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
What Is a Flatworm?
Flatworms are acoelomates, which means they have
no coelom.
A coelom is a fluid-filled body cavity that is lined with
tissue derived from mesoderm.
The digestive cavity is the only body cavity in a
flatworm.
Flatworms have bilateral symmetry.
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27–1 Flatworms
What Is a Flatworm?
Three germ layers of a flatworm
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27–1 Flatworms
Form and Function in Flatworms
Form and Function in Flatworms
All flatworms rely on diffusion for respiration,
excretion, and circulation.
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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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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
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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27–1 Flatworms
Groups of Flatworms
Turbellarians
Turbellarians are free-living flatworms.
Most live in marine or fresh water.
Most species live in the sand or mud under
stones and shells.
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27–1 Flatworms
Groups of Flatworms
Flukes
Flukes are parasitic flatworms. Most
flukes infect the internal organs of their
host.
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27–1 Flatworms
Form and Function in Flatworms
Life Cycle of a Blood Fluke
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27–1 Flatworms
White Tail Deer Liver Fluke
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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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Form and Function in Flatworms
Scolex
Structures of a Tapeworm
Young
proglottids
Mature
proglottids
Uterus
Zygotes
Testes
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Ovary
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Form and Function in Flatworms
Proglottids are the segments that make up most of
the worm's body.
Mature proglottids contain both male and female
reproductive organs.
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27–1
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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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