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Biology
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27–1 Flatworms
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27–1 Flatworms
What Is a Flatworm?
What Is a Flatworm?
Flatworms are soft, flattened worms
that have tissues and internal organ
systems.
They are the simplest animals to have
three embryonic germ layers, bilateral
symmetry, and cephalization.
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27–1 Flatworms
Form and Function in Flatworms
Feeding
Flatworms have a digestive cavity with a single
opening through which both food and wastes pass.
Near the mouth is a muscular tube called a
pharynx.
Flatworms extend the pharynx out of the mouth.
The pharynx then pumps food into the digestive
cavity.
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Form and Function in Flatworms
Most parasitic worms do not need a complex
digestive system.
They obtain nutrients from foods that have already
been digested by their host.
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27–1 Flatworms
Form and Function in Flatworms
Respiration, Circulation, and Excretion
Flatworms do not need a circulatory system to
transport materials.
Flatworms rely on diffusion to
• transport oxygen and nutrients to their internal
tissues, and
• to remove carbon dioxide and other wastes
from their bodies.
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Form and Function in Flatworms
Flatworms have no gills or respiratory organs, heart,
blood vessels, or blood.
Some flatworms have flame cells which are
specialized cells that remove excess water from the
body.
Flame cells may filter and remove metabolic wastes.
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27–1 Flatworms
Form and Function in Flatworms
Response
In free-living flatworms, a head encloses ganglia,
or groups of nerve cells, that control the nervous
system.
Two long nerve cords run from the ganglia along
both sides of the body.
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Form and Function in Flatworms
Many free-living flatworms have eyespots.
Eyespots are groups of cells that can detect
changes in light.
Most flatworms have specialized cells that detect
external stimuli.
The nervous systems of free-living flatworms allow
them to gather information from their environment.
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27–1 Flatworms
Form and Function in Flatworms
Eyespot
Head
Digestive Structures
of a Planarian
Digestive
cavity
Mouth
Pharynx
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27–1 Flatworms
Form and Function in Flatworms
Ganglia
Nerve
cords
Excretory,
Nervous, and
Reproductive
Structures
of a Planarian
Excretory
system
Ovary
Testes
Flame cell
Excretory tubule
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Form and Function in Flatworms
Movement
Free-living flatworms move in two ways.
Cilia on their epidermal cells help them glide
through the water and over the bottom of a stream
or pond.
Muscle cells controlled by the nervous system
allow them to twist and turn.
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27–1 Flatworms
Form and Function in Flatworms
Reproduction
Most free-living flatworms are hermaphrodites that
reproduce sexually.
A hermaphrodite is an individual that has both male and
female reproductive organs.
Two worms join in a pair and deliver sperm to each other.
The eggs are laid in clusters and hatch within a few weeks.
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Form and Function in Flatworms
Asexual reproduction takes place by fission, in which
an organism splits in two.
Each half grows new parts to become a complete
organism.
Parasitic flatworms often have complex life cycles
that involve both sexual and asexual reproduction.
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Groups of Flatworms
Groups of Flatworms
The three main groups of flatworms are
• turbellarians
• flukes
• tapeworms
Most turbellarians are free-living.
Most other flatworm species are parasites.
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Groups of Flatworms
Turbellarians
Turbellarians are free-living carnivore
flatworms. Most live in marine or fresh
water.
Most species live in the sand or mud under
stones and shells.
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Groups of Flatworms
Flukes
Flukes are parasitic flatworms. Most
flukes infect the internal organs of their
host.
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Form and Function in Flatworms
Tapeworms
Tapeworms are long, flat, parasitic
worms that are adapted to life inside the
intestines of their hosts.
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27–1
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27–1
Flatworms are the simplest animals to have
a. two germ layers.
b. bilateral symmetry.
c. radial symmetry.
d. two openings in the digestive system.
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An individual that has both male and female
reproductive organs is known as a
a. turbellarian.
b. proglottid.
c. hermaphrodite.
d. parasite.
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The function of flame cells in flatworms is to
a. digest food and move it to various parts of
the body.
b. detect the presence of chemicals in the
surroundings.
c. remove excess water and metabolic wastes
d. move reproductive cells into position for
fertilization.
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A flatworm that lacks a digestive tract is the
a. planarian.
b. free-living flatworm.
c. tapeworm.
d. fluke.
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Turbellarians differ from most other flatworms
because they
a. live freely on land.
b. live freely in fresh and salt water.
c. are marine parasites.
d. are land-dwelling parasites.
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