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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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4. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. 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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13. 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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14. 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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20. 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
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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