VS.
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Flatworms demonstrate a bilaterally
symmetrical body plan
They are dorsoventrally flattened and
lack a skeleton
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They have a highly branched gastrovascular
cavity .
There is only one opening which serves the
function of both the mouth and anus (not
excretion)
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They have three germ layers:
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Ectoderm: Outside
Endoderm: Inside
Mesoderm: Middle layer of tissue between the
ectoderm and the endoderm
Though Flatworms have three germ
layers they are acoelomates and have no
body cavity
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Flatworms do have true organs and primitive
organ systems that are used for digestion and
excretion
• Flatworms are also the most primitive
organisms to show cephalization.
• The head region of Flatworms contains
a concentration of nerve tissue called
ganglia (singular=ganglion) that
resembles a primitive brain.
Cephalization
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They have two nerve cords that run from
the ganglia in the head region along the
ventral side of the worm to the tale
region
In the head region of Flatworms are two
eye-spots
They also have lobes on the side of their
head called auricles
Auricles
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Flatworms have a mouth/anus which is
connected to the gastrovascular system
through a long muscular pharynx.
Small invertebrates or the remains of
dead animals are taken into the
mouth/anus by the muscular pharynx
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The food is then digested in the highly
branched gastrovascular cavity
The nutrients moves from the gut into the body
cells by diffusion
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Respiration occurs by diffusion
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Nutrients and oxygen in the gut are simply
absorbed into the body cells by the process of
diffusion
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Most undigested food is released directly
out of the anus/mouth: Pharynx
A flame cell is a specialized excretory cell
Flame cells function like a kidney, removing
waste materials.
The beating of cilia resembles a flame, giving
the cell its name.
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Flatworms are able to sense and respond to at least
three forms of stimuli:
 Sense and respond to light: The eye-spots can detect
light and allow the Flatworms to respond to it
 Sense and respond to chemicals: Pits on the side of
their head regions can sense chemicals in the water
and allow the Flatworms to respond (like
“smelling”)
 Sense and respond to touch: The auricles on either
side of the head region can sense touch and allow the
Flatworms to respond
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The ganglia in the head region relay messages
from the sensory organs down the nerve cords
to the rest of the body. The nerve cords can
control muscles in the body which allow the
Flatworms to move or eat.
Auricles
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The flatworms move across a surface
using cilia on their ventral surface
They can also move by contracting
circular and longitudinal muscles that
lay just below the ectoderm. These
muscles are controlled by the nerve
cords.

Asexual Reproduction:
 Flatworms can asexually reproduce through
a process called fission. The anterior and
posterior ends hold a surface and the
midsection constricts. This results in two
new flatworms, one from the anterior end of
the original flatworm and the other from the
posterior end of the original flatworm.
• Flatworms can also regenerate parts
they have lost.
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Sexual Reproduction:
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Flatworms are hermaphroditic
After two flatworms have copulated they release
sacs of fertilized eggs and attach them to a surface
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The tapeworm has an anterior end called
a scolex with complicated hooks for
attaching to the intestines of its host.
The tapeworm does not have a mouth or
digestive system. Instead they bath in
the pre-digested fluids of their host and
absorb nutrients directly into their body
cells
Hooks
Scolex
Suckers
Proglottid
s
Phylum Platyhelminthes
The “flat” “worms”
www.onacd.ca
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 two 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
•
the largest tapeworm ever reported was in
a sperm whale and was 30 meters in
length!
Tapeworm
Scolex