Flatworms, Roundworms, &
Segmented Worms
Phylum Platyhelminthes
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Flatworms
Flat and thin bodies
Bilateral symmetry
Most are parasitic
Phylum Platyhelminthes
• Flatworms include planarians, flukes, and
tapeworms
• 20,000 species of flatworms
Miscellaneous Information- Flatworms
• Movement- use of moving bodies of water or
damp soil, or larger animals transporting them
• Nutrition- parasitic; the flatworm will take in food
through same opening it eliminates waste
• Circulatory system- Gastrovascular cavity with
one opening
• Digestion and excretion occur through same
opening
• Reproduction- flatworm splits in two forming a
new flatworm
Tapeworms Information
• Can get tapeworm
infection by ingestion of
food or water
contaminated with
tapeworm eggs or
larvae
• Can cause intestinal
infections
• Symptoms- poor
appetite, diarrhea,
weakness,
gastrointestinal
discomfort
Phylum Nematoda
• Roundworms
• Ex. Pinworms and
hookworms
• 90,000 species of
nematodes
• Bilateral Symmetry
• Live in wet soil or water
• Eat dead leaves and
other materials
• Some eat insects that
destroy plant roots
• Some destroy plant
roots
Flatworm Head
• Head holds
attachment hooks for
host attachment
• 2 eyespots to help
detect light
• Also contains brain
called 2 simple brains
called ganglia- simple
bundles of nerves
Phylum Nematoda
• Some
roundworms
can live in
humans and
make them sick
• Roundworms
have a complete
digestive tract
with 2 openings
Nervous System of Roundworms
• Two nerve cords that transmit impulses in the
roundworm
Miscellaneous InformationRoundworms
• Reproduction- sexually, eggs deposited in soil
after fertilization
• Bilateral symmetry
• No formal respiration, circulation, skeletal
systems
Phylum Nematoda
Phylum Annelida
• Annelids are segmented worms
• Bodies are divided into many sections or
segments
• Live in moist soil, freshwater, or saltwater
• 15,000+ species of annelids
• Examples: earthworms and leeches
Phylum Annelida
• Earthworms
– Have bristlelike Setae on each segment- helps in
movement
– Tunnel through soil to eat small pieces of food
– Tunnels will loosen the soil and allow air to enter
which helps plants to grow
Leeches and Us
• Used in medicinal treatment
• Abscesses, painful joints, glaucoma,
myasthenia, and to heal venous diseases and
thrombosis
Phylum Annelida
Miscellaneous InformationSegmented Worms
• Nervous system- ganglia nerve centers in each
segment that are connected by nerve cords to brain
• Closed circulatory system- like humans
• Gas exchange through skin- need for worm to live near
water
• Digestion- complete internal digestive tract that runs
length of body
– Gizzard- muscular sac and hard particles help grind soil and
food before they pass into intestine
Nephridia- collect/remove waste from each segment
Pg. 731 is a good reference for earthworm body
Miscellaneous InformationSegmented Worms
• Reproduction- Hermaphrodites
• Produce both eggs and sperm
Phylum Annelida
• Leeches
– Eat small invertebrates
– Can attach to skin of vertebrates and feed on its
blood
– Anesthetics in chemicals of bite to prevent pain
– Leech secretes chemical so blood does not clot
Bilateral Symmetry and Body Plans
• All bilaterally symmetrical animals developed
from 3 embryonic cell layers:
– Ectoderm
– Endoderm
– Mesoderm
– 3 types- Acoelomates, Pseudocoelomates,
Coelomates
Acoelomates
• Develop from the 3 layers but have no body
cavities
• Digestive tract extends throughout body
• May have been first group of animals to
evolve
• Ex. Flatworms
Acoelomates
Pseudocoelomates
• Develop from the 3 layers
• BUT also has a space that develops between
the endoderm and mesoderm- called a
pseudocoelom (difference between flatworm
and roundworm)
Pseudocoelomates
Coelomates
• Body cavities form from Coelom (fluid filled
space that is completely surrounded by
mesoderm where specialized organs and
organ systems, serves to cushion and protect
organs)
• Humans, insects, fishes
• Example: Earthworm
• Greatest diversity among animals
Coelomates
Earthworm
Compare and Contrast
• Flatworm and Earthworm digestive tracts
• One opening to flatworm’s digestive tract
(pharynx)
• Earthworm’s digestive tract has 2 openings
(mouth and anus)
3 types of worms
Segmented Worms
Roundworms
Flatworms
Phylum Annelida
Phylum Nematoda
Phylum Platyhelminthes
Segmented/sectioned
bodies, bilateral symmetry
Bilateral symmetry
Bilateral symmetry, flat and
thin bodies
Moist soil, freshwater, or
saltwater
Wet soil or water
Parasitic- live inside a host or
in water
Complete digestive tract- 2
openings (mouth and anus)
Complete digestive tract
with 2 openings (mouth
and anus)
One digestive opening
(pharynx)
Coelomate- form from
coelom (fluid filled space
surrounded by mesoderm)
Pseudocoelomate- develop Acoelomate- develop from 3
from 3 layers, with space
layers but have no body
between endoderm and
cavities
mesoderm called
Pseudocoelom
Earthworms, leeches
Pinworms, hookworms
planarians, flukes and
tapeworms