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Overview of Animal Diversity
Chapter 32
General Features of Animals
Animals are a diverse group of consumers
that share major characteristics
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All are heterotrophs
All are multicellular
Cells do not have cell walls
Most are able to move
All are very diverse in form and habitat
Most reproduce sexually
Have a characteristic patterns of embryonic
development
• Cells of all animals (except sponges) are organized
into tissues
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Evolution of the Animal Body Plan
Five key transitions in animal evolution
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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
– Parazoa (sponges) lack any definite symmetry
– Eumetazoa have a symmetry defined along
imaginary axes drawn through the animal’s
body
There are two main types of symmetry
– Radial symmetry
– Bilateral 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
2D plane perpendicular to that axis
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Evolution of the Animal Body Plan
• Bilateral symmetry
– Body has right and left halves that are mirror
images
– Body has distinct anterior/posterior and
dorsal/ventral divisions
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Evolution of the Animal Body Plan
• Bilaterally symmetrical animals have
two main advantages over radially
symmetrical animals
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)
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Evolution of the Animal Body Plan
3. Evolution of a body cavity
• Three basic kinds of body plans
a. Acoelomates have no body cavity
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Evolution of the Animal Body Plan
b. Pseudocoelomates have a body cavity
between mesoderm and endoderm
• Called the pseudocoel
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Evolution of the Animal Body Plan
c. Coelomates have a body cavity entirely within
the mesoderm
• Called the coelom
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Evolution of the Animal Body Plan
• The body cavity made possible the
development of advanced organs 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
– Blastula indents to form a 2-layer-thick ball called
a gastrula with:
• Blastopore - Opening to outside
• Archenteron - Primitive body cavity
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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 embryological features:
– Cleaveage pattern of embryonic cells
• Protostomes - Spiral cleavage
• Deuterostomes - Radial cleavage
– Developmental fate of cells
• Protostomes - Determinate development
• Deuterostomes - Indeterminate development
– Origination of coelom
• Protostomes - Forms simply and directly from the mesoderm
• Deuterostomes - Forms indirectly from the archenteron
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Evolution of the Animal Body Plan
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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
• The traditional animal phylogeny is being
reevaluated using molecular data
• Myzostomids are marine animals that are
parasites of echinoderms
• Have no body cavity and only incomplete
segmentation and so have been allied with
annelids
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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
• It seems that key morphological characters
used in traditional classification are not
necessarily correct
• 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
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The latter is then broken down into
Lophotrochozoa and Platyzoa
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A New Look At Metazoans
Examples can be found in Table 32.2 of Raven et al.
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Evolutionary Developmental Biology
• Most taxonomists agree that the animal
kingdom is monophyletic
• Three prominent hypotheses have been
proposed for the origin of metazoans from
single-celled protists
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Evolutionary Developmental Biology
1. The multinucleate hypothesis
2. The colonial flagellate hypothesis
3. The polyphyletic origin hypothesis
• Molecular systematics using rRNA sequences
settles this argument in favor of the colonial
flagellate hypothesis
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