Phylum Platyhelminthes- Flatworms
 Bilateral symmetry
 Acoelomate – no developed middle layer
 solid body without a space in the center
 Gastrovascular cavity with one opening
 Some respire (breathe) directly through skin

Platyhelminthes
 More advanced nervous system than
Porifera/Cnidaria
 Shows Cephalizationdevelopment of the head end
 Development of a BRAIN

Platyhelminthes

Platyhelminthes tapeworm

Platyhelminthes flukes

Nematoda (Roundworms)
 NON-segmented!!
 Bilateral Symmetry
 Pseudocoelomate – have a “false” middle layer
 a fluid filled body cavity
 Complete digestive tract (two openings)
 Many are parasitic
 Abundant (as many as 90000 roundworms in one rotten apple!)
 C. elegans first multicellular animal to have its complete DNA sequenced
 Also have Cephalization (Brain)

Nematoda

Life cycle of Nematode animal parasite

Annelida (Segmented worms)
 Coelomate – have a true middle layer (most advanced)
 Segmented
 Bilateral Symetry
 Complete digestive tract
 Closed circulatory system
(earthworms have 5 hearts)
 Paired setae (bristles) for movement
 Cephalization

Earthworms - Oligochaetes
 True body cavities and segmentation are important evolutionary adaptations!!

Marine worms - Polychaetes
 Many setae brushlike structures
 Some have tentacles and eyes

Marine Worms - Polychaetes

Leeches - Hirudinea
 No or very few setae
 Mostly parasitic one-fourth carnivorous