1. Introduction to Platyhelminthes
Biology 11
Mrs. Trevelyan

2. Platyhelminthes 1. PRESCRIBED LEARNING OUTCOMES and VOCABULARY
2. VIDEOS:
None 
3. TEXTBOOK PGS
Section 26.3 pgs

4. Overview…

5. Examples of Members:

6. Marine Flatworms are Beautiful!
Video: Marine Flatworm Swimming

7. Three Classes of Flatworms
1. Turbellaria: Free-living flatworms, mostly predators, living in water or moist environments
2. Trematoda: Flukes, all are parasitic and have complex life cycles
2. Cestoda: Tapeworms, all are parasitic and have complex life cycles

8. 1. Unifying Characteristics
Bilateral symmetry, no segmentation
First to have 3 germ layers!
Outer layer (epidermis) is soft and ciliated (has tiny hairs) or covered with a cuticle
Parasitic species have a mouth with an external sucker or hook for attachment to a host
Digestive system is incomplete with only one opening
Muscle layer is well developed (can swim)
No skeleton, circulatory or respiratory systems
Hermaphroditic, use internal fertilization

9. 2. Structure
Cephalisation has occurred (have a head with a concentration of nerve cells)
Has 3 cell layers (ectoderm , mesoderm and endoderm) with specialized cells and tissues
Development of more complex tissues and organs (e.g. Pharynx, eyespot, muscle layers etc)

10. 2. Structure
Some freshwater species have primitive eyespots and can detect the presence of light
E.g. Planarians
This eyespot is called an OCELLI and is NOT a true eye

11. 3. Digestion
One opening (“mouth” with a gastrovascular cavity that is often highly branched throughout the body)
Free-living are carnivores or scavangers- pharynx comes out of mouth and “vacuums” up food, also “spits out” solid waste
Parasitic flatworms anchor themselves to their host and live off of host tissue/blood
Video: Planarian eating

12. Digestion- some are PARASITIC
Tapeworm in an intestine
Most do not have a complete digestive system because they absorb digested material directly

13. 4. Respiration/Excretion
No Respiratory organs
Exchange gases by diffusion (hence small, flat shape)
EXCRETION
Solid wastes exit the body through the
“mouth”
Gas wastes exit the body through diffusion
Freshwater flatworms have FLAME CELLS
that excrete excess water from the body

14. 5. Nervous System More advanced than cnidarians
Have cephalization and cerebral ganglia (brain-like structure)
Nerve cords are organized like a ladder (no netting)

15. 6. Reproduction Flatworms are hermaphroditic (both male and female)
Can reproduce aseuxally or sexually
ASEXUAL REPRODUCTION:
Free-living freshwater flatworms can do this by:
Fission- organism splits in two
Regeneration- a single part can grow into a new worm

16. 6. Reproduction SEXUAL REPRODUCTION
Freshwater free-living flatworms use internal fertizilation
Flatworms “join up” in pairs to reproduce sexually. One delivers sperm, one receives sperm. Eggs are laid and hatch in a few weeks
Video: Marine Flatworms Penis Fencing:

17. 6. Reproduction
Parasitic flatworms have a much more complex life cycle, often with more than one host (e.g. infects snails and humans)
See and know figure on page 710

18. 7. Additional Interesting Facts
flatworms sometimes growing to lengths of 40 feet
Penis fencing- nuff said