Radial vs. Bilateral Symmetry
Cephalization
• Bilateral Symmetry usually has led to
cephalization—the process by which sensory
organs and appendages became localized in
the head end of animals.
Evolutionary Trends
If we analyze the basic body plans of animals,
we find that they illustrate evolutionary
trends.
Four major “advances” (in order):
1.
2.
3.
4.
Multicellular body plan (tissues)
Bilaterally symmetrical body plan
“Tube-within-a-tube” body plan
Coelomate body plan
3 Major Bilateral Body Plans
1.
2.
3.
•
Acoelomates
Pseudocoelomates
Coelomates
Each plan consists of 3 cell layers:
endoderm, mesoderm, ectoderm
Body Cavities
• http://www.youtube.com/watch?v=AJ_qZ6ld6
A0
Acoelomates
• These animals have
no other cavity than
the gut.
• They are often called
the “solid worms.”
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.
Coelomates
• These animals have a “true
coelom” lined with
mesodermal peritoneum.
• Most animals are
coelomate.
Platyhelminthes
Introduction to Planaria
• http://www.youtube.com/watch?v=w0QzSYQGsnA
Flattened Body, one opening
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
Bilateral Symmetry
Sexual or Asexual Reproduction
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
Respiratory system:
Circulatory system:
• No organized systems for these
• O2, CO2 diffuse through the
ectoderm
Excretory system:
• Flame cells - primitive excretory structures that get rid
of excess water
• Metabolic wastes diffuse through the skin
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
Planaria
nervous system
auricle
Musculoskeletal system:
• Muscles and cilia on ventral surface for locomotion
http://www.youtube.com/watch?v=wn3xluIRh1Y
Free-living forms are hermaphrodites
• Asexual reproduction - fission/regeneration
If you cut a
part off, it
grows into a
new worm!
• Sexual reproduction - exchanging sperm between 2 worms
• Parasitic worms reproduce sexually only
• Each worm produces huge numbers of sperm and eggs
Planaria
reproductive
system
Budding
Some are parasites
Tape worm mouth parts
Class Monogenea
• Parasitic flatworms with one main host
Most infect the gills of fish
Classes of Flatworms
1) Class Turbellaria
▫ Free-living, usually small, freshwater
▫ Predators or scavengers
▫ Example member: Planaria
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
Class Trematoda blood fluke
Schistosoma
primary
host:
human
Intermediate host: snail
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
proglottid
eggs) (beef tapeworm)
• No digestive
• (absorb through skin- tegument)
• or nervous systems
scolex
• Highly developed reproductive
system
Cestode (tapeworm) l.c.