Patterns of Organization
1. ASYMMETRICAL, RADIAL, OR BILATERAL
2. UNICELLULAR, DIPLOBLASTIC, OR
TRIPLOBLASTIC
3. ACOELOMATE, PSUEDOCOELOMATE, OR
COELOMATE
The Three Arrangement of Appendages
 Asymmetrical – appendages are arranged in no set
pattern.
 Examples – sea sponges
& many protists
 These animals do not have
complex sensory,
communication, or
locomotor functions.
Arrangement of Appendages
 Radial – appendages are arranged in a circular or
spherical pattern
 Examples – octopus, sea stars, sea urchins, etc.
 Advantages – can sense predators or prey coming
from any direction.
 Disadvantages – these animals are usually sedentary
or have minimal locomotion.
Arrangement of Appendages
 Bilateral – Animals with a distinct left and right
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side, front and back, and head and foot regions.
Examples – us, dogs, whales, squirrels, tigers, etc.
Advantages – have excellent locomotion.
Disadvantages – can only see the world primarily in
one direction.
The evolution of a distinct “head” with many sensory
inputs is called cephalization.
Unicellular, Diploblastic, or Triploblastic
 Unicellular – the whole animal is just one tiny cell.
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Example – protists
Diploblasts – Animals who only have an endoderm and
ectoderm, no mesoderm. Only have tissue level
organization, no real organs. Examples – jellyfish, hydra.
Picture
Triploblasts – animals with all three layers of tissue and
true organ systems. Usually display bilateral symmetry.
Examples – us, horses, cows, llamas, etc.
Triplobalsts are further classified based upon whether or
not they have an internal body cavity!
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Advantages of Having an Internal Body Cavity
 Body cavity – a fluid-filled space in which the
internal organs can be suspended and separated
from the body wall.
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1. provides more room for organs to grow and develop.
2. allows for an increase in overall body size.
3. provides more surface area for gas, nutrient, and waste
exchange.
4. acts as a hydrostatic skeleton.
Acoelomates, Pseudocoelomates, & Coelomates
 Acoelomates – triploblasts with no body cavity,
flatworms.
 Pseudocoelomates – triploblasts with a body cavity
that is NOT connected to their mesoderm,
roundworms. Pseudo means “false”.
 Coelomates – triploblasts with a body cavity that IS
connected to their mesoderm, most animals.