains a chemical that dilates
blood vessels, which improves the blood
flow and allows the wound to heal more
quickly.
Segmented Worms
2
Value of Segmented Worms
• Earthworms help aerate the soil by constantly
burrowing through it.
• Earthworms speed up the return of nitrogen
and other nutrients to the soil for use by
plants.
• Researchers are developing drugs based on
the chemicals that come from leeches
because leech saliva prevents blood clots.
• Marine worms and their larvae are food for
many fish, invertebrates, and mammals.
Segmented Worms
2
Origin of Segmented Worms
• Some scientists hypothesize that segmented
worms evolved in the sea.
• The fossil record for segmented worms is
limited because of their soft bodies.
• The tubes of marine worms are the most
common fossils of the segmented worms.
• Some of these fossils date back about 620
million years.
Segmented Worms
2
Origin of Segmented Worms
• Mollusks and segmented worms may have a
common ancestor.
• Their larvae are similar and are the best
evidence
that they
have a
common
ancestor.
Section Check
2
Question 1
Which letter corresponds with the setae?
A. A
B. B
C. C
D. D
Section Check
2
Answer
The answer is A. Setae are the bristlelike
structures segmented worms use to hold onto
soil and to move.
Section Check
2
Question 2
Which letter corresponds with the structures
that pump blood through the body?
A. D
B. E
C. F
D. G
Section Check
2
Answer
The answer is D. Earthworms have five aortic
arches that pump blood throughout their
bodies.
Section Check
2
Question 3
How are leeches valuable medically?
A. Chemicals in their saliva prevent blood
from clotting.
B. Leeches act as blood transfusers.
C. Leeches prevent pain from occurring in
major wounds.
D. Leeches filter and purify blood.
Section Check
2
Answer
The answer is A. Leeches are sometimes used
after surgery to keep blood flowing to the
repaired area.
Arthropods
3
Characteristics of Arthropods
• There are more than a million different
species of arthropods, (AR thruh pahdz).
• The jointed appendages of arthropods can
include legs, antennae, claws, and pincers.
• Arthropods also have bilateral symmetry,
segmented bodies, an exoskeleton, a body
cavity, a digestive system with two openings,
and a nervous system.
Arthropods
3
Characteristics of Arthropods
• Most arthropods species have separate sexes
and reproduce sexually.
• Arthropods
are adapted to
living in
almost every
environment.
Arthropods
3
Segmented Bodies
• The bodies of arthropods are divided into
segments.
• Some arthropods have
many segments, but
others have segments
that are fused together
to form body regions,
such as those of insects,
spiders, and crabs.
Arthropods
3
Exoskeletons
• All arthropods have a hard, outer covering
called an exoskeleton.
• It covers, supports, and protects the internal
body and provides places for muscles to
attach.
• In many land-dwelling arthropods, such as
insects, the exoskeleton has a waxy layer
that reduces water loss from the animal.
Arthropods
3
Exoskeletons
• An exoskeleton cannot grow as the animal
grows.
• From time to time, the exoskeleton is shed
and replaced by a new one in a process called
molting.
• While the animals are molting, they are not
well protected from predators because the
new exoskeleton is soft.
Arthropods
3
Insects
• More species of insects exist then all other
animal groups combined.
• More than 700,000 species of insects have
been classified, and scientists identify more
each year.
• Insects have three
body regions—a
head, a thorax, and
an abdomen.
Arthropods
3
Head
• An insect’s head has a pair of antennae, eyes,
and a mouth.
• The antennae are used for
touch and smell.
• The eyes are simple or
compound. Simple eyes
detect light and darkness.
• Compound eyes contain
many lenses and can detect
colors and movement.
Arthropods
3
Thorax
• Three pairs of legs and one or two pairs of
wings, if present, are attached to the thorax.
• Insects are the only
invertebrate animals
that can fly.
• Flying allows insects to
find places to live, food
sources, and mates.
• Flight also helps them escape from their
predators.
Arthropods
3
Abdomen
• The abdomen is where the reproductive
structures are found.
• Insects have an open circulatory system that
carries digested food to cells and removes
wastes.
• Insects have openings called spiracles
(SPIHR ih kulz) on the abdomen and thorax
through which air enters and waste gases
leave the insect’s body.
Arthropods
3
From Egg to Adult
• Many insects go through changes in body
form called
metamorphosis
(me tuh MOR fuh
sihs).
• Grasshoppers,
silverfish, lice, and
crickets undergo
incomplete
metamorphosis.
Arthropods
3
From Egg to Adult
• Many insects—butterflies, beetles, ants, bees,
moths, and flies—undergo complete
metamorphosis.
• The stages of
complete
metamorphosis
are egg, larva,
pupa, and adult.
Click box to view movie.
Arthropods
3
Obtaining Food
• Grasshoppers and ants have large mandibles
(MAN duh bulz) for chewing plant tissue.
• Butterflies and honeybees are equipped with
siphons for
lapping up
nectar in
flowers.
Arthropods
3
Obtaining Food
• Praying mantises eat other
animals.
• External parasites, such as
mosquitoes, fleas, and lice,
drink the blood and body
fluids of other animals.
• Silverfish eat things that
contain starch and some
moth larvae eat wool
clothing.
Arthropods
3
Insect Success
• Most insects have short life spans, so genetic
traits can change more quickly in insect
populations than in organisms that take
longer to reproduce.
• Because insects generally are small, they can
live in a wide range of environments and
avoid their enemies.
Arthropods
3
Insect Success
• Many species of insects can live in the same
area and not compete with one another for
food, because many are so specialized in
what they eat.
• Protective coloration, or camouflage, allows
insects to blend in with their surroundings.
Arthropods
3
Arachnids
• Spiders, scorpions, mites, and ticks are
examples of arachnids (uh RAK nudz).
• They have two body regions—a head-chest
region called the cephalothorax (se fuh luh
THOR aks) and an
abdomen.
Arthropods
3
Arachnids
• Arachnids have four pairs of legs but no
antennae.
• Many arachnids are adapted to kill prey with
venom glands, stingers, or fangs.
• Others are parasites.
Arthropods
3
Scorpions
• Arachnids that have a sharp, venom-filled
stinger at the end of their abdomen are called
scorpions.
• Unlike other arachnids,
scorpions have a pair of
well-developed
appendages—pincers—
with which they grab
their prey.
Arthropods
3
Spiders
• Because spiders can’t chew their food, they
release enzymes into their prey that help
digest it, then sucks it back into its mouth.
Arthropods
3
Spiders
• Oxygen and carbon
dioxide are
exchanged in book
lungs.
• Openings on the
abdomen allow
these gases to move
into and out of the
book lungs.
Arthropods
3
Mites and Ticks
• Most mites are animal or plant parasites,
but some are not like the mites that live in
the follicles of human eyelashes.
• Most mites are so small that they look like
tiny specs to the unaided eye.
• Ticks attach to their host’s skin and remove
blood through specialized mouthparts.
• Diseases carried by ticks include Lyme
disease and Rocky Mountain spotted fever.
Arthropods
3
Centipedes and Millipedes
• Two groups of arthropods—centipedes
and millipedes—have long bodies with
many segments and many legs, antennae,
and simple eyes.
• They can be found in damp environments,
including in woodpiles, under vegetation,
and in basements.
• Centipedes and millipedes reproduce
sexually. They make nests for their eggs
and stay with them until the eggs hatch.
Arthropods
3
Centipedes and Millipedes
• Centipedes hunt for their prey, which
includes snails, slugs, and worms.
• They have a pair of venomous claws that
they use to inject venom into their prey.
• Millipedes feed on
plants and decaying
material and often are
found under the damp
plant material.
Click image to view movie.
Arthropods
3
Crustaceans
• Crabs, crayfish,
shrimp, barnacles,
pill bugs, and water
fleas are crustaceans.
• Crustaceans have
one or tow pairs of antennae and mandibles,
which are used for crushing food.
• Most crustaceans live in water, but some live
in moist environments on land.
Arthropods
3
Crustaceans
• Crustaceans have five pairs of legs.
• The first pair of legs are claws that catch
and hold food.
• The other four pairs are walking legs.
• They also have five pairs of appendages
on the abdomen called swimmerets.
• If a crustacean loses an appendage, it will
grow back, or regenerate.
Arthropods
3
Value of Arthropods
• Arthropods are a source of food for many
animals, including humans.
• Bees, butterflies, moths, and flies pollinate
crops.
• Bees manufacture honey, and silkworms
produce silk.
• Many insects and spiders are predators of
harmful animal species.
Arthropods
3
Value of Arthropods
• Not all arthropods are useful to humans.
Almost every cultivated crop has some
insect pest that feeds on it.
• Many arthropods—mosquitoes, tsetse flies,
fleas, and ticks—carry human and other
animal diseases.
• In addition, weevils, cockroaches, carpenter
ants, clothes moths, termites, and carpet
beetles destroy food, clothing, and property.
Arthropods
3
Controlling Insects
• One common way to control problem insects
is by insecticides.
• However, many insecticides also kill helpful
insects.
• Another problem is that many toxic substances
that kill insects remain in the environment and
accumulate in the bodies of animals that eat
them.
Arthropods
3
Controlling Insects
• Different types of bacteria, fungi, and viruses
are being used to control some insect pests.
• Other biological controls include using sterile
males or naturally occurring chemicals that
interfere with the reproduction or behavior of
insect pests.
Arthropods
3
Origin of Arthropods
• Because of their hard body parts, arthropod
fossils are among the oldest and bestpreserved fossils of many-celled animals.
• Scientists hypothesize
that arthropods
probably evolved
from an ancestor of
segmented worms.
Section Check
3
Question 1
The word arthropoda means_______.
A. many bristles
B. head-footed
C. little rings
D. jointed foot
Section Check
3
Answer
The answer is D. Crabs are an example of an
arthropod.
Section Check
3
Question 2
Which is NOT one of the three body regions of
an insect?
A. abdomen
B. appendage
C. head
D. thorax
Section Check
3
Answer
The answer is B. Insect bodies consist of a
head, thorax, and abdomen.
Section Check
3
Question 3
Which is an arachnid?
A. butterfly
B. conch
C. earthworm
D. tick
Section Check
3
Answer
The answer is D. Arachnids have two body
regions and four pairs of legs, but no antennae.
Echinoderms
4
Echinoderm Characteristics
• Echinoderm (ih KI nuh durm) are found in
oceans all over the world.
• Echinoderms have a hard endoskeleton
covered by a thin, bumpy, or spiny epidermis.
• They are radically symmetrical, which
allows them to sense food, predators, and
other things in their environment from all
directions.
Echinoderms
4
Echinoderm Characteristics
• All echinoderms have a
mouth, stomach, and
intestines.
• They feed on a variety
of plants and animals.
• Others feed on dead and decaying matter
called detritus (de TRI tus) found on the ocean
floor.
• Echinoderms have no head or brain, but they
do have a nerve ring that surrounds the mouth.
Echinoderms
4
Water-Vascular System
• A characteristic unique to echinoderms is
their water-vascular system.
• It allows them to
move, exchange
carbon dioxide
and oxygen,
capture food,
and release
wastes.
Echinoderms
4
Water-Vascular System
• The water-vascular system is a network of
water-filled canals with thousands of tube feet
connected to it.
Echinoderms
4
Water-Vascular System
• Tube feet are hollow, thin-walled tubes that
each end in a suction cup.
• As the pressure
in the tube feet
changes, the
animal is able to
move along by
pushing out and
pulling in its tube
feet.
Echinoderms
4
Types of Echinoderms
• Approximately 6,000 species of echinoderms
are living today.
• Of those, more than one-third are sea stars.
• The arms are lined with thousands of tube feet.
• Sea stars use their tube feet to open the shells
of their prey.
Echinoderms
4
Types of Echinoderms
• Sea stars reproduce sexually when females
release eggs and males release sperm into
the water.
• Females can produce
millions of eggs in one
season.
• Sea stars also can repair
themselves by regeneration.
• If a sea star loses an arm, it can grow a new
one.
Echinoderms
4
Brittle Stars
• Brittle stars have fragile, slender, branched
arms that break off easily.
• This adaption helps a brittle star survive
attacks by predators.
• Brittle stars quickly regenerate lost parts.
• They live hidden under rocks or in litter on
the ocean floor.
• Brittle stars use their flexible arms for
movement instead of their tube feet.
Echinoderms
4
Sea Urchins and Sand Dollars
• Sea urchins, sea biscuits, and sand dollars
are disk- or globe-shaped echinoderms
covered with spines.
• Some sea urchins have sacs
near the end of the spines that
contain toxic fluid that is
injected into predators.
• The spines also help in
movement and burrowing.
Echinoderms
4
Sea Cucumbers
• Sea cucumbers are soft-bodied echinoderms
that have a leathery covering.
• They have
tentacles around
their mouth and
rows of tube feet
on their upper and
lower surfaces.
Echinoderms
4
Value of Echinoderms
• Echinoderms are important to the marine
environment because they feed on dead
organisms and help recycle materials.
• Sea urchins control the growth of algae in
coastal areas.
Echinoderms
4
Value of Echinoderms
• Sea urchin eggs and sea cucumbers are used
for food in some places.
• Many echinoderms are used in research and
some might be possible sources of medicines.
• Sea stars are important predators that control
populations of other animals.
Echinoderms
4
Origin of Echinoderms
• A good fossil record exists for echinoderms.
• Echinoderms date back more than 400
million years.
• The earliest echinoderms might have had
bilateral symmetry as adults and may have
been attached to the ocean floor by stalks.
Echinoderms
4
Origin of Echinoderms
• Scientists hypothesize that echinoderms more
closely resemble animals with backbones than
any other group of invertebrates.
• This is because echinoderms have complex
body systems and an embryo that develops
the same way that the embryos of animals
with backbones develop.
Section Check
4
Question 1
In echinoderms, the _______ is a network of
water-filled canals with thousands of tube feet
connected to it which allows for movement,
exchange of carbon-dioxide and oxygen,
capture of food and release of waste.
Section Check
4
Answer
The answer is water-vascular system. The
water-vascular system is unique to
echinoderms.
Section Check
4
Question 2
Which letter corresponds with the tube feet?
Section Check
4
Answer
The letter C represents the tube feet. Tube feet
are hollow, thin-walled tubes that each end in
suction cup. They allow sea stars to move.
Section Check
4
Question 3
Which is a survival tactic of an echinoderm?
A. arms break off easily
B. jet propulsion
C. pinchers
D. venom
Section Check
4
Answer
The answer is A. Brittle stars have arms that
break off easily if they are grabbed by a
predator, allowing for escape. The lost parts
regenerate quickly.
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