1. B IL A T E R A L
S YM M E T R Y

2. In t r o d u c t io n t o
B ila t e r ia
Protostome Bilateria
Subkingdom: Eumetazoa
• Tissues, symmetry, digestive tract, muscles and
nerves
Group Bilateria
Bilateral symmetry, triploblastic, organs and organ
system

4. B ila t e r a l
S ym m e tr y
B ila t e r ia n - Organisms having Bilateral
Symmetry
Animals that are similar to the front and back, upside
and downside.
An animal is considered to have Bilateral Symmetry if
only one plane of bisection of the midsagittal or
midplane produces right and left mirror halves.
Streamlining is allowed by Bilateral Symmetry.

5. E n c o u n t e r in g
R e s ourc e s
Most animals, including Humans, are bilaterally
symmetric. They belong to the group called Bilateria.
Bilateral Symmetry is also known as Plane Symmetry.
Bilateral Symmetry may have evolved when radial
animals began to move up the surface.
The oldest known bilateral animal is the
Ve r n a n im a lia c u la .

6. E x a m p le s
dddd

7. B ila t e r ia B o d y
D e s ig n
Bilateral Symmetry
Cephalized
• Sensory organs concentrated in the head
Triploblastic
Three basic tissue layers

8. B ila t e r a l
S ym m e tr y

9. B ila t e r a l A n im a l
L in e s
Must have evolved in Precambrian
First fossils are Cambrian
Spiralian (3rd Cleave Division is spiral)

12. C e p h a liz a t io n
The differentiation of the front/anterior end of an
organism into a definite head is called Cephalization. It
is considered an evolutionary advancement, and is
accompanied by the concentration of nerve tissues as
well as feeding organisms at the head region with the
purpose of integrating actions/movements in the
nervous system.

13. Ta r g e t i n g
R e s ourc e s
Cephalization allows the anterior end of the animal to
first encounter food, predators and other external
factors concerning their environment. This is because
the brain as well as sensory organs can be found on
the anterior end. This helps protect and alarms the
organism 24/7.

14. M o t ile B a c t e r ia
Many, however, not all bacteria is able to display
motility (ex. self-propelled motion) under
circumstances. There are many different types of
motility:
Flagella (sing. Flagellum)
Copious Slime
Internal Structure (ex. Axial Filament)

15. S e s s ile B a c t e r ia
Bacteria that is not freely able to move about.

16. M u s c u la t u r e
Bilaterians have complete digestive tracts along with a
separate mouth and anus. (Except for some red forms
of bilaterians.) They have an internal body cavity
called a Coelome.
They are Triploblastic which means they have three
tissue layers consisting of the: ectoderm, mesoderm
and endoderm.

17. C o m p a r t m e n t a liz
a t io n
Compartmentalization or Cellular Compartments
comprise of all the closed parts within the Cytosol of
the (Eukaryotic) cell of an organism. It has 5 types
namely the:
Nuclear compartments including the nucleus.
Intercisternal Space
Organelles
Cytosol

18. R e g u la t e d
C o mp a rtme nts
Epithelia is maintains the regulated compartments
Coelenteron or the gut has evolved in cnidarians and
has multiple functions in the taxon.
The bilaterian gut is dedicated to digestion. Therefore
other spaces must assume the responsibility for the
other functions.
Hemal System = new role for the connective tissue.

19. C o e lo m s
A new fluid filled cavity
It is lined by Mesothelium or Middle Epithelium
Contains a third regulated compartment
It is filled with a fluid called Coelomic Fluid
Functions as a Hydrostat, Transport, Excretion,
Reproduction
Animals with coeloms are called Coelomates

20. C o e lo m F o r m a t io n

21. A lt e r n a t iv e
C o e lo m ic
C o n d it io n s

22. B ila t e r ia
Most animals are considered to be Bilaterians. They
are also known to be the most familiar.
They can be found almost anywhere: seas, land,
freshwater and even on air.
It is important to consider separately the small and
large Biletarians.

23. D e v e lo p m e n t :
C le a v a g e
Radial/Regulative: Cleavage planes are either parallel
or perpendicular to the polar axis of the egg.
Spiral/Determinate: Cleavage planes are oblique to
the polar axis of the egg.
Bilateral/Determinate: Cleavage planes are oblique to
the polar axis of the egg.

24. C n id a r ia
Comes from the Greek word “Cnidos” which means
stinging nettle
Diverse in form
Armed with stinging cells called Nematocysts;
inherited from a common ancestor
Lives in the world’s oceans

25. H e m a l S ys te m
A system of tubes and ducts which
morphologically parallels that of the water
vascular system in echinoderms, but lies just
orally to it. Its function is not known, but it
seems clear that it does not perform roles
typically associated with a blood-vascular system
in other animals. Possible functions ascribed to
it are internal defense and endocrine, but direct
evidence for either is lacking.

26. B ila t e r ia B o d y
F e a ture s
Bilateral Symmetry
Triploblasty
A Coelome
A Through-Gut
CNS or Central Nervous System