Animal Origins and the Evolution of Body Plans

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Animal Origins and the Evolution of Body Plans
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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• Traits that distinguish the animals
• All are multicellular and undergo development
  from a single cell.
• All are heterotrophs use internal digestion
  processes.
• Most can move have specialized muscle tissues.

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What Evidence Indicates the Animals Are
Monophyletic?

• The animals are monophyletic. They share many
  derived traits. Evidence
• Gene sequences, such as those for rRNA, support
  monophyly.
• Similar organization and function of Hox genes.
• Hox (homeobox) genes are that control development
  along the anterior-posterior axis

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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• Cells have unique junctions tight junctions,
  desmosomes, and gap junctions.
• Animals have a common set of extracellular matrix
  molecules, including collagen and proteoglycans.

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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• The ancestor of the animal clade was probably a
  colonial flagellated protist.
• Functional specialization of cells in the colony
  arose and cells continued to differentiate.
• Coordination among cells may have been improved
  by regulatory molecules eventually leading to
  larger, more complex animals.

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Figure 31.1 A Current Phylogenetic Tree of Animals
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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• Patterns of embryonic development were
  traditionally used to study animal phylogeny.
• Cleavage patterns (first few divisions of the
  zygote) distinguish some animal groups.
• The patterns are influenced by configuration of
  the yolk.

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Figure 43.3 Patterns of Cleavage in Four Model
Organisms (Part 1)
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Figure 43.3 Patterns of Cleavage in Four Model
Organisms (Part 2)
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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• Reptiles have incomplete cleavage the dividing
  cells form an embryo on top of a yolk mass.
• Sea urchins have a complete cleavage pattern
  known as radial cleavage.
• Lophotrochozoans have spiral cleavage, a derived
  form of radial cleavage (spiralians).

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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• Distinct layers of cells form in early
  development.
• Diploblastic animals have two cell
  layersectoderm and endoderm.
• Triploblastic have three cell layersecto-,
  endo-, and mesoderm.

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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• In many animals, gastrulation results in a hollow
  ball one cell thick, with an indent.
• Opening to the cavity formed by the indent is the
  blastopore.
• In triploblastic animals, there are two patterns
  of development after this point.

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Figure 31.2 Gastrulation Illuminates Evolutionary
Relationships
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31.1 What Evidence Indicates the Animals Are
Monophyletic?

• Protostomes (mouth first) the blastopore
  develops into the mouth.
• Deuterostomes (mouth second) the blastopore
  develops into the anus the mouth develops later.
  This is thought to be the ancestral condition.

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31.2 What Are the Features of Animal Body Plans?

• Body plan the general structure, arrangement of
  organ systems, and integrated functioning of
  parts.
• Four key features
• Symmetry
• Body cavity
• Segmentation
• External appendages

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31.2 What Are the Features of Animal Body Plans?

• The regulatory genes that govern development of
  these key features are widely shared across
  animal groups.
• Thus, animals also share body plans.

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31.2 What Are the Features of Animal Body Plans?

• Symmetry is overall shape.
• An animal is symmetrical if it can be divided
  into similar halves on at least one plane.
• If notasymmetricmany sponges.

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31.2 What Are the Features of Animal Body Plans?

• Spherical symmetry body parts radiate out from a
  single point common in unicellular protists.
• Radial symmetry one main axis around which body
  parts are arranged ctenophores and cnidarians.

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Figure 31.3 Body Symmetry
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31.2 What Are the Features of Animal Body Plans?

• Animals that move in one direction have bilateral
  symmetry can be divided into similar halves on
  only one plane.
• The plane runs from the anterior end to the
  posterior end (tail).
• A plane at right angles to the midline divides
  animals into dorsal and ventral (belly) surfaces.

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31.2 What Are the Features of Animal Body Plans?

• Bilateral symmetry is associated with
  cephalization concentration of sensory organs
  and nerve tissues at the anterior end or head.
• The anterior end encounters the environment
  first. Cephalization has been evolutionarily
  favored.

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31.2 What Are the Features of Animal Body Plans?

• Based on presence of an internal, fluid-filled
  body cavity, animals can be divided into three
  types
• Acoelomate lack a fluid-filled body cavity.
  Space between gut and body wall is filled with
  cells called mesenchyme.

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31.2 What Are the Features of Animal Body Plans?

• Pseodocoelomate body cavity is a pseudocoel, a
  fluid-filled space in which organs are suspended.
  Muscles are only on the outside.
• Coelomate body cavity is a coelom, lined with a
  layer of muscle tissue called peritoneum which
  also covers the organs. More control over
  movements of fluids in the body cavity.

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Figure 31.4 Animal Body Cavities (Part 1)
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Figure 31.4 Animal Body Cavities (Part 2)
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Figure 31.4 Animal Body Cavities (Part 3)
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31.2 What Are the Features of Animal Body Plans?

• Body cavities can act as hydrostatic skeletons.
• When muscles contract, it pushes fluid to another
  part of the cavity, which causes that region to
  expand.
• If the animal has both circular and longitudinal
  muscles, it has even greater control over
  movement.

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31.2 What Are the Features of Animal Body Plans?

• Segmentation facilitates specialization of body
  regions.
• Also allows animal to alter body shape and
  control movements precisely.
• Segmentation evolved independently several times.
• Radiation of the arthropods was based on changes
  in a segmented body plan.

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Figure 31.5 Segmentation
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31.2 What Are the Features of Animal Body Plans?

• Appendages
• Locomotion is important for finding food, finding
  mates, and avoiding predators.
• Many echinoderms have tube feet.
• Limbs are highly specialized for rapid,
  controlled movements.
• Arthropods and vertebrates have jointed limbs.

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31.3 How Do Animals Get Their Food?

• Some animals rely on photosynthetic endosymbionts
  for food, but most must actively obtain food from
  the environment.
• The need to locate food has favored evolution of
  sensory structures and nervous systems to
  receive, process, and coordinate information.

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31.3 How Do Animals Get Their Food?

• To acquire food, energy must be expended.
• Sessile animals stay in one place, they must move
  the environment and food to themselves.
• Motile animals move through the environment.

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31.3 How Do Animals Get Their Food?

• Feeding strategies
• Filter feeders
• Herbivores
• Predators
• Parasites
• Detritivores

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31.3 How Do Animals Get Their Food?

• Filter feeders use straining devices to filter
  out small organisms and organic molecules from
  air or water.
• Many sessile, aquatic animals rely on water
  currents to bring food to them.
• Motile filter feeders, such as the flamingo,
  bring the food-containing medium to them.

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Figure 31.6 Filter Feeding Strategies
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31.3 How Do Animals Get Their Food?

• Some sessile filter feeders use energy to move
  water past food-capturing structures.
• Sponges bring water into their bodies by beating
  flagella of specialized cells called choanocytes.
• These cells link animals with choanoflagellate
  protists and fungi into a clade called
  opisthokonts.

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Figure 31.7 Even Sessile Filter Feeders Expend
Energy
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31.3 How Do Animals Get Their Food?

• Herbivores eat plants or parts of plants. Often
  the plant is not killed.
• Many kinds of herbivores may feed on a single
  plant.
• Land plants have tissues that are difficult to
  digest and chemicals that must be detoxified.
  Herbivores tend to have long, complex guts for
  digestion of plant materials.

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31.3 How Do Animals Get Their Food?

• Predators capture and subdue relatively large
  animals the prey.
• Predators have structures such as sharp teeth and
  claws, and well-developed sensory organs to
  detect prey.
• Many predators have toxic chemicals, (e.g., snake
  venom).
• Cnidarians have toxins in specialized stinging
  cells called nematocysts.

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Figure 31.10 Nematocysts Are Potent Weapons
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31.3 How Do Animals Get Their Food?

• Omnivores eat both plants and animals, (e.g.,
  humans, raccoons).
• Many animals change diets in different life
  stages, (e.g., birds that eat seeds as adults,
  but feed young on insects).

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31.3 How Do Animals Get Their Food?

• Parasites live in or on another animalthe host
  they obtain nutrients by consuming some part of
  the host.
• Parasites often much smaller than host, usually
  do not kill the host.
• Parasites often have complex life cycles.

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31.3 How Do Animals Get Their Food?

• Endoparasites live inside the host. Often have no
  digestive system, absorb food directly from host
  (e.g., flatworms).
• Ectoparasites live on the outside of the host.
  Often have mouthparts to pierce or suck hosts
  fluids (e.g., fleas and ticks).

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31.4 How Do Animal Life Cycles Differ?

• The life cycle encompasses embryonic development
  and all life stages.
• In direct development, newborns are very similar
  to adults but in most animals, newborns differ
  dramatically.
• A larva is an immature stage that differs from
  the adult. Many insects undergo metamorphosis, or
  radical changes between larva and adult stage.

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Figure 31.11 A Life Cycle with Metamorphosis
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31.4 How Do Animal Life Cycles Differ?

• Larvae and adults may feed on different foods,
  (e.g., caterpillars that eat leaves and adult
  butterflies that eat nectar).
• The larva may be specialized for feeding, the
  adult specialized for reproduction. Adults of
  some insects do not feed at all.

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31.4 How Do Animal Life Cycles Differ?

• All life cycles have a dispersal stage the
  animal moves or is moved from where it was born.
• Many sessile animals disperse during egg or
  larval stage.

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31.4 How Do Animal Life Cycles Differ?

• Sessile marine animals have a larvae called a
  trochophore. Others have a bilaterally
  symmetrical larva called a nauplius.
• Both types feed in the plankton before settling
  and becoming sessile.

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Figure 31.12 Planktonic Larval Forms of Marine
Animals
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31.4 How Do Animal Life Cycles Differ?

• Most motile animals disperse as adults, (e.g.,
  butterfly adults can fly to a new plant to lay
  eggs).

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31.4 How Do Animal Life Cycles Differ?

• Trade-offs characteristics of an animal in one
  life stage may improve ability for one activity,
  but reduce performance in some other activity.
• For example, energy devoted to building a shell
  cannot be used for growth.

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31.4 How Do Animal Life Cycles Differ?

• Trade-offs in reproduction
• Females can produce large numbers of small eggs
  (with small energy stores), or small numbers of
  large eggs (with large energy stores).

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31.4 How Do Animal Life Cycles Differ?

• The larger the energy store, the longer the
  animal can develop.
• In birds, incubation periods vary. In some
  species, young are helpless when hatched
  (altricial) and must be cared for by parents.
• Some species incubate longer, and hatchlings can
  forage right away (precocial).

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31.4 How Do Animal Life Cycles Differ?

• Parasites must expend energy to overcome the
  hosts defenses, and they must disperse to new
  hosts.
• Some parasite eggs pass out with hosts feces,
  and may be ingested by other hosts.
• Many parasites have more than one host, which may
  facilitate transfer among hosts.

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Figure 31.15 Reaching a New Host by a Complex
Route
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31.5 What Are the Major Groups of Animals?

• The Bilateria is a large monophyletic group that
  includes all animals except sponges, ctenophores,
  and cnidarians.
• Traits that support this monophyly are bilateral
  symmetry, three cell layers, presence of at least
  seven Hox genes.

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31.5 What Are the Major Groups of Animals?

• Bilateria are divided into protostomes and
  deuterostomes. These groups have been evolving
  separately for about 500 million years.

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Table 31.1 Summary of Living Members of the
Major Groups of Animals (Part 1)
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Table 31.1 Summary of Living Members of the
Major Groups of Animals (Part 2)
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31.5 What Are the Major Groups of Animals?

• Sponges are the most simple animals they have no
  cell layers and no organs.
• All other animals are called eumetazoans, with
  symmetry, a gut, nervous system, special cell
  junctions, and tissues in distinct cell layers.

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31.5 What Are the Major Groups of Animals?

• Sponges are not a clade, but have similar body
  organization that is ancestral.
• They have some specialized cells, but no tissues
  or organs.

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31.5 What Are the Major Groups of Animals?

• Sponges have skeletal elements called spicules.
• rRNA gene analysis suggest there are three
  groups.
• Glass sponges and demosponges have spicules of
  silicon dioxide.
• Calcareous sponges have spicules of calcium
  carbonate.

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31.5 What Are the Major Groups of Animals?

• The sponge body plan is an aggregation of cells
  around a water canal system.
• There is an extracellular matrix of collagen,
  adhesive glycoproteins, and other molecules.
• Most sponges are filter feeders a few trap prey
  on protruding hook-shaped spicules.

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Figure 31.7 Even Sessile Filter Feeders Expend
Energy
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31.5 What Are the Major Groups of Animals?

• Sponges reproduce asexually by budding and
  fragmentation.
• In sexual reproduction, water currents carry
  sperm from one individual to another.

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31.5 What Are the Major Groups of Animals?

• The ctenophores (comb jellies) have a radially
  symmetrical, diploblastic body plan.

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31.5 What Are the Major Groups of Animals?

• The two cell layers are separated by a thick,
  gelatinous mesoglea, and a complete gut.
• Ctenes are comb-like rows of fused cilia move
  through water by beating the cilia.
• All are marine and feed on small planktonic
  organisms. Simple life cyclefertilized egg
  develops into a small ctenophore.

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Figure 31.17 Comb Jellies Feed with Tentacles
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31.5 What Are the Major Groups of Animals?

• Cnidarians include jellyfish, sea anemones,
  corals, and hydrozoans.

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31.5 What Are the Major Groups of Animals?

• The cnidarian gut is a blind sac called the
  gastrovascular cavity. Functions in digestion,
  circulation, gas exchange, and as a hydrostatic
  skeleton.

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31.5 What Are the Major Groups of Animals?

• Cnidarian life cycle has two stages
• Sessile polyp stage stalk attaches to substrate
  polyps may reproduce by budding and form a
  colony.
• Motile medusa stage free-swimming produce
  gametes. Fertilized egg develops into a
  free-swimming ciliated larva or planula, which
  later settles to the bottom and grows into a
  polyp.

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Figure 31.18 The Cnidarian Life Cycle Has Two
Stages
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31.5 What Are the Major Groups of Animals?

• Cnidarians have simple nerve nets, and epithelial
  cells with muscle fibers that enable movement.
• They are predators, using toxins in the
  nematocysts to subdue prey.
• Corals and anemones have photosynthetic protists
  in their tissues that provide some of their
  nutrition.

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31.5 What Are the Major Groups of Animals?

• There are about 11,000 species of cnidarians,
  most are marine.
• Three of the clades are described
• Scyphozoans
• Anthozoans
• Hydrozoans

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31.5 What Are the Major Groups of Animals?

• Scyphozoans jellyfish
• Medusa stage dominates the life cycle.
  Individuals are male or female.
• Fertilized egg becomes a planula larva that
  settles quickly and grows into a polyp. The polyp
  buds off small medusae.

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31.5 What Are the Major Groups of Animals?

• Anthozoans sea anemones, sea pens, and corals.
• Sea anemones are solitary.
• Sea pens are colonial, with two types of polyps.
  Primary polyps anchor in the sediments, secondary
  feeding polyps are produced by budding.

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31.5 What Are the Major Groups of Animals?

• Corals are also colonial. Polyps of most species
  secrete a matrix of organic molecules on which
  they deposit calcium carbonateforms a skeleton.
• Living polyps form a layer on top of a growing
  mass of skeletal remains. Forms coral reefs and
  islands.

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What Are the Major Groups of Animals?

• Corals grow in clear, nutrient-poor tropical
  waters.
• They have photosynthetic, endosymbiotic protists.
  The corals receive nutrition from the protists
  the corals provide nutrients such as nitrogen,
  and provide a place to live.
• Coral reefs are threatened by global warming and
  polluted runoff from land.

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What Are the Major Groups of Animals?

• Hydrozoans
• In some species, polyps dominate, others have
  only medusae.
• Most are colonial the polyps are connected and
  share a single gastrovascular cavity.
• Some polyps are specialized for feeding, others
  produce medusae.

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Hydrozoans Often Have Colonial Polyps