A comparative overview of the Animal Kingdom

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A comparative overview of the Animal Kingdom
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General Features of Animals

• Animals are the consumers of the Earth
• They are a very diverse group
• -However, they share major characteristics
• -Are heterotrophs
• -Are multicellular
• -Have cells without cell walls
• -Most are able to move

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General Features of Animals

• -Are very diverse in form and habitat
• -Most reproduce sexually
• -Have a characteristic pattern 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 can be noted in animal
  evolution
• 1. Tissues
• 2. Symmetry
• 3. Body cavity
• 4. Development
• 5. 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 animals 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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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

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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 pseudocoel
• -Coelomates 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 two-layer- thick
  ball 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 fundamental embryological features
• -1. Cleaveage pattern of embryonic cells
• -Protostomes Spiral cleavage
• -Deuterostomes Radial cleavage
• -2. Developmental fate of cells
• -Protostomes Determinate development
• -Deuterostomes Indeterminate development

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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

• 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

• 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

• 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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Evolutionary Developmental Biology

• Molecular analysis may also explain the Cambrian
  explosion
• -The enormous expansion of animal diversity in
  the Cambrian period (543 to 525 MYA)
• -The homeobox (Hox) developmental gene complex
  evolved
• -Provided a tool that can produce rapid
  changes in body plan

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