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CHAPTER 32
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General Features of Animals
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3
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Evolution of the Animal Body Plan
• Five key transitions can be noted in animal
evolution
1.
2.
3.
4.
5.
Tissues
Symmetry
Body cavity
Development
Segmentation
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Evolution of the Animal Body Plan
1. Evolution of tissues
-Parazoa (Sponges – the simplest animals)
lack defined tissues and organs
-Have the ability to disaggregate and
aggregate their cells
-Eumetazoa (all other animals) have
distinct and well-defined tissues
-Have irreversible differentiation for
most cell types
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Evolution of the Animal Body Plan
2. 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
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Evolution of the Animal Body Plan
-Radial symmetry
-Body parts arranged around central axis
-Can be bisected into two equal halves in
any 2-D plane
-Bilateral symmetry
-Body has right and left halves that are
mirror images
-Only the sagittal plane bisects the
animal into two equal halves
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Top
Back
Front
Bottom
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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
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
3. Evolution of a body cavity
-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
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-Called the coelom
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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
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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
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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
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Evolution of the Animal Body Plan
Deuterostomes differ from protostomes in three
other fundamental embryological features:
-1. Cleavage pattern of embryonic cells
-Protostomes = Spiral cleavage
-Deuterostomes = Radial cleavage
-2. Developmental fate of cells
-Protostomes = Determinate development
-Deuterostomes = Indeterminate
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development
Evolution of the Animal Body Plan
-3. Origination of coelom
-Protostomes = Forms simply and directly
from the mesoderm
-Deuterostomes = Forms indirectly from
the archenteron
Deuterostomes evolved from protostomes
more than 500 MYA
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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 animals
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Traditional Classification of Animals
Multicellular animals, or metazoans, are
traditionally divided into 36 or so distinct phyla
based on shared anatomy and embryology
Metazoans are divided into two main branches:
-Parazoa = Lack symmetry and tissues
-Eumetazoa = Have symmetry and tissues
-Diploblastic = Have two germ layers
-Triploblastic = Have three germ layers
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A New Look At Metazoans
Recent analysis of the translation machinery
revealed that myzostomids have no close
link to the annelids at all
-Instead, they are
more closely allied
with the flatworms
(planaria and
tapeworms)
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A New Look At Metazoans
Therefore, key morphological characters
used in traditional classification are not
necessarily conservative
Molecular systematics uses unique
sequences within certain genes to identify
clusters of related groups
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A New Look At Metazoans
Most new phylogenies agree on two
revolutionary features:
1. Separation of annelids and arthropods
into different clades
2. Division of the protostome group into
Ecdysozoa and Spiralia
-The latter is then broken down into
Lophotrochozoa and Platyzoa
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A New Look At Metazoans
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Evolutionary Developmental Biology
Molecular analysis may also explain the
Cambrian explosion
-The enormous expansion of animal
diversity in the Cambrian period (543–525
MYA)
-The homeobox (Hox) developmental gene
complex evolved
-Provided a tool that can produce rapid
changes in body plan
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Evolutionary Developmental Biology
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