Platyhelminthes:
Flatworms
Phylum Platyhelminthes
• Phylum Platyhelminthes - Flatworms
• Soft, unsegmented, flattened worms that have tissues and internal
organ systems
• Have three embryonic germ layers
• Simplest animals with bilateral symmetry
• Exhibit cephalization
• More developed organ systems than Poriferans and Cnidarians
• Acoelomates – “without a coelom” – a coelom is a fluid-filled body
cavity that is lined with mesoderm
Anatomy of a Flatworm
Eyespot
Ganglia
Head
Nerve
cords
Gastrovascular
cavity
Flatworms use a pharynx to suck
food into the gastrovascular
cavity. Digested food diffuses
from the cavity into other cells of
the body. Eyespots in some
species detect light.
Mouth
Excretory
system
Freshwater flatworms have
simple ganglia and nerve
cords that run the length of
the body. The excretory
system consists of a network
of tubules connected to flame
cells that remove excess
water and cell wastes.
Ovary
Testes
Pharynx
Most flatworms are
hermaphrodites, having male
reproductive organs (testes)
and female reproductive
organs (ovaries) in the same
organism.
Flame cell
Excretory
tubule
Feeding in Flatworms
• Can be carnivores or scavengers
• Gastrovascular cavity with one opening at
the end of a pharynx
• Digested food diffuses to other body
tissues
Respiration, Circulation, and
Excretion in Flatworms
• No circulatory system or respiratory
organs – rely on diffusion to transport
nutrients and waste
• Expel undigested material through
pharynx
• Flame cells filter and remove excess water
and wastes from body
Response in Flatworms
• Nervous system much more developed
than Sponges or Cnidarians
• Definite head
• Simple ganglia control nervous system
• Two nerve cords run from ganglia along
both sides of body
• Ocelli, or eyespot, detects changes in the
amount of light in their environment
Movement in Flatworms
• Cilia on epidermal cells
• Muscle cells allow to twist and turn to be
able to react rapidly
Reproduction in Flatworms
• Sexual – hermaphrodites – two worms join
as a pair and deliver sperm to each other
• Asexual – fission – splits in two
Penis Fencing in Flatworms
Groups of Flatworms
• 3 major classes of flatworms
– Class Turbellaria
– Class Trematoda
– Class Cestoda
Class Turbellaria
• Free-living flatworms found in marine or
freshwater
• Bottom dwellers
• Planarians – 1/2 cm to 6 m long
Examples of Class Turbellaria
Parasitic Flatworms
• Feed on blood, tissue, or cells; some just
absorb nutrients through their “skin”
• Examples: Tapeworms, Flukes
• Very complicated sexual reproductive life
cycles
Class Trematoda – Flukes
• Parasitic and infect internal organs of host
• Blood flukes Schistosoma mansoni
• Humans – primary host – burrow through
exposed skin
• Many more eggs than free-living flatworms
• Eggs pass out with feces of host
• Tropical areas – Southeast Asia, Africa
Examples of Class Trematoda
Class Cestoda – Tapeworms
• Long, flat parasitic worms
• Head of an adult called a scolex – used to attach to
intestinal wall where it absorbs nutrients
• 18m long, host may lose weight and become weak
• Behind scolex is a narrow neck region that produces
proglottids that break off the tail and rupture in intestine
or feces and release embryos
• Eggs consumed by intermediate host
• Larvae form cysts in muscle tissue
• Tissue then consumed by humans
Life Cycle of Tapeworm
Examples of Class Cestoda
Nematoda:
Roundworms
Phylum Nematoda
• Phylum Nematoda - Roundworms
• Slender, unsegmented worms
• Pseudocoelom – body cavity only partially lined with
mesoderm
• Two openings, mouth and anus, in digestive system –
“tube-within-a-tube” – food moves only in ONE direction
• Microscopic in size to 1m long
• Can be decomposers or dangerous parasites
• Might be the most common multicellular animal
• Most are free-living, found everywhere
Feeding in Roundworms
• Mainly carnivores, some eat algae or
decaying organic matter
Respiration, Circulation, and
Excretion in Roundworms
• Exchange gases and excrete metabolic
wastes through diffusion
• No internal transport system
Response in Roundworms
• Simple nerve network, ganglia
Movement in Roundworms
• Muscles and fluid = hydrostatic skeleton
Reproduction in Roundworms
• Sexual – internal fertilization
• Ascarid worms – complex life cycles
involving two or three different hosts or
several organs within a single host
• Stomach – lung – throat – intestine - feces
Examples of Nematodes
Roundworms and Human
Disease
• Hookworms – similar to Ascaris lifecycle,
except they enter through the skin
• Filarial Worms – elephantiasis (swelling
of body) – transmitted by mosquitoes
ORGANISM
DISEASE
ROUTE OF INFECTION
DESCRIPTION
Trichinella
Trichinosis
Eating undercooked
meat containing
larval cysts
Larvae burrow into tissues of
host, causing pain
Filiarial
worms
Elephantiasis
Bite of insect carrying Worms block passage of fluids
filarial worms
within lymph vessels, causing
tissues to swell
Ascaris
Ascaris
infection/
infestation
Eating unwashed
food contaminated
with Ascaris
Worms in intestine block normal
passage of food and absorption
of nutrients
Hookworms
Hookworm
infection/
infestation
Bare skin in contact
with contaminated
soil
Worms attach to intestinal wall
and suck blood, causing
weakness and poor growth