Platyhelminthes
Flatworms
Phylum Platyhelminthes
Learning Outcomes
By the end of this unit you should be able to:
• Describe the unifying characteristics of
members of the phylum Platyhelminthes
• Describe how platyhelminthes carry out
their life functions
• Describe physical changes that were
necessary for flatworm to become
parasitic
• Describe the ecological roles of
platyhelminthes
Phylum Platyhelminthes
• Name means ‘flatworms’
• Examples: Planaria, flukes, tapeworms
Bilateral Symmetry
• Platyhelminthes have bilateral symmetry
• One place to cut them in half and have
each side be a mirror image of the other
General Characteristics
• 3 cell (body) layers:
ectoderm (outside),
mesoderm (middle),
and endoderm
(inside)
• They have an organ
level of development
(reproductive,
muscular, and
excretory organs)
General Characteristics (2)
• Exhibit cephalization (they have a distinct
head with a concentration of nervous
tissue at the anterior end)
• Mesoderm layer develops into the organs
• These organisms have anterior (head) and
posterior (tail) ends as well as dorsal
(top/back) and ventral (bottom/stomach)
surfaces
Class Turbellaria (Planaria)
• This group contains the ‘free-living’
flatworms
• Most members of class Turbellaria are
marine but some are found in fresh water
• Some are terrestrial
A marine flatworm
A predatory flatworm
Planaria Ingestion
• Eats a diet of small
animals and dead,
decaying materials
• Mouth on the ventral
surface allows
muscular pharynx to
be extended and
inserted into prey
Planaria Digestion
• Once the pharynx is inserted into the prey
it secretes enzymes that partially
digest/break down the food
• The pieces of partially digested food are
sucked up through the pharynx and into
the mouth
• Further digestion occurs in the many
branches of the gastrovascular cavity
(GVC)
Planaria Digestion (2)
• The gastrovascular
cavity (GVC) has a
large number of
branches which
increases the surface
are so that more
nutrients can be
absorbed.
Planaria Elimination
• Undigested food is eliminated out the
mouth and through the pharynx (just like in
phylum cnidaria)
Planaria Excretion
• The excretory ‘system’ is composed of
flame cells, excretory tubules, and
excretory pores
Planaria Excretion (2)
• The cilia of flame cells beat and remove
excess water and nitrogenous waste from
tissue cells
• The waste is drawn into excretory tubules
and exits out the excretory pores located
in the Planaria’s head
• The development of an excretory system
is necessary because of the greater
number of cells and the density of the
tissue
Circulation & Respiration
• The gastrovascular cavity functions in both
circulation and respiration
• GVC circulates nutrients and diffused
oxygen to the tissues
Planaria Nervous System
• Exhibits cephalization – contains group of
nerve tissue called ganglia at the anterior
(head) end
• Ganglia attaches to 2 ventral nerve cords
• Anterior end contains eye spots that are
sensitive to light and pointed lobes that are
sensitive to touch
Planaria Muscular System
• 2 layers of muscle tissue
• Longitudinal and circular layers of muscle
tissue allow the planaria to contract, twist,
and turn
• Muscles are stimulated by the nervous
system to allow coordinated movement
Asexual Reproduction
• Planaria reproduce
asexually by binary
fission
• Planaria can also
regenerate missing or
damaged parts
Sexual Reproduction
• Planaria are hermaphrodites (both male
and female in the same individual)
• 2 planaria will trade sperm with each other
(egg stays with the worm who produced it)
• Fertilization is internal
• The zygote (fertilized egg) is released into
the water
Class Trematoda
• Parasitic flukes such as sheep liver fluke
and blood flukes
Class Cestoda
• Parasitic tapeworms
• Live in many different hosts
Tapeworm Anatomy
• Head is called a
scolex
• Reproductive organs
are in specialized
segments called
proglottids
Tapeworm Anatomy (2)
Parasitic Adaptations of
Tapeworms & Flukes
• Have suckers &/or hooks so they can
attach to the host
• Have a tough tegument (tapeworms) or
cuticle (flukes) to prevent being digested
by their host
• Absent or reduced digestive, circulatory,
and muscular systems (don’t need to
expend energy on these)
Parasitic Adaptations of
Tapeworms (2)
• Produce 10’s to 1000’s of eggs (helps to
ensure that some will survive to become
adult worms)
• Have complex lifecycles that involve more
than one host which makes prevention
more difficult