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Biology
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27–1 Flatworms
Photo Credit: ©Carolina Biological Supply Company/Phototake
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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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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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What Is a Flatworm?
Three germ layers of a flatworm
Flatworms are the simplest animals to have three embryonic germ layers—ectoderm, endoderm, and mesoderm.
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6. Form and Function in Flatworms
All flatworms rely on diffusion for respiration, excretion, and circulation.
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7. 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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8. Form and Function in Flatworms
Eyespot
Head
Digestive Structures of a Planarian
Digestive cavity
Mouth
Pharynx
All flatworms, including this planarian, have organ systems that perform essential life functions. The digestive cavity is branched throughout the body and opens to the outside through the pharynx. Photo Credit: ©Carolina Biological Supply Company/Phototake
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9. Form and Function in Flatworms
Ganglia
Nerve cords
Excretory, Nervous, and Reproductive Structures of a Planarian
Excretory system
Ovary
Testes
All flatworms, including this planarian, have organ systems that perform essential life functions. The excretory system (in purple) consists of many flame cells (in red) that maintain water balance and may remove waste. The nervous system (in dark gray) consists of ganglia and two nerve cords that run the length of the body. The reproductive system (in green) has testes and ovaries, or male and female reproductive organs, along both sides of the body.
Flame cell
Excretory tubule
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10. 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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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 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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14. Form and Function in Flatworms
Life Cycle of a Blood Fluke
Flukes usually infect the internal organs of their host. The life cycle of the blood fluke Schistosoma mansoni involves two hosts: humans and snails.
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15. White Tail Deer Liver Fluke
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16. 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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17. Form and Function in Flatworms
Scolex
Structures of a Tapeworm
Young proglottids
Mature proglottids
Tapeworms are parasitic flatworms that live in the intestines of their host. A tapeworm attaches to the host using hooks or suckers on its scolex. A single tapeworm is made of many proglottids. The youngest proglottids are at the anterior (head) end, and the largest and most mature proglottids are at the posterior (tail) end. After eggs have been fertilized, proglottids break off and release zygotes that are then passed out of the host in feces.
Uterus
Zygotes
Testes
Ovary
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18. 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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27–1
Flatworms are the simplest animals to have
two germ layers.
bilateral symmetry.
radial symmetry.
two openings in the digestive system.
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27–1
An individual that has both male and female reproductive organs is known as a
turbellarian.
proglottid.
hermaphrodite.
parasite.
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27–1
The function of flame cells in flatworms is to
digest food and move it to various parts of the body.
detect the presence of chemicals in the surroundings.
remove excess water and metabolic wastes
move reproductive cells into position for fertilization.
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27–1
A flatworm that lacks a digestive tract is the
planarian.
free-living flatworm.
tapeworm.
fluke.
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27–1
Turbellarians differ from most other flatworms because they
live freely on land.
live freely in fresh and salt water.
are marine parasites.
are land-dwelling parasites.
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