1. Radial vs. Bilateral Symmetry

2. Cephalization
Bilateral Symmetry usually has led to cephalization—the process by which sensory organs and appendages became localized in the head end of animals.

3. Evolutionary Trends
If we analyze the basic body plans of animals, we find that they illustrate evolutionary trends.
Four major “advances” (in order):
Multicellular body plan (tissues)
Bilaterally symmetrical body plan
“Tube-within-a-tube” body plan
Coelomate body plan

4. 3 Major Bilateral Body Plans
Acoelomates
Pseudocoelomates
Coelomates
Each plan consists of 3 cell layers: endoderm, mesoderm, ectoderm

5. Body Cavities

6. Acoelomates These animals have no other cavity than the gut.
They are often called the “solid worms.”

7. Pseudocoelomates
These animals have a body cavity (the pseudocoelom) which is not completely lined with mesoderm.
The “tube within a tube” body plan.
This category is also composed of mostly worms.

8. Coelomates
These animals have a “true coelom” lined with mesodermal peritoneum.
Most animals are coelomate.

9. Platyhelminthes

10. Introduction to Planaria

11. Flattened Body, one opening

12. Introduction to Flatworms
Phylum Platyhelminthes: platy = flat, helminth = worm
Simplest animals with bilateral symmetry
Body has 3 cell layers
ectoderm (outer)
mesoderm (middle)
endoderm (inner)
Show the beginnings of cephalization (i.e. a head)
Can be free-living or parasitic
ectoderm
mesoderm
endoderm
gastrovascular cavity

13. Bilateral Symmetry

14. Sexual or Asexual Reproduction

15. Form and Function of Flatworms
Digestive system:
Gastrovascular cavity where food is broken down
Two-way digestive system
Food and wastes enter/exit the pharynx
e.g.Planaria
Pharynx = opening to the gastrovascular cavity

16. Respiratory system:
Circulatory system:
Excretory system:
Flame cells - primitive excretory structures that get rid of excess water
Metabolic wastes diffuse through the skin
No organized systems for these
O2, CO2 diffuse through the ectoderm

17. Planaria nervous system
Fairly well-developed
Has primitive “brain” and
2 ventral nerve cords
Ocelli (aka eyespots) to
detect light/dark
Two auricles sensitive to touch
and chemicals
Musculoskeletal system:
Muscles and cilia on ventral surface for locomotion
auricle
Planaria nervous system

18. If you cut a part off, it grows into a new worm!
Free-living forms are hermaphrodites
Asexual reproduction - fission/regeneration
Sexual reproduction - exchanging sperm between 2 worms
Parasitic worms reproduce sexually only
Each worm produces huge numbers of sperm and eggs
If you cut a part off, it grows into a new worm!

19. Planaria reproductive system

20. Budding

21. Some are parasites

22. Tape worm mouth parts

23. Class Monogenea Parasitic flatworms with one main host
Most infect the gills of fish

24. Classes of Flatworms Free-living, usually small, freshwater
Class Turbellaria
Free-living, usually small, freshwater
Predators or scavengers
Example member: Planaria

25. Classes of Flatworms
2) Class Trematoda (flukes)
Parasitic (internal parasites)  often infect blood and organs
Often have more than 1 host
Reduced digestive and nervous systems
Example member: blood flukes (Schistosoma);
has 2 hosts (p. 687)
humans = primary host
snails = intermediate host

26. Class Trematoda - blood fluke Schistosoma
primary host: human
Intermediate host: snail

27. Classes of Flatworms 3) Class Cestoda (tapeworms)
Long and flat parasites
Head is called scolex
Narrow neck region that divides to form proglottids (contain huge numbers of sperm and eggs) (beef tapeworm)
No digestive
(absorb through skin- tegument)
or nervous systems
Highly developed reproductive
system
proglottid
scolex

28. Cestode (tapeworm) l.c.