An Introduction to Animals

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An Introduction to Animals

• Chapter 31

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An Introduction to Animals

• Animals are distinguished by two traits they
  eat and they move
• Largest and most abundant predators, herbivores,
  and detritivores in every ecosystem
• Find food by tunneling, swimming, filtering,
  crawling, creeping, slithering, walking, running,
  or flying

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Why Do Biologists Study Animals?
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Why Do Biologists Study Animals?

• They are interesting
• Animals are heterotrophs
• The dominant consumers in both aquatic and
  terrestrial habitats
• 10-50 million species of animals
• Range in size and complexity
• Most species-rich and morphologically diverse
  lineage of multicellular organisms

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• Humans depend on wild and domesticated animals
• Food
• Transportation
• Power

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How Do Biologists Study Animals?
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How Do Biologists Study Animals?

• Animals are a monophyletic group of
  multicellular eukaryotes
• 34 major animal phyla are recognized

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Defining Animals

• Four features define an animal's body plan, or
  basic architecture
• 1) The number of tissue types in embryos
• 2) The type of body symmetry
• 3) The presence or absence of a fluid-filled
  cavity
• 4) The way in which the earliest events of embryo
  development proceed

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The Evolution of Tissues

• All animals other than sponges have tissue
• Structural and functional units of cells
• Diploblasts are animals whose embryos have two
  types of tissues
• Triploblasts are animals whose embryos have three
  types of tissues
• Germ layers develop into distinct adult tissues

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Symmetry and Cephalization

• Animals with radial symmetry have at least two
  planes of symmetry.
• Organisms with bilateral symmetry have a single
  plane of symmetry
• Face their environment in one direction.

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Cephalization

• Bilateral symmetry allowed cephalization
• The evolution of a head, or anterior region,
• Where structures for feeding, sensing the
  environment, and processing information are
  concentrated

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Evolution of a Body Cavity

• Animals may or may not have coelom
• an internal, fluid-filled body cavity
• The coelom forms from within the mesoderm and is
  lined with cells from the mesoderm.

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Coelom

• Creates a medium for circulation, along with
  space for internal organs
• A hydrostatic skeleton is a fluid-filled chamber
• Allows an animal to move even without fins or
  limbs

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The Protostome and Deuterostome Patterns of
Development
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Coelomates

• All coelomates, except echinoderms, are
  bilaterally symmetric and have three embryonic
  tissue layers.
• Can be divided into two groups
• protostomes (arthropods, mollusks, and segmented
  worms)
• deuterostomes (vertebrates and echinoderms)
• Three events in early development (cleavage,
  gastrulation, and coelom formation) differ in
  protostomes and deuterostomes

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Phylogeny of Animals
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Phylogeny of Animals Based on Morphology

• A group of protists called the choanoflagellates
  are the closest living relatives of animals
• Porifera (sponges) are the most ancient animal
  phylum
• Radially symmetric phyla are next because their
  body plans are simple
• The acoelomates and pseudocoelomates appeared
  first, followed by the coelomates.

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Phylogeny of Animals Based on Morphology
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Using the Fossil Record

• Most major groups of animals appear in the
  fossil record about 580 million years ago
• Fossil record is generally consistent with the
  morphological phylogeny

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Evaluating Molecular Phylogenies

• Phylogenetic tree based on genes for ribosomal
  RNA and several proteins

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What Themes Occur in the Diversification of
Animals?
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What Themes Occur in the Diversification of
Animals?

• Basic features of the body plan do not vary from
  species to species
• Diversity mostly because of the evolution of
  methods for feeding and moving

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Feeding

• The feeding tactics observed in animals can be
  broken into five general types
• suspension feeding
• deposit feeding
• herbivory
• predation
• parasitism

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Suspension (Filter) Feeding

• Filter feeders
• Capture food by filtering out particles suspended
  in water or air
• Found in a wide variety of animal groups
• Evolved many times independently

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Deposit Feeding

• Deposit feeders eat their way through a
  substrate
• Food consists of soil-dwelling bacteria,
  protists, fungi, and archaea, along with
• Depending on what they eat, deposit feeders can
  also be considered herbivores (plant eaters),
  parasites, detritivores (detritus eaters), or
  predators.

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Herbivory

• Herbivores are animals that digest algae or
  plant tissues
• Have complex mouths with structures that make
  biting and chewing or sucking possible.
• The radula of mollusks functions like a rasp or
  a file
• The mandibles of grasshoppers are used to
  process leaves or stems.

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Predation

• Sit-and-wait predators rarely move at all until
  prey is captured
• Stalkers pursue their prey

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Parasitism

• Parasites are much smaller than their victims
• Often harvest nutrients without causing death
• Endoparasites live inside their hosts
• Often wormlike in shape and can be extremely
  simple morphologically
• Ectoparasites live outside their hosts
• Have grasping mouthparts that allow them to
  pierce the hosts exterior and suck the fluids
  inside

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Movement

• Many animals are sit-and-wait predators
• Some are sessile throughout their lives
• Vast majority of animals move under their own
  power either as juveniles or as adults
• Movement has three functions in adults (1)
  finding food, (2) finding mates, and (3) escaping
  from predators.

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Types of LimbsUnjointed and Jointed

• The evolution of the limbmade controlled, rapid
  movement possible
• Types of limbs
• Unjointed limbs are saclike
• Jointed limbs move when muscles that are attached
  to a skeleton contract or relax
• Exoskeleton of arthropods serves the same purpose
  as internal skeleton

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Reproduction and Life Cycles

• At least some species in most animal phyla can
  reproduce asexually as well as sexually
• Can occur with internal or external fertilization

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Reproduction and Life Cycles

• Eggs or embryos may be retained in the females
  body during development, or they may be laid
  outside the body
• Species in the former group are viviparous those
  in the latter are oviparous

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Reproduction and Life Cycles

• Some species are ovoviviparous
• The female retains eggs inside her body during
  early development
• The growing embryos are nourished by yolk inside
  the egg and not by nutrients transferred directly
  from the mother
• The vast majority of animals are oviparous.

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Metamorphosis

• The change from juvenile to adult body type
• A juvenile is called a larva if it looks
  substantially different from the adult form
• Nymph if it looks like a miniature adult.

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Complete Metamorphosis

• Holometabolous (complete) metamorphosis
• Two-step process, from larva to pupa to adult
• Involves dramatic changes in morphology
• Larval stages specialized for eating and growing
  that are known by such names as maggot, grub, or
  caterpillar

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Complete Metamorphosis

• Extremely common in marine animals
• Most cnidarians have two distinct body types
  during their life cycle a polyp alternates with
  a free-floating medusa stage

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Incomplete Metamorphosis

• Hemimetabolous (incomplete) metamorphosis
• Limited morphological difference between juvenile
  and adult
• One-step process of sexual maturation
• Nymphs resemble adults but are smaller and go
  through a series of molts until they reach full
  size

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Key Lineages of Animals
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Choanoflagellates(Collar Flagellates)

• Closest living relatives to animals
• Aquatic suspension feeders and reproduce by
  simple fission.

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Porifera (Sponges)

• Suspension feeders
• Cells are totipotent, meaning that an isolated
  cell has the capacity to develop into a complete
  adult.

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Cnidaria (Jellyfish, Corals, Anemones, Hydroids,
Sea Fans)

• Cnidarians are radially symmetric diploblasts
• Specialized cnidocytes used for prey capture
• Most life cycles have a sessile polyp form and a
  mobile medusa form

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Ctenophora (Comb Jellies)

• Ctenophores are transparent, ciliated,
  gelatinous diploblast predators
• Live in marine habitats
• Most species have male and female organs and
  routinely self-fertilize externally

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Acoelomorpha

• Lack a coelom
• Bilaterally symmetric worms that have distinct
  anterior and posterior ends
• Feed on detritus and prey on small animals