Biology 11 –
Kingdom Animalia
Study of the Primitive Worms
Phylum Platyhelminthes: Flatworms
Phylum Nematoda: Roundworms
BIOL 316
Phylum Platyhelminthes
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Phylum Platyhelminthes – the Flatworms
The Planarians
The Tapeworms
The Flukes
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Importance of the Flatworms to Evolution

Bilateral body plan – left side mirror image of right

Cephalization – development of the head


sense organs and nerves concentrated in head region
Centralization of nervous system:

Nerve cord accompanies cephalization and bilateral symmetry.

Advanced muscle tissue present;

Primitive excretory system

allows removal of metabolic wastes
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Flatworms (Phylum Platyhelminthes)
Definition:
Flatworms are soft, flattened worms that have tissues and
internal organ systems.
They are the simplest animals to have 3 embryonic germ
layers, bilateral symmetry, and cephalization

Flatworms are acoelomates, meaning “without a coelom”
 coelom = a fluid-filled body cavity that is lined with
mesoderm
 mesoderm = middle germ layer that gives rise to
muscles, circulatory, reproductive, and excretory systems
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Phylum Platyhelminthes Characteristics

Bilateral symmetry; anterior and
posterior ends.

Body flattened

Well-developed epidermis

No internal body cavity other than
digestive tube (acoelomate)


Parenchyma (connective tissue)
fills space between organs.
Incomplete digestive system

Still one opening

This critter eats and poops out of its
mouth – mmmm, pass the
popcorn
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Cross Section of a Flatworm
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Advantage of Being Flat

Surface area to volume
ratio is important for
organisms.
3mm


Many substances enter and
leave through body
surface.
Faster absorption and
release of wastes
3mm
3mm
Volume = 3 x 3 x 3 = 27mm2
Area = 6(3 x 3) = 54mm3
9mm
3mm
1mm
Volume = 9 x 3 x 1 = 27mm2
Area = 2(9 x 1) + 2(3 x 1) + 2(3x9) = 78mm3
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Nutrition and Digestion

Typical digestive system includes a
mouth, a pharynx, and an intestine.

Parasitic Tapeworms and Flukes lack a
digestive system
 Parasitic flatworms, such as the
Tapeworm absorb molecules from host.
Free living flatworms (Planarians) are
carnivorous; suck prey into pharynx with
mouth.
 Undigested food goes out mouth. Yes,
folks, they eat and excrete out the same
hole 

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Flatworm Respiratory, Circulatory, and
Excretory System

Lack circulatory, respiratory, and excretory
systems.

Flatworms rely on diffusion to transport oxygen and
nutrients throughout the body, to remove CO2 and
other wastes

Some flatworms have flame cells – specialized cells
that remove excess water and metabolic wastes
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Nervous System and Sense Organs

Have a nerve-net.

Most have more complex system.
 Cerebral ganglion = primitive brain
coordinates impulses.
 “Ladder” system carries impulses
throughout body
Many sense organs present
 Eyespots
 Chemoreceptors, touch sensors
and rheoreceptors (sense
direction) also common

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Note: Free-living vs. Parasitic


Free-living (eg Planarians) have well developed digestive
and nervous systems to seek-out and process food
Parasitic Tapeworms – rely on host’s digestive system to
process food, which absorb. No need for advancement
of nervous system – not needed.
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Reproduction – in Free-living Flatworms
such as Planarians

May reproduce sexually or
asexually

Asexual:
 Binary fission – split in two

Sexual:
 Both sexes present in same
flatworm (hermaphrodites).
 Fertilization internal, eggs
produced
 Embryos emerge as juveniles
resembling adults.
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Reproduction in theTapeworm!

Lack digestive system

Sucker and hooks on anterior
end for attachment

Neck produces new segments
 Called proglottids

Each proglottid contains male
and female gonads

Reproductive output high
 Many proglottids per worm
 Many eggs per proglottid
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Groups of Flatworms



Turbellarians
Free-living, e.g. planarian
Flukes (Trematoda)
Parasites
Tapeworms (Cestoda)
Parasites
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Tapeworm - Life Cycle
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Flukes -- Life Cycle of a Liver Fluke
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The Coelom or Body Cavity Advances
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Another Look at the Coelom
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Phylum Nematoda (Roundworms)

Definition:


Roundworms are unsegmented worms that have
pseudocoeloms and digestive systems with two openings – a
mouth and an anus
Pseudocoelom = “false coelom”
 Is a body cavity between the endoderm and mesoderm tissues,
partially lined with mesoderm tissue
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Form and Function in Roundworms




Feeding: free-living carnivores; or eat algae, fungi,
decaying organic matter
Respiration: gases diffuse through body walls
Circulation: no internal transport system 
diffusion
Excretion: metabolic wastes diffuse through body
walls
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Form and Function in Roundworms



Response: ganglia from the head and run through
length of body; sense organs to detect chemicals of
other organisms
Movement: muscles and fluid of the pseudocoelom
function as a hydrostatic skeleton; allows
movement (swimming or pushing through soil)
Reproduction: sexual reproduction; internal
fertilization, were male deposits sperm inside the
female’s reproductive tract
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Roundworm Ecological Roles




Omnivores that catch and eat algae, fungi,
decaying matter
Recyclers
Parasites of Plants – eg. The Golden
Nematode attacks potato crops, causing
severe devastation
Parasites of animals
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Ascaris – Parasitic Roundworm of Pigs and
Humans
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Unfortunately, they can Grow quite Big!
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Parasites: Your Pet's Enemies
Tiny parasites that pose a gigantic threat
Heart worms inside the dog's heart, can
grow to a length of 14".
What they do:
Restrict blood flow and cause organ damage.
Symptoms include coughing. labored
breathing, heart failure. Untreated, heart
worm disease can kill a dog. Death may
occur within 72 hours.
How they spread:
Transmitted by over 70 species of
mosquitoes. Virtually all dogs are at risk.
Defending against them: Once a month
prevention is recommended, contact your
local veterinarian for advice.
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Adult roundworms live in the dog's
small intestine.
What they do:
Cause diarrhea, vomiting, stunted growth,
rough coat, bloated belly.
How they spread:
Almost all puppies get roundworms from their
mothers. Dogs can also pick up roundworms
from the soil.
Defending against them: Once a month
prevention is recommended, contact your local
veterinarian for advice.
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Enlarged hook worm mouth
showing 'teeth'.
What they do:
Attach to the dog's intestinal lining, leaving
bleeding internal wounds. Cause blood loss,
anemia, and diarrhea. As few as 100 hook
worms can kill a puppy.
How they spread:
Eggs pass through the feces of an infected
dog into the soil where they are easily
swallowed or can penetrate through a dog's
feet.
Defending against them: Once a month
prevention is recommended, contact your
local veterinarian for advice.
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Symptoms:
Larvae digested
enter bloodstream
and burrow into
organs, causing
absolutely
horrendous pain
and in some cases
death.
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Filarial (threadlike worms) are nematodes that
may block the passage of Fluids in the Body

Disease: Elephantitis

The disease is caused by
parasitic worms, including
Wuchereria bancrofti, Brugia
malayi, and B. timori, all
transmitted by mosquitoes.
Lymphatic filariasis currently
affects 120 million people
worldwide, and 40 million

Blocks Lymphatic nodes
which drain fluid to and from
the blood.
Tropics, Africa and Asia

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Phylum Annelida (Earthworms)


Annelida  annellus, meaning “little ring”
Definition:
 Annelids are worms with segmented bodies.
They have a true coelom that is lined with
tissue derived from mesoderm.
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Body Plan

The body of an annelid is divided into segments
separated by septa (singular: septum)
 Septum – internal walls between each segment
 Most segments contain the same organs (eyes,
antennae, sense), while others are specialized
 Most segments have bristles called setae
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Feeding and Digestion

Annelids may be filter feeders or predators
(carnivorous)
Annelids use their
pharynx (with jaws) to
bring food into the
esophagus. The food
then go into the crop
(food storage) and then
to the gizzard (food is
ground up). Then food is
absorbed in the
intestine.

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Circulation

Annelids have a closed circulatory system.

Closed circulatory system:
 Blood is contained in a network of blood vessels
•Blood circulates through a
dorsal vessel and a
ventral vessel, and
smaller blood vessels
through each segment
•Dorsal vessel helps
pump blood
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Respiration

Annelids can respire through:
 Gills (worms that live in water)
 Diffusion through skin (earthworms)
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Excretion


Digestive waste passes
out anus
Metabolic waste is
filtered out by
nephridia
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Response


Most annelids have a well-developed nervous
system with a brain and nerve cords
Other sense organs:
 Sense receptors, chemical receptors, sensors
that detect gravity
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Movement

Hydrostatic skeleton
 Contraction of longitudinal and circular muscles

Annelids can burrow, use setae, or parapodia
(paddlelike appendages for swimming or crawling)
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Reproduction

Sexual reproduction
May have separate sexes and external fertilization
OR
 Annelids are hermaphrodites and exchange sperm with
another worm
 Two worms attach, exchange sperm, and store it in
sacs
 A clitellum (a thick band) secretes a mucus ring
where the egg and sperm are released and fertilization
takes place inside this ring
 The ring falls off the worm and forms a cocoon, where
the eggs will hatch

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Ewwwwwwwwwwwwwwwwwwwwwwwww!
Ah, but they are sooooooo in love, don’t you think?
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Class Oligochaeta


Earthworms
Streamlined bodies, few setae, live in soil or water
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Class Hirudinea
• Leeches
• External parasites that suck blood and body fluids of host
• Medical uses: reduce swelling, helps blood clotting
• Have suckers at one or both ends for attachment. Produce
a chemical in saliva called HIRUDIN, which is an anticoagulant
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Leech Anatomy 101
Mmmm – lunch! 
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Leeches Used in Medicine
Successful
reattachment of
severed ear, as
blood continued
to flow and carry
nutrients to the
damaged region
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Class Polychaeta


Sandworms, blood worms
Marine annelids with parapodia (paired paddlelike
appendages)
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Ecology of Annelids

Annelids are important for our environment
because they:
 Burrow through the soil, aerating and mixing it
 Decompose plant matter
 Food source (for birds, moles, toads, snakes,
etc.)
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