WORMS

•Platyhelminthes

•Nematodes

•Annelids

Annelids

WORMS

•Plathyhelminthes

•Nematodes

•Annelids

Annelids

Phylum Platyhelminthes

The Flatworms

Phylum Platyhelminthes

• About 20,000 species» About 80% of parasites are from this

phylum

• Divided into two major groups

• Free-living flatworms (Planaria)• Parasitic (Tapeworm and fluke)

Planaria

FlukeTapeworm (Taenia)

Flatworms• Thin and soft body divided in

segments but without locomotive organs

• Range in size– Some microscopic free-living

forms– Parasitic forms over 20 meters

long• Show many advances over

Cnidaria

General Characteristics– Bilateral symmetry

»the most primitive bilateral animals

– Development of definite anterior and posterior ends

– Tissue-organ level of organization

– No locomotor organs– Flattened body shape

»Dorso-ventrally

Planaria

Tapeworm (Taenia)

Fluke

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)

Reproduction in Planaria• Asexual Reproduction

– Fragmentation. Head separates and forms tails.

– Unbelievable potential for regeneration!!– Tails CAN regenerate heads!!

Reproduction in Planaria

• Sexual Reproduction– Hermaphrodites . . . Again!

– REMEMBER: self-fertilization does not occur

– Fertilization is INTERNAL

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

Taenia solium

Tapeworms

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