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» 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

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

– 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

Platyhelminthes fact:

The longest flatworm ever

found, a tapeworm, was

over

90 FEET LONG . . .