Phylum Platyhelminthes
The “flat” “worms”
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4 Classes of Phylum Platyhelminthes
TREMATODA – flukes
MONOGENEA – small,
parasitic flatworms
CESTODA – tapeworms
TURBELLARIA - small, freeliving flatworms
Identifying Characteristics of the
Phylum Platyhelminthes
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Acoelomate
Exhibit bilateral symmetry
Have a bilateral nervous system with cephalization
at the head end. Some species exhibit eyespots
sensitive to light
Possess a Gastrovascular Cavity (GVC) and
primitive organ systems for digestion and excretion
Do not have a circulatory or respiratory system but
do take in O2 through their body surface
(integumentary exchange)
Hermaphroditic : can reproduce sexually (do not self
fertilize) or asexually by regeneration
Are motile and utilize an undulation form of motion
Found in marine, freshwater and damp terrestrial
habitats
Free-living Flatworms
Planaria : Dugesia tigrina
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Freshwater, free-living flatworm
Moves by beating cilia and gliding
on a film of mucus
3-12mm in size
Have a single opening to their
stomachs in the middle of their
bodies
Possesses 2 eyespots (ocelli) that
are sensitive to light
Carnivorous (eat daphnia and
midges)
Common to most parts of the
world
Reproduce by asexual
reproduction and capable of
regeneration (see next slide)
Planaria Regeneration
Planarians will spontaneously detach the tail end of their
bodies and each half will regenerate into a full size
flatworm
 Planarians can be cut either transversally (shown above)
or dorsally and most will regenerate into a full size worm
Super Cool Fact: the smallest piece of planarian to ever
regenerate in a lab into a new planarian was 1/279th of a
planarian! That’s approximately 10,000 cells!
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Parasitic Flatworms
The pork tapeworm – Taenia solium
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Infects pigs and humans
Lives in the intestine of its host
and passes eggs through the feces
Highly adapted to constant
internal environments
Lacks sensory organs, coordination
for mobility and a digestive system
(more room for reproductive
structures!)
Have a modified epidermis
“tegument” which protects against
the digestive enzymes and the
immune systems of the host
Can reach 7m in length in humans!
Are flat and long which maximizes
absorption of nutrients from the
host
The tapeworm “up close and personal….”
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The head end has a
scolex with four suckers
(two seen here) that help
it attach to the intestine
of its host
The body is separated
into sections called
“proglottids” that house
highly developed
reproductive systems
(darkened areas)
capable of producing
hundreds of thousands
of eggs and sperm
Life cycle of the Pork Tapeworm
Super cool Flatworm Fact
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the largest tapeworm ever reported was in
a sperm whale and was 30 meters in
length!
Tapeworm
Scolex