ANIMAL DIVERSITY AND THE EVOLUTION OF BODY PLANS
GENERAL FEATURES OF ANIMALS
• Heterotrophy - obtain energy and organic
molecules by ingesting other organisms
• Multicellularity - Many have complex bodies
• No cell walls - They lack rigid cell walls and are
usually flexible
• Active movement - Move more rapidly and in more
complex ways
• Diversity of form - Vary greatly in form, ranging in
size from organisms too small to see with the
unaided eye to enormous2
GENERAL FEATURES OF ANIMALS
• Diversity of habitat - Grouped into 35–40 phyla,
most that occur only in the sea but many occur in
fresh water and on land
• Sexual reproduction - Most animals reproduce
sexually. Animal eggs, which are nonmobile.
• Embryonic development - Zygote first undergoes a
series of mitotic divisions that produces a ball of
cells
• Tissues - Cells of most animals are organized into
structural and functional units called tissues3
4
Some characteristics shared by animals include
mobility, diversity of shapes and sizes, and
development that involves complex cleavage
patterns
a. c.
b.
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a: © Royalty-Free/Corbis; b: © Edward S. Ross; c: © Cabisco/Phototake
EVOLUTION OF THE ANIMAL BODY PLAN
• Five key transitions can be noted in animal evolution
1. Symmetry
2. Tissues
3. Body cavity
4. Patterns of Development
5. Segmentation
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1. Evolution of symmetry
Sponges also lack
any definite symmetry
Eumetazoa have a
symmetry defined
along an imaginary
axis drawn through
the animal’s body
There are two
main types of symmetry 6
Evolution of the Animal Body Plan
Radial symmetry (i.e. jellyfish, anemones, corals)
Body parts arranged around central axis
Can be bisected into two equal halves in any
2-D plane
Bilateral symmetry (i.e. worms, octopus, spiders)
Body has right and left halves that are mirror images
Only the sagittal plane bisects the animal into two
equal halves
7
Evolution of the Animal Body Plan
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a.
Radial Symmetry
b.
Bilateral Symmetry
Ventral
Dorsal
Frontal plane
Sagittal plane
Anterior
Posterior
Transverse
plane8
Top
Bottom
Back
Front
Comparison of
radial and
bilateral
symmetry
EVOLUTION OF THE ANIMAL BODY PLAN
Bilaterally symmetrical animals have two main
advantages over radially symmetrical ones
1. Cephalization
Evolution of a definite brain area
2. Greater mobility
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EVOLUTION OF THE ANIMAL BODY PLAN
2. Evolution of tissues
Parazoa (Sponges – the simplest animals) lack
defined tissues and organs
• Have the ability to disaggregate and aggregate
their cell
Eumetazoa (all other animals) have distinct and well-
defined tissues
Have irreversible differentiation for most cell
types10
EVOLUTION OF THE ANIMAL BODY PLAN
3. Evolution of a body cavity
Eumetazoa produce three germ layers
Outer ectoderm (body coverings and nervous system)
Middle mesoderm (skeleton and muscles)
Inner endoderm (digestive organs and intestines)
Body cavity = Space surrounded by mesoderm tissue that is
formed during development
11
EVOLUTION OF THE ANIMAL BODY PLAN
Three basic kinds of body plans
Acoelomates = No body cavity
Pseudocoelomates = Body cavity between
mesoderm and endoderm
• Called the pseudocoelom
Coelomates = Body cavity entirely within the
mesoderm
• Called the coelom
12
13
Three body plans
for bilaterally
symmetrical
animals
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Flatworm
Pseudocoelomate
RoundwormPseudocoelom
Coelomate
AnnelidCoelom
Ectodermally
derived tissue
Endodermally
derived tissue
Mesodermally
derived tissue
Digestive
cavity
Ectodermally
derived tissue
Endodermally
derived tissue
Mesodermally
derived tissue
Acoelomate
Endodermally
derived tissue
Ectodermally
derived tissue
Digestive
cavity
Mesodermally
derived tissue
Digestive
cavity
EVOLUTION OF THE ANIMAL BODY PLAN
The body cavity made possible the development of
advanced organ systems
Coelomates developed a circulatory system to flow
nutrients and remove wastes
Open circulatory system: blood passes from vessels
into sinuses, mixes with body fluids, and reenters the
vessels
Closed circulatory system: blood moves
continuously through vessels that are separated from
body fluids 14
EVOLUTION OF THE ANIMAL BODY PLAN
4. Evolution of different patterns of development
- The basic Bilaterian pattern of development:
- Mitotic cell divisions of the egg form a
hollow ball of cells, called the blastula
- Blastula indents to form a two-layer-
thick ball with:
- Blastopore = Opening to outside
- Archenteron = Primitive body cavity
15
EVOLUTION OF THE ANIMAL BODY PLAN
Bilaterians can be divided into two groups:
Protostomes develop the mouth first from or near the
blastopore
• Anus (if present) develops either from blastopore or another
region of embryo
• Deuterostomes develop the anus first from the
blastopore
• Mouth develops later from another region of the embryo
16
EVOLUTION OF THE ANIMAL BODY PLAN
5. Evolution of segmentation
- Segmentation provides two advantages
1. Allows redundant organ systems in adults such as
occurs in the annelids
2. Allows for more efficient and flexible movement
because each segment can move
independently
- Segmentation appeared several times in the evolution of
animals17
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