Chapter 36: Flatworms,
Roundworms, and Rotifers
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Objectives:
State the distinguishing characteristics of flatworms
Describe the anatomy of a planarian
Compare and contrast free-living and parasitic flatworms
Diagram the life cycle of a fluke
Describe the life cycle of a tapeworm
Describe the body plan of a pseudocoelomate
Explain the relationship between humans and three types of parasitic
roundworms
Describe the anatomy of a rotifer
36-1: Platyhelminthes
• Members of the phylum Platyhelminthes are
called flatworms.
• Their bodies develop from three germ layers and
are more complex than those of sponges,
cnidarians, and ctenophores.
• Flatworms have Bilateral symmetry, with dorsal
and ventral surfaces, right and left sides, and
anterior and posterior ends.
What Is a Flatworm?
• Flatworms are soft, flattened worms
that have tissues and internal organ
systems.
• They are the simplest animals to have
three embryonic germ layers, bilateral
symmetry, and cephalization.
What Is a Flatworm?
•Flatworms are acoelomates, which means
they have no coelom.
•A coelom is a fluid-filled body cavity that is
lined with tissue derived from mesoderm.
•The digestive cavity is the only body cavity in
a flatworm.
•Flatworms have bilateral symmetry.
What Is a Flatworm?
•Three germ layers of a flatworm
Form and Function in Flatworms
• Flatworms are thin and most of their cells
are close to the external environment.
• All flatworms rely on diffusion for
respiration, excretion, and circulation.
Form and Function in Flatworms
•Free-living flatworms have organ systems
for digestion, excretion, response, and
reproduction.
•Parasitic species are typically simpler in
structure than free-living flatworms.
Form and Function in Flatworms
•Feeding
– Flatworms have a digestive cavity with a
single opening through which both food and
wastes pass.
– Near the mouth is a muscular tube called a
pharynx.
– Flatworms extend the pharynx out of the
mouth. The pharynx then pumps food into the
digestive cavity.
Form and Function in Flatworms
•Most parasitic worms do not need a
complex digestive system.
•They obtain nutrients from foods that have
already been digested by their host.
Form and Function in Flatworms
•Respiration, Circulation, and Excretion
– Flatworms do not need a circulatory system to
transport materials.
– Flatworms rely on diffusion to
• transport oxygen and nutrients to their internal
tissues, and
• to remove carbon dioxide and other wastes from
their bodies.
Form and Function in Flatworms
•Flatworms have no gills or respiratory
organs, heart, blood vessels, or blood.
•Some flatworms have flame cells which
are specialized cells that remove excess
water from the body.
•Flame cells may filter and remove
metabolic wastes.
Form and Function in Flatworms
•Response
– In free-living flatworms, a head encloses
ganglia, or groups of nerve cells, that control
the nervous system.
– Two long nerve cords run from the ganglia
along both sides of the body.
Form and Function in Flatworms
•Many free-living flatworms have eyespots.
•Eyespots are groups of cells that can
detect changes in light.
•Most flatworms have specialized cells that
detect external stimuli.
•The nervous systems of free-living
flatworms allow them to gather information
from their environment.
Form and Function in Flatworms
•Digestive Structures
of a Planarian
•Excretory, Nervous,
and Reproductive
Structures
of a Planarian
Ganglia
Nerve
cords
Excretory system
Ovary
Testes
Flame cell
Excretory tubule
Form and Function in Flatworms
•Movement
– Free-living flatworms move in two ways.
– 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.
Form and Function in Flatworms
•Reproduction
– Most free-living flatworms are hermaphrodites
that reproduce sexually.
– A hermaphrodite is an individual that has both
male and female reproductive organs.
– Two worms join in a pair and deliver sperm to
each other.
– The eggs are laid in clusters and hatch within a
few weeks.
Form and Function in Flatworms
• Asexual reproduction takes place by fission,
in which an organism splits in two.
• Each half grows new parts to become a
complete organism.
• Parasitic flatworms often have complex life
cycles that involve both sexual and asexual
reproduction.
Classes of Flatworms
•What are the characteristics of the three
Classes of flatworms?
Classes of Flatworms
• The three main groups of flatworms are
– turbellarians
– Trematoda- flukes
– Cestoda- tapeworms
• Most turbellarians are free-living.
• Most other flatworm species are parasites.
Classes of Flatworms
•Class Turbellarians
– Turbellarians are free-living flatworms. Most
live in marine or fresh water.
– Most species live in the sand or mud under
stones and shells.
Classes of Flatworms
•Class Trematoda- Flukes
– Flukes are parasitic flatworms. Most flukes
infect the internal organs of their host. Usually
don’t kill the host.
– Host are typically Animals and humans
– No special sense organs
– Mostly aquatic
– Most less than 1cm long
– Schistosoma- Genus of flukes that cause
Schistosomiasis.
Form and Function in Flatworms
• Flukes can infect the blood or organs of the
host.
• Some flukes are external parasites.
• In the typical life cycle of parasitic flukes, the
fluke lives in multiple hosts.
Form and Function in Flatworms
•Life Cycle of a Blood Fluke
Form and Function in Flatworms
•A blood fluke’s
primary host is a
human.
•Blood flukes infect
humans by burrowing
through the skin.
Human
intestine
Tailed
larva
Form and Function in Flatworms
•Once inside the
human, they are
carried to the blood
vessels of the
intestines.
•In the intestines the
flukes mature and
reproduce.
•Embryos are released
and are passed out of
the body with feces.
Adult
fluke
Embryo
Form and Function in Flatworms
•If the embryos reach
water, they develop
into swimming larvae
that infect a snail (the
intermediate host).
•An intermediate host
is an organism in
which a parasite
reproduces asexually.
Embryo
Ciliated
larva
Life Cycle of a Blood Fluke
Form and Function in Flatworms
•Larvae that result from
asexual reproduction are
released from the snail
into the water to begin
the cycle again.
Life Cycle of a Blood Fluke
Form and Function in Flatworms
•Class Cestoda- Tapeworms
– Tapeworms are long, flat, parasitic worms that
are adapted to life inside the intestines of their
hosts.
– No organs for locomotion, senses, or digestion.
They absorb from the hosts digestive system.
– Can be up to 40 feet long.
Form and Function in Flatworms
• Tapeworms have no digestive tract and
absorb digested food directly through their
body walls.
• The head of an adult tapeworm, called a
scolex, is a structure that can contain suckers
or hooks.
• The tapeworm uses its scolex to attach to the
intestinal wall of its host.
Form and Function in Flatworms
•Structures of a Tapeworm
Form and Function in Flatworms
• Proglottids are the segments that make up
most of the worm's body.
• Mature proglottids contain both male and
female reproductive organs.
• Sperm produced by the testes (male
reproductive organs), can fertilize eggs of
other tapeworms or of the same individual.
Form and Function in Flatworms
• After the eggs are fertilized, the proglottids
break off and burst to release the zygotes.
• The zygotes are passed out of the host in
feces.
• The eggs ingested by an intermediate host
hatch and grow into larvae.
• Larvae burrow into the intermediate host’s
muscle tissue.
Form and Function in Flatworms
• Larvae form a dormant protective stage called
a cyst.
• If a human eats incompletely cooked meat
containing these cysts, the larvae become
active and grow into adult worms within the
human’s intestines, beginning the cycle again.
36-2: Nematoda and Rotifera
• Members of the Phyla Nematoda and Rotifera
have bilateral symmetry and contain a fluid-filled
space.
• This space holds the internal organs and serves as
a storage area for eggs and sperm.
• It also supports the body and provides a structure
against which the muscles can contract.
Phylum Nematoda
• Nematoda is made up of roundworms, worms
with long slender bodies that taper at both ends.
• Roundworms are Pseudocoelomates
• Pseudocoelomates is a hollow fluid filled cavity
that has mesoderm lining the outside and
endoderm on the inside. (coelomates have
mesoderm lining the entire cavity)
What Is a Roundworm?
• Roundworms are un-segmented worms
that have pseudocoeloms and digestive
systems with two openings—a mouth
and an anus.
What Is a Roundworm?
•This cavity is partially lined with tissue derived
from the mesoderm and is called a
pseudocoelom, meaning, “false coelom.”
What Is a Roundworm?
• Roundworms have a digestive tract with two
openings.
• Food moves in one direction through the
digestive tract of roundworms.
• Any food that is not digested leaves the body
through the anus.
Form and Function in Roundworms
• Roundworms have specialized tissues and
organ systems that carry out essential
physiological functions.
Form and Function in Roundworms
•Feeding
– Many free-living roundworms use grasping
mouthparts and spines to catch and eat other
small animals.
Form and Function in Roundworms
•Respiration, Circulation, and Excretion
– Roundworms exchange gases and excrete
metabolic waste through their body walls.
– They depend on diffusion to carry nutrients and
waste through their bodies.
Form and Function in Roundworms
•Response
– Roundworms have simple nervous systems,
consisting of several ganglia.
– Several nerves extend from ganglia in the
head and run the length of the body.
– These nerves transmit sensory information
and control movement.
Form and Function in Roundworms
•Movement
– Fluid in the pseudocoelom and muscles
extending the length of their bodies function
as a hydrostatic skeleton.
– Aquatic roundworms contract muscles to
move like snakes through the water.
– Soil-dwelling roundworms push their way
through the soil by thrashing around.
Form and Function in Roundworms
•Reproduction
– Roundworms reproduce sexually.
– Most species have separate sexes.
– Roundworms reproduce using internal
fertilization.
– Parasitic roundworms often have life cycles
that involve two or three different hosts or
several organs within a single host.
Roundworms and Human Disease
•What roundworms cause human disease?
Roundworms and Human Disease
•Parasitic roundworms include trichinosiscausing worms, filarial worms, ascarid
worms, and hookworms.
Roundworms and Human Disease
•Trichinosis-Causing Worms
– Adult Trichinella worms live and mate in the
intestines of their hosts.
– Female worms carrying fertilized eggs burrow
into the intestinal wall and release larvae.
– Larvae travel through the bloodstream and
burrow into organs and tissues.
Roundworms and Human Disease
• The larvae form cysts and become inactive in
the host’s muscle tissue.
• Trichinella completes its life cycle only when
another animal eats muscle tissue containing
these cysts.
• Humans can get trichinosis by eating raw or
incompletely cooked pork.
Roundworms and Human Disease
•Filarial Worms
– Filarial worms are threadlike worms that live
in the blood and lymph vessels of birds and
mammals.
– They are transmitted by biting insects.
– Large numbers of filarial worms may block the
passage of fluids within lymph vessels and
cause swelling.
Roundworms and Human Disease
•Ascarid Worms
– Ascaris lumbricoides is a serious parasite of
humans and many other vertebrate animals.
– It absorbs digested food from the host’s small
intestine.
– Ascaris lumbricoides is commonly spread by
eating foods that are not washed properly.
Roundworms and Human Disease
Ascaris Life Cycle
1 Human ingests food or
water containing Ascaris
eggs.
Roundworms and Human Disease
Ascaris Life Cycle
2 The eggs travel to
the small intestine
and develop into
larvae.
Roundworms and Human Disease
Ascaris Life Cycle
3 Larvae enter blood
vessels and are
carried to the lungs
Roundworms and Human Disease
Ascaris Life Cycle
4 Larvae are coughed
up and swallowed.
They then travel to
the small intestine
where they develop
to maturity
Roundworms and Human Disease
Ascaris Life Cycle
5 Eggs are released
and leave the host
in faces.
Roundworms and Human Disease
•Hookworms
– Hookworm eggs hatch and develop in the soil.
– They use sharp tooth like plates and hooks to
burrow into the skin and enter the
bloodstream.
Roundworms and Human Disease
• Hookworms travel through the blood of
their host to the lungs and down to the
intestines.
• There, they suck the host’s blood, causing
weakness and poor growth.
Phylum Rotifera
• Most Rotifers are transparent, free-living
animals that live in fresh water.
• When they are in dry conditions the look like
grains of sand and then will go back to their
normal state when wet.
• Have cilia near the mouth. Looks like a pair of
rotating wheels. They sweep food- algae,
bacteria, and protozoans- into their digestive
tracks.
Rotifers