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Biology 320 Invertebrate Zoology Fall 2005


Biology 320 Invertebrate Zoology Fall 2005 Highlights from Chapter 4 Introduction to Metazoa Metazoans Multicellular organisms Really we are referring to members ... – PowerPoint PPT presentation

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Title: Biology 320 Invertebrate Zoology Fall 2005

Biology 320Invertebrate ZoologyFall 2005
  • Highlights from Chapter 4
  • Introduction to Metazoa

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

Basic Properties
  • Eukaryotic
  • Typically large
  • Many costs / benefits associated with increased
  • 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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  • 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

Cleavage in a Sea Urchin Embryo
  • 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

  • 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

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

Cells, Tissues, and Skeletons
  • Remember, protozoans posses very little cellular
  • 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

  • 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)

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

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

  • 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

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
  • Multicellularity provides the ability to

Origins of Metazoans
  • Colonial theory
  • Metazoa is derived from colonial flagellated
  • 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
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