Overview of Animal Diversity

1
Overview of Animal Diversity

• Chapter 32

2
General Features of Animals

• Animals are a diverse group of consumers that
  share major characteristics
• 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

3
Evolution of the Animal Body Plan

• Five key transitions in animal evolution
• Tissues
• Symmetry
• Body cavity
• Development
• Segmentation

4
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

5
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 animals 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
• Cephalization
• Evolution of a definite brain area
• Greater mobility

9
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
• Acoelomates have no body cavity

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Evolution of the Animal Body Plan

• Pseudocoelomates have a body cavity between
  mesoderm and endoderm
• Called the pseudocoel

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Evolution of the Animal Body Plan

• 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
18
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
• Separation of annelids and arthropods into
  different clades
• 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
Examples can be found in Table 32.2 of Raven et
al.
25
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