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Overview of Animal Diversity
Chapter 31
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General Features of Animals
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Heterotrophs
Multicellular
Able to move from place to place
Diverse in form and habitat
Sexual reproduction
Characteristic embryonic development
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Diverse Kingdom
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Traditional classification of animals
– multicellular animals, metazoans,
traditionally divided into 35 distinct phyla
 First branch - tissues
 Parazoa lack definite symmetry and do
not possess tissues or organs.
 Eumetazoa have definite shape and
symmetry and usually have organs and
organ systems.
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Traditional Classification of Animals
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Second branch - symmetry
– Eumetazoan branch has two principles
branches.
 Radiata - radial symmetry
 Bilateria - bilateral symmetry
– Further branches were assigned by
comparing key shared features of the body
plan.
 body cavity
 coelom
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Broad Groupings of Kingdom Animalia
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Key Transitions in Body Plan
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Evolution of tissues
– first key transition in animal body plan
Evolution of bilateral symmetry
– radial symmetry - regular arrangement of
parts around central axis
– bilateral symmetry - right and left halves
form mirror images
 dorsal vs. ventral
 anterior vs. posterior
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Radial and Bilateral Symmetry
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Bilateral Symmetry
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Bilaterally symmetrical eumetazoans produce
three germ layers.
– ectoderm
– endoderm
– mesoderm
Much of the nervous system is in the form of
major longitudinal nerve cords.
– ultimately led to evolution of definite head
 cephalization
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Three Body Plans
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Key Transitions in Body Plan
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Evolution of a body cavity
– Presence of a body cavity allows digestive
tract to be larger and longer.
 storage of undigested food
 more complete digestion
 more space for gonads to expand
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Key Transitions in Body Plan
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Kinds of body cavities
– acoelomates - no body cavity
– pseudocoelomates - possess pseudocoel
– coelomates - possess coelom
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Coelomates
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Coelom poses circulation problem
– solved by circulatory system
 open circulatory system
 Blood passes from vessels into
sinuses, mixes with body fluid, and
reenters vessels in another location.
 closed circulatory system
 Blood is physically separated from
other body fluids and can be separately
controlled.
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Key Transitions in Body Plan
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Advantages of a coelom
 Allows contact between mesoderm and
endoderm, so that primary induction can
occur during development.
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Key Transitions in Body Plan
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Evolution of deuterostome development
– mitotic egg division leads to blastula
 indents to form blastopore, opening to the
archenteron
Bilaterians can be divided into protostomes
(mouth-first) and deuterstomes (mouthsecond).
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Protostome and Deuterostome Differences
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Cleavage
– spiral
– radial
Fate of embryonic cells
– determinate - predetermined fate
– indeterminate - identical daughter cells
Fate of blastopore
– mouth or anus develops near blastopore
Formation of coelom
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Embryonic Development
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Key Transitions in Body Plan
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Evolution of segmentation
– advantages
 Each segment may go on to develop a
more or less complete set of adult organs.
 Locomotion is far more effective when
individual segments can move
independently due to flexibility of
movement.
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Animal Classification is Being Reevaluated
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New look at metazoan family tree
– New taxonomical comparisons using
molecular data have come to new,
different conclusions.
 hint that 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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Roots of the Animal Family Tree
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Origins of metazoans
– Most taxonomists agree the animal
kingdom is monophyletic.
 Three prominent hypotheses exist for
origin from single-celled protists.
 multinucleate hypothesis
 colonial flagellate hypothesis
 polyphyletic origin hypothesis
– debate over cause of Cambrian explosion
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