Learning
Outcome G3
Phylum Platyhelminthes,
Nematoda and Annelida
Learning Outcome G3
 Analyze
the increasing complexity of the
Phylum Platyhelminthes, the Phylum
Nematoda and the Phylum Annelida
Student Achievement
Indicators
Students who have fully met this learning outcome will be able to:
 Examine members of the Phylum Platyhelminthes and describe their
unifying characteristics.
 Describe how platyhelminthes carry out their life functions
 Examine members of the phylum Nematoda and describe their unifying
characteristics.
 Describe how nematodes carry out their life functions
 Examine members of the Phylum Annelida and describe their unifying
characteristics
 Describe how annelids carry out their life functions
 Describe the physical chanes that were necessary for flatworms and
roundworms to become parasitic
 Evaluate the characteristics of a successful parasite
 Describe human disorders that are caused by non-segmented worms.
 Compare platyhelminthes, nematodes and annelids with respect to
evolutionary changes
 Describe the ecological role of platyhelminthes, nematodes and annelids
The Flatworms
Classification
 Kingdom Animalia
 Phylum Platyhelminthes
Characteristics
 Exhibit
bilateral symmetry
 Have a mesoderm
 Have a basic primitive system for digestion and
excretion
 Most primitive organism to show cephalization
 Can be free-living or parasitic
 Parasitic flatworms live on or on the body of a
host from which they obtain their nutrients
 Do not have a coelom
Adaptations for a Free-Living
Lifestyle
 Live
in moist or submerged habitats in both
marine and fresh water.
 Most are small, less than 1 cm in length but
can be much larger
 There are very large terrestrial forms that may
reach up to 60 cm in length
 Example – of a free-living flatworm is planaria
Digestive System
 Like
cnidarians, free-living flatworms have a
“blind” digestive system, with the mouth as
the only entrance or exit.
 Parasitic flatworms do not have a digestive
system because the obtain pre-digested
nutrients from their host
Excretory System
 Very
primitive
 Do have a rudimentary excretory system
consisting of a network of fine tubules running
though the body and opening to the outside
via tiny pores
Nervous System
 Very
primitive
 Has a collection of nerve tissue that
resembles a primitive brain.
 Two nerve chords, each made up of tiny
nerve cells, arise for the “brain “ and run
along the ventral side of the body towards
the tail
 The nervous system appears to coordinate
body movement and receives input from
simple sensory cells, allowing more complex
movement than previous groups.
Reproductive System
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Simple
A reproductive system is present only during the
breeding season and degenerates at other times
of the year
Fertilized eggs are released in tiny capsules that
contain a small number of eggs and thousands
of nutritive yolk cells.
These capsules attach to objects in the water,
and within two or three weeks these eggs hatch
into juveniles.
Juvenile flatworms resemble adult flatworms
Some planarians can reproduce asexually by
fragmentation
Adaptations for a Parasitic
Lifestyle
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May look very different from free-living forms
Can be found both on the inside and outside of
an animal’s body
Parasites living within a host, can not grow very
large, because no parasite wants kills its host.
The parasite wants to obtain as much food as
possible without killing its host
Human tapeworms can reach a length of 7 cm
Many hosts, including humans maintain a
constant internal environment
Parasites living within such a host are highly
adapted to specific and reasonably stable
environments.
Adaptations for a Parasitic
Lifestyle
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Parasitic worms require little sensory information, so sensory
receptors are reduced or absent
Parasites are capable of producing hundreds of thousands of
eggs.
The absence of a digestive system leaves more room for eggs
Intestinal parasites live and prosper in one of nature’s most
dangerous environments.
These parasites have a modified epidermis called a tegument
which protects them from digestive enzymes and immune
responses made by the host.
The tegument is folded to increase surface area and make
absorption of nutrients easier.
Some parasites such as tapeworms are protected by a nonliving cuticle which is secreted by epidermal cells.
Like the tegument the cuticle is designed to protect the
parasite.
Roundworms
Classification
 Kingdom Animalia
 Phylum Nematoda
 The phylum Nematoda is often put into a
larger group known as the
pseudocoelomates or the Aschelminthes.
Characteristics
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A bridge between “lower” and “higher” invertebrates
They have a pseudocoelom “fake-coelom”
Simple organism that has a complete digestive tract, with a
mouth at one end and an anus at the other.
The development of an anus means that food can be
digested as it moves through the system and the removal of
wastes does not interfere with the intake of more food
Has a simple nervous system; there is a aggregation of
nerve cells and lateral or ventral nerve chords that extend
along the body.
Undergo sexual reproduction
There is not respiratory or circulatory system due to the
organism’s small size
Oxygen and carbon dioxide are exchanged by simple
diffusion through the skin.
Free-Living Species
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Small harmless worms
Some are parasites
Example – trichinosis
This parasite infects humans when they eat undercooked
port infected with Trichinella spirillis
These worms form small cysts in the muscle tissue of the host
and may remain dormant for many years
When the host tissue is eaten (pork) the worms become
active again.
The adults are small and inhabit the human digestive tract
However, 100 fertilized eggs are released by each adult
female
The developing young burrow through the intestinal wall
and are transported to all organs of the body.
These tiny worms cause muscle pain and extreme fever
Life Cycle of a Tapeworm
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Live in the intestines of vertebrates
Produce large numbers of eggs, and have an
intermediate host which ensures that the tapeworm is
carried from one host to another
The tapeworm has scolex, which is a structure at the
anterior end that has suckers, a hook or both.
The scolex enables the tapeworm to attach to the
intestinal wall of the host.
The long body of the tapeworm also gives it a large
surface area to absorb the digested food from the
host.
Behind the scolex are mature segments called
proglottids, which contain a full set of sex organs.
Proglottids are reproductive units that can mate with
each other by the worm doubling back on its self
Life Cycle of a Tapeworm
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The segments at the end of the worm are full of eggs
Proglottids at the extreme end of worm drops off and
pass out with the host’s feces taking the eggs with the
feces
These eggs can then infect an intermediate host
Once the intermediate host ingest the tapeworm eggs,
the eggs mature into embryos and bury into the
muscle tissue of the host’s gut wall.
Each embryo forms a cyst about the size of a pea;
these cysts are consumed when humans eat
improperly cooked meat.
In the human intestine, a young tapeworm emerges
from the cyst
Life Cycle of Ascaris
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A roundworm
Lives in various organs within a single hose
Example - human or dog
After fertilization the female lays its eggs in the host’s intestines
The eggs pass out of the host with the feces
A new infection occurs when a human or other animal
consumes food contaminated with the eggs
The eggs then hatch in the small intestine of the new host
The newly hatched eggs burrow though the intestinal wall and
enter the bloodstream and are carried to the live, heart and
lungs.
These roundworms continue to move in the lungs and after
about 1o days they migrate up the windpipe and are
swallowed and transported back to the small intestines
The mature worms mate and a new mass of eggs is excreted in
the feces.
Invertebrates with Coeloms
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The presence of a coelom indicates a true body cavity
Most bilaterally symmetrical animals have a coelom
Roundworms and flatworms do not have a coelom
The coelom develops within the embryonic mesoderm
and has definite lining of epithelial cells known as
peritoneum.
The peritoneum not only lines the inner surface of the
body but ait also surrounds the organs and forms the
mesenteries that suspend the internal organs in their
upward position
Mesenteries are loose connective tissues that suspend
internal organs upright.
Invertebrates with Coeloms
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The development of a coelom in “higher”
invertebrates was probably associated with the
increase in the animal’s size
With a body cavity there is more space to
accommodate organs
The fluid within the body cavity may further aid in
waste removal and the circulation of food and
oxygen.
The body cavity provides space where organs
such as lungs, heart, stomach and intestines have
room to expand and contract and to slide by
each other as the animal moves.
Coelomates are thought to have developed from
flatworms
Invertebrates with Coeloms
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After the development of a true coelom and a
dual opening digestive tract, the development of
animals follows to two evolutionary lines: the
protostomes and the deuterstomes.
Both lines have a coelom and a complete
digestive tract, but they differ in the way that their
embryo’s develop
Deuterostomes have radial cleavage which
means the first opening to develop becomes the
anus and the second become the mouth.
Protostomes show spiral cleavage and the first
opening is the anus second is the mouth.
The Segmented Worms
Classification
 Kingdom Animalia
 Phylum Annelida
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Class Oligiochaeta
Class Polychaeta
Class Hirudinea
Characteristics
 Segments
and have a true coelom
Class Oligiochaeta
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Common earthworm
Very important to agriculture, burrow in soil, and promote the
mixing and churning of soil.
This increases drainage and helps break down organic
material so it can be used by future crops
Well adapted for burrowing
Cylindrical in shape with tapered ends
Each segment is separated by a septum or a wall which is
formed by a double layer of peritoneum
A combination of circular and longitudinal muscles allows the
body of the earthworm to contract.
The contractions move along the length of the worm in a
process called peristalsis
This class has four chitonous bristles known as setae which are
used to extend and anchor the worm as it burrows and assists
is locomotion.
Digestive System
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Earthworms eat soil and soil enters the mouth and
is sent backwards to the crop by contractions of
the muscular pharynx.
The soil is stored here until to moves into a
“grinding” organ called the gizzard.
In the gizzard with the help of sand contained in
the soil, dead and decaying organic material is
ground into tiny pieces.
Wastes are expelled though the anus, this waste
materials known as castings and help fertilize
surrounding soil.
In the intestine digested food is absorbed into the
blood via an extensive capillary network
Circulatory System
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Has a closed circulatory system, which means
blood flow is in closed vessels which branch to all
parts of the animals.
These branches become progressively smaller until
they become capillary beds
There is no heart but thickened muscular blood
vessels in the anterior regions act as a pump to
help move blood.
Both gases and molecules of digested food are
carried to the body cells by a watery fluid known
as blood.
As in higher animals the blood is red due to
presence of hemoglobin
Excretory System
 Very
primitive
 Paired nephridia are found in each of the
approximately 100 segments
 The nephridia open into the coelom in one
segment and the pass through the septum
into the next posterior segment.
 In that segment the nephriduium ends in an
exit pore in the body walls.
 Waste materials are excreted through this
pore to the outside
Nervous System
A
ventral nerve cord runs the length of
ceolom, with small ganglia giving rise to
nerves in each segment.
 There is a concentration of sensory cells at the
anterior end and along the body are
scattered cells that appear receptive to
chemical, mechanical and light stimulation.
Reproductive System
Hermaphroditic
 Part way along the body is a prominent
swollen segment called the clitellum which
contains reproductive cells
 When the eggs are laid the clitellum secretes
a cocoon which envelops and protects them
 The young develop in the cocoon and leave
it as fully formed worms
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Class Polychaeta
A
diverse group of marine worms
Class Polychaeta
 Some
live beneath stones, shells and in the
ocean within tiny tubes they create
 Some are transparent and free-swimming
 A few are parasitic species that live within the
coelomic cavities of other polychaetes
 Consist of several segments like earthworms
 Have a fleshy flattened projection on each
side of segment known as parapodia (side
feet)
 These parapodia function in movement and
gas exchange.
Class Hirudinea
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The leeches
Live in lakes, slow-moving streams, ponds,
marshes and on moist vegetation in humid
environments such as jungles
75% of known species are blood sucking external
parasites
Theses leeches have salivary glands that
produce a chemicals known a hirudin which
prevents the host’s blood from clotting
Very slow digestive process and this allows
leaches to tolerate up to one year with no food.