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» Dorso-
ventrally
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?!?
• Specialized adaptations for parasitic lifestyle– Thick cuticle to
protect against digestive enzymes
– Structures to allow parasite to attach to host
Liver Fluke
Tapeworm
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
– Capsule surrounding larva is digestable, allowing the infection of a new host
Proglottids
PlatyhelminthesAdvances in Adaptation
• Symmetry» Bilateral
• Cephalization» Distinct anterior advancements» Coordinated movement
• Asexual and Sexual Reproduction» Regeneration » Internal fertilization» parasitism
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