Phylum Platyhelminthes
T H E F L AT W O R M S
Body Plans
 Up until now, we have talked about animals with diploblastic
body plans. What does it mean to be diploblastic?
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It means that you have two major cell layers: the endoderm (inside layer)
and ectoderm (outside layer).
 What two animal Phyla have we studied that are diploblastic?
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Phylum Porifera (sponges) and Phylum Cnidaria (jellyfish, coral, and their
relatives)
 However, from here on out, we will be exploring the
triploblastic world. What does it mean to be triploblastic?
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It means that you have three major cell layers: the endoderm, the
ectoderm, and the mesoderm (middle layer). What does YOUR mesoderm
consist of?
Body Plans Continued
 We will first start off with triploblasts that DO NOT have a
coelem (body cavity). Remember that these animals are
called Acoelemates. The prefix a- means NOT or NO, so
“acoelemates” means no coelem (no body cavity).
Why are flatworms special?
 What makes these guys different from Phylum Porifera
and Phylum Cnidaria?
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Phylum Platyhelminthes have bilateral symmetry instead of
asymmetry or radial symmetry.
Phylum Platyhelminthes have a mesoderm which allows them to
develop true muscles. Phylum Porifera and Phylum Cnidaria did not
have a mesoderm and thus did not have true muscles.
Phylum Platyhelminthes has a true nervous system, meaning a brain
with attached nerve cords. Neither Phylum Porifera nor
Phylum Cnidaria had a brain.
Where do they live?
 Flatworms can live just about anywhere. There are terrestrial
flatworms (meaning they live on the ground or underground).
There are freshwater flatworms (meaning they live in streams
and ponds). There are marine flatworms (meaning they live in
the oceans). And there are parasitic flatworms. Where do
you think parasitic flatworms live?
 If they are free living (meaning NOT a parasite), they fall into
the class called Turbellaria.
 If they are parasitic, they fall into either the class called
Trematoda (flukes) or the class called Cestoidea (tapeworms).
 Let’s start with Class Turbellaria. We’ll get to the other two
classes in due time.
Class: Turbellaria
What and How do they eat?
 They can be either predators, scavengers,
or herbivores.
 Believe it or not, a flatworm’s mouth is
NOT near his head, but actually in the middle
of his body on the ventral side.
 Inside the mouth opening is a tongue-like
organ called the pharynx. The flatworm
finds food, then secretes enzymes from the
pharynx onto the food, which break it down
into pieces small enough for the worm to eat.
It would be the equivalent of you cutting
your food into bite-size pieces that actually
fit in your mouth.
Exchanges with the Environment
 Even though flatworms have
certain organs, they do NOT
have lungs or a bladder. So
how do they breathe and how
do they get rid of wastes?
 DIFFUSION
 In the case of removing wastes,
specialized cells called flame
cells are responsible for the
diffusion of wastes and
maintaining homeostasis.
Nervous System and Sense Organs
 Flatworms are the first animals to have a true nervous system,
meaning that they have a brain and have 3 distinct types of
nerve chords.
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Sensory nerves respond to an external stimulus and send a signal to the
brain.
Motor nerves receive a signal from the brain and route it to its destination
in the body.
Association nerves serve as connections between sensory and motor
nerves.
 Sense Organs
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Flatworms have two eyespots on the top of their head that detect light.
Most flatworms will move AWAY from light.
They also have auricles on the side of their head. The auricles look like
what we would call ears, but actually function more like two tiny noses,
and are used to search out food.
Turbellarian Reproduction
 Like the sponges we studied before, turbellarians are
monoecious. What does it mean to be monoecious?
 Turbellarians can be reproduce sexually or asexually if a
partner is not available.
 Asexual reproduction occurs through a process called
transverse fission. This is where the worm can separate
itself into two halves and each half will become a new
worm.
Class Trematoda: Flukes
 Most flukes are flat and oval.
 They feed on host cells. Almost all adult flukes are
parasites of vertebrates, while immature flukes can be
parasites of vertebrates or invertebrates.
 Have at least two different forms: an adult and one or
more larval stages.
Life Cycle of a Trematode
 1. An egg hatches in freshwater and a ciliated larval form called a miracidium swims
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out.
2. The miracidium swims until it finds a good intermediate host, usually a snail. Once
inside the snail, it develops into its next larval form, a sporocyst. This is where asexual
reproduction occurs.
3. The asexual stage results in the next larval form, the cercaria. A cercaria has a
digestive tract, suckers, and a tail. Cercaria leave the snail and swim freely until they
find the second host. This host could be an invertebrate, a vertebrate, or even a
plant.
4. The final host eats the invertebrate, vertebrate, or plant infested with the cercaria.
Once inside the final host, usually a vertebrate, the cercaria develops into a juvenile.
5. The juveniles mature into adults inside the final host. As adults, they are able to
engage in sexual reproduction and produce lots of fertilized eggs. The eggs then get
pooped out by the host and end up in the water supply ready to begin the cycle again.
Life Cycle of a Typical Fluke
Class Cestoidea Characteristics: Tapeworms
 Tapeworms almost always parasitize the
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digestive tract.
Can be anywhere from 1mm to 25m in length.
There have been rare cases where tapeworms
have been even longer than 25m.
Tapeworms lack a digestive system of their own.
Why might this be?
Adult tapeworms consist of a long series of
repeating units called proglottids. Each
proglottid contains its own complete set of
reproductive structures.
Instead of a true mouth, tapeworms have a
sucker-like organ called a scolex, which it uses
to attach itself to the digestive tract of its host.
Cestoidea Reproduction
 Tapeworms are monoecious. What does this mean
again?
 In tapeworms however, if no appropriate mate is present,
they CAN self-fertilize!
Where would we find a tapeworm?
 In any vertebrate animal, but most commonly in
 Beef
 Pork
 Fish
 Lamb
 Dogs
 From all these sources, tapeworms find their way into US!!!!
Tapeworm Life Cycle
How Do I Know If I Have a Tapeworm or other
Infectious Worm?
 If you live in North America or Eastern Europe, you probably
don’t have an infectious or parasitic worm. Why not?
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We wear shoes when we go outside.
Our meat supply is raised under stringent standards and inspected
before it reaches our table, AND we COOK our meat, which kills any
unnoticed eggs.
Our pets are usually not raised on a farm with access to infected
vegetation, AND are usually medicated and/or vaccinated for
worms. (One exception-- newborn animals.)
During winter months, parasitic and infectious worms cannot
survive here outside of a host, it’s just too cold.
But…What If????
 On the off chance that you do become infected, 9 times out of
ten, you won’t know! Most people infected with a tapeworm
show no signs or symptoms until the tapeworm reaches
sufficient size to cause an intestinal blockage. In this case, it
needs to be removed surgically.
 Sometimes, people experience nausea, vomiting, diarrhea,
anemia due to B12 deficiency, and weight loss. And if it’s a
fluke, you will experience the same types of symptoms.
 In even rarer cases, pieces of a tapeworm can break off and
infect other areas of your body, such as your liver, heart, and
even your brain. But again, this is EXTREMELY rare!
Removal