Overview of Animal Diversity

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Overview of Animal Diversity

• Chapter 32

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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

• 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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Evolutionary Developmental Biology