Phylum Platyhelminthes
The Flatworms
Phylum Platyhelminthes
• About 20,000 species
» About 80% of parasites are from this phylum
• Divided into three major groups
• Free-living flatworms
• Parasitic tapeworms
• Parasitic flukes
Flatworms
• Range in size
– Some
microscopic
free-living
forms
– Parasitic
forms over 20
meters long
• Show many
advances over
Cnidaria
General Characteristics
• The following
characteristics are
shared by ALL
flatworms:
– Bilateral symmetry
» the most
primitive
bilateral
animals
– Development of definite
anterior and posterior
ends
– Tissue-organ level of
organization
» Simple organ
systems
– Flattened body shape
» Dorsoventrally
Body Plan of Flatworms
• Acoelomate
– No Body cavity
• NOT a tube-within-a-tube
– One simple tube
• GVC present
• Incomplete digestive tract
– One opening
– Food and wastes enter /
exit through same
opening
» Two way
flow
Planaria
(class Turbellaria)
• Typical flatworms
• 5-25 mm in length
• Freshwater organisms
– Streams and ponds
– Beneath rocks, leaves,
logs, etc.
• Planaria are carnivores
– Smaller animals
– Dead organisms
Internal Structure of a Planarian
• Simple digestive system
– Mouth
• Pharynx – tube that can be extended through the mouth
– GVC
• With a primitive branched intestine
• Extracellular and intracellular digestion (cells lining intestine)
Getting Around …
Movement Towards Cephalization
• Planaria can move about
freely
– Muscles
– Underside covered with cilia
• Presence of eyespots
– Small brain located beneath
• Development of primitive
nervous system
– Two nerve cords extend the
length of the body
– Transverse nerve cords
extend across the body
Reproduction in Planaria
• Asexual Reproduction
– Fission
» Head separates from tail end
– Unbelievable potential for regeneration!!
– Tails CAN regenerate heads!!
Reproduction in Planaria
• Sexual Reproduction
– Hermaphrodites . . . Again!
– REMEMBER: self-fertilization does not occur
– Fertilization is INTERNAL
– Fertilized eggs are shed in protective
capsules
– Eggs hatch into tiny new planarians!
What’s Different About Flukes
and Tapeworms?!?
Liver Fluke
Tapeworm
• Specialized
adaptations for
parasitic lifestyle
– Thick cuticle to
protect against
digestive
enzymes
– Structures to
allow parasite to
attach to host
Tapeworms
scolex
Suckers
Hooks
Specialized Reproductive
Measures
• Each square making up the
body is a reproductive sac
– > 100,000 fertilized eggs
– Each square will
eventually break off and
is released in the feces
of the host
» Proglottids
Proglottids
– Capsule surrounding
larva is digestable,
allowing the infection of
a new host
Platyhelminthes
Advances in Adaptation
• Symmetry
» Bilateral
• Cephalization
» Distinct anterior advancements
» Coordinated movement
• Asexual and Sexual Reproduction
» Regeneration
» Internal fertilization
» parasitism
Platyhelminthes
fact:
The longest
flatworm ever
found, a
tapeworm, was
over
90 FEET LONG . . .