Bilateral symmetry
 Exhibits cephalization
 Have tissue and organ
systems
 The simplest organism
with 3 germ layers
 “acoelomates”

Acoelomates
are animals
without a
coelum – a
fluid-filled body
cavity between
the endoderm
and the
ectoderm.
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Digestion begins in the gastrovascular cavity.
Smaller particles are absorbed by the intestinal
walls where digestion is completed.
The gastrovascular cavity branches throughout
the body, so nutrients are absorbed directly
from the intestinal cells
Wastes are expelled through the mouth
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Free-living, aquatic
flatworms eat
plankton or recently
dead animals
They use the
pharynx to suck the
food into the
gastrovascular cavity
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Oxygen diffuses directly into their flat bodies
Carbon dioxide and metabolic wastes diffuse
out directly

Fresh-water
flatworms have
flame cells to get
rid of excess water
(as contractile
vacuoles did in
amoeba)
No brain but several
groups of nerve cells
called ganglia in their
“head”
 Two ventral nerve
cords run down sides
of body

Free-living
flatworms have
ocelli to detect
light
 Chemoreceptors
test water for food
 Motion detectors
are scattered
around the body

Parasitic flatworms
do not have
developed nervous
systems
 Do they need
them?

Asexual – by fission
 Common among
free-living flatworms
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Most free-living flatworms are
hermaphrodites
The worms join and exchange sperm
Eggs are laid in clusters and hatch in a few
weeks
Most flatworms are
able to regenerate
structures if necessary
 One worm breaks into
small pieces as a form
of reproduction
 Each piece creates a
new worm
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Most familiar free-living flatworm
Parasitic flatworm
 Have two different
hosts in their life cycle
 Found in tropical areas
 E.g. Schistosoma
mansoni causes
schistomiasis which
affects millions of
people worldwide.
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Most flukes are
hermaphroditic
Produce huge
numbers of eggs
Cause the intestinal
cells to burst, leaking
blood and eggs into
the intestine
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The male is about
6-10 mm
Female is longer
and thinner and
lives in the groove
which runs the
length of the male’s
body
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Eggs are passed out with the feces
Where there are poor sewer systems, the eggs
get into the water system
Swimming larvae hatch from the eggs

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Swimming larvae find
their intermediate
host – a snail
They reproduce
asexually in the snail
and digest it’s tissues
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The new worms swim until they find a
human
They bore into the skin, eat their way to
the blood vessels and travel to the
intestines, making the human very ill
Blood flukes often kill their human host
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Have a scolex with
several suckers and a
ring of hooks
Attach to intestinal
walls
Absorb nutrients
through their body
walls
Can be up to 18
meters long
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Constantly produce
proglottids
Contain male and
female reproductive
organs
Proglottids rupture full
of eggs, or break off
Eggs and proglottids
are passed out with the
feces
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Food or water contaminated with eggs is
ingested by an intermediate host (cow, pig,
fish)
Larvae from eggs burrow into muscle tissue of
host and create a cyst

Eating uncooked meat containing the cysts
will begin the cycle again