Biology 320 Invertebrate Zoology Fall 2005

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Biology 320Invertebrate ZoologyFall 2005

• Highlights from Chapter 4
• Introduction to Metazoa

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Metazoans

• Multicellular organisms
• Really we are referring to members of Kingdom
  Animalia, or animals
• Believed to have evolved from protozoans
  specifically choanoflagellates

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Basic Properties

• Eukaryotic
• Typically large
• Many costs / benefits associated with increased
  size
• Motile
• Usually even to some degree in sessile animals
• Many have dual life stages motile larvae and
  sessile adults
• Polarity
• Anterior posterior
• Oral - aboral

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Ontogeny

• Means origin of being
• Fancy way of saying development
• Zygote formed by union of egg and sperm nuclei
• Soon after multicellular embryo is formed in
  process known as cleavage (division)
• One important, early stage is known as a blastula
• Hollow ball of cells

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Cleavage in a Sea Urchin Embryo
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• Soon after, one wall of the blastula invaginates
• Process known as gastrulation
• Altered embryo is referred to as a gastrula
• In most individuals, primary germ layers are
  established by grastrulation
• Ectoderm outer layer
• Endoderm inner layer
• Mesoderm middle layer
• Organisms possessing ectoderm and endoderm are
  said to be diploblastic
• Organisms possessing all three germ layers are
  triploblastic

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• If there is indirect development, a larva forms
• Lives independently of adult
• Very different from adult
• Occupies different ecological niche
• Producing larvae that occupy a different niche
  allows for resource partitioning
• Example caterpillar to butterfly
• Eventually undergoes metamorphosis to become a
  juvenile

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• Some undergo direct development
• No larval phase
• Juvenile looks just like a miniature adult
• Example Grasshoppers
• Believed to be a derived trait rather than
  primitive

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Cells, Tissues, and Skeletons

• Remember, protozoans posses very little cellular
  specialization
• Most protozoans rely on organelles to carry out
  all cellular functions
• Different tissue types allow for a partitioning
  of labor
• Damaged or destroyed cells can be regenerated,
  however if a Paramecium is badly damaged, the
  whole organism dies

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• Some metazoans lack true tissues
• Main tissue types present in early metazoans are
  derived from ectoderm and endoderm
• Epithelial tissue line and cover body surfaces
  and organs
• Connective tissue most common and widely
  distributed primary tissue type. Consists of
  cells in an extracellular matrix (almost always
  containing collagen fibers)

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Functional Consequences of Body Size

• On average, metazoans range in size from 0.5mm to
  approximately one meter
• Surface area to volume ratio (SAvol) is
  drastically affected by an increase in body size
• Area is squared
• Volume is cubed
• Body size is negatively correlated with SAvol
• This effects the exchange of substance such as
  gases, nutrients and wastes

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• However, some metazoans have structural
  adaptations to increase SAvol
• Microvilli
• Pseudopodia
• Tissues arranged in sheets with metabolically
  inert ECM between
• Vermiform or flat body plan
• Body plans with fractal relationships example
  tubes in sponges

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• Rates of diffusion slow drastically over great
  distances
• Effective diffusion distance is approximately
  0.5mm for most animals
• Body diameters of greater than 1.0mm may be
  diffusion-limited
• Some animals such as flatworms carry out all gas
  exchange via simple diffusion, but others require
  a circulatory system

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• Generally, metabolic rates are positively
  correlated with body size
• However, 1g of shrew tissue consumes more power
  than 1g of elephant tissue
• Poikilotherms (cold blooded animals) consume 8
  times more mass-specific energy than protozoans
• Homeotherms (mammals and birds) consume 29 times
  the power of a poikilotherm of equal mass
• Possessing a circulatory system, and maintaining
  a constant body temperature are both
  metabolically expensive

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Possible Advantages of Larger Body Size

• Mass-specific decrease in metabolic rate
• Less danger of predation by protozoa
• Predation upon protozoans by metazoans
• Motile metazoans move at greater speeds that
  protozoans
• Multicellularity provides the ability to
  regenerate

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Origins of Metazoans

• Colonial theory
• Metazoa is derived from colonial flagellated
  protozoans
• Supported by morphological and molecular data
• Accepted by the authors of your text
• rRNA data indicated that Volvox evolved 50 to
  75mya and is not an ancestor to metazoans
  (believed to have evolved 600mya)
• Choanoflagellates are believed to be the sister
  taxon