Platyhelminthes
By: Thomas Sullivan
Platyhelminthes
Flatworms:
 Flatworms are only a few
millimeters thick.
 Flatworms are soft, flat, have
tissues and internal organ
systems.
 They are the simplest
animals to have 3 embryonic
germ layers, bilateral
symmetry, and cephalization.
 The three embryonic germ
layers include ectoderm,
endoderm, and mesoderm.
Facts on Flatworms
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All Flatworms:
Have Acoelomate, which means without coelom.
A coelom is a fluid filled body cavity that is lined with mesoderm.
Have only one body cavity, which is the digestive cavity is lined with
endoderm.
 Have Bilateral symmetry, which means that flatworms has 2 wellformed sides that can be identified as left and right.
 The cells of a flatworm are close to the external environment that allows
materials to pass easily into and out of their bodies.
 Have Organ systems for digestion, excretion, response, and
reproduction.
 Have Reliance on diffusion for respiration and circulation.
How & What Flatworms Eat
 Free-living flatworms can be carnivores
that feed off of tiny aquatic animals or
they can be scavengers that feed off of
recently dead animals.
 Flatworms have a gastrovascular cavity
with a single opening (a mouth) where
food and waste can pass.
 A muscular tube called the pharynx near
the mouth, is extended out of the mouth,
then pumps food inside.
 The food is digested and is diffused into
all other body tissues from the
gastrovascular cavity.
A. gastrovascular
cavity
B. gastrodermis
C. mesoglea
D. nematocysts
E. epidermis
Respiration, Circulation, and
Excretion
 A circulatory system is not needed by many flatworms
because of their size.
 Instead, they rely on diffusion to transport oxygen and
nutrients to their internal tissues, and to remove carbon
dioxide and other wastes.
 Flatworms have no gills and no heart, blood vessels, or
blood.
 Some flatworms have flame cells that function in excretion.
 They are cells that filter and remove excess water, ammonia
and urea from body.
Response
 More Complex than cnidarians or
sponges.
 Ganglia; Groups of nerve cells that
control the the nervous system.
 Two long cords run down sides of
body and shorter nerves run across.
 Eyespot; detects light change.
 Flatworms have specialized cells that
detect stimuli.
 The nervous system locates food.
Movement
 Flatworms move in two way.
 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 so that they can
react quickly to environmental
stimuli.
A. ciliated
epidermis
Reproduction
 Flatworms are hermaphrodites that
reproduce sexually.
 Hermaphrodite is an individual that has
both male and female reproductive organs.
 During a sexual reproduction, two worms
deliver sperm to each other.
 Eggs are laid in clusters and hatch within a
few weeks.
 Asexual reproduction takes place by
fission. An organism splits in two and
grows new parts.
 Broken pieces form new worms.
Groups of Flatworms
 Flatworms are enormously diverse group with many
different worms. The three main groups are turbellarians,
flukes, and tapeworms.
 Tubellarians- Member of class Turbellaria Tubellarians are
Free-living flatworms, live in marine or fresh water.
 Flukes- Member of class Trematoda
 Flukes are parasitic flatworms. Most flukes infect the
internal organs of their host.
 Tapeworms- Member of class Cestoda
 Tapeworms are long, flat, parasitic worms that are adapted
to life inside the intestines of their hosts.