Phylum Cnidaria

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Phylum Cnidaria

• Jellyfish, Hydra, Coral, Anemones

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Organization

• Developed from two layers (ENDODERM ECTODERM)
• Has MESOGLEA a jelly-like substance with some
  cells.
• Has specialized nerve muscle tissue
• ASCOELEMATES

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Motility

• Two basic body plans are both RADIALLY SYMETRICAL
• Polyp sessile, vase-shaped, with mouth on dorsal
  surface (top) and tentacles that point upwards.
• Medusa mobile, umbrella-shaped, with a mouth on
  ventral side (underside) and tentacles that point
  downward. Moves with the current.

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Feeding

• Must capture or poison prey with NEMATOCYSTS (a
  barbed, thread-like tube cell) that are
  discharged by CNIDOCYTES when organism is
  touched.
• Contraction of tentacles bring prey to the mouth.
• Cells lining gastrovascular cavity excrete
  enzymes for extracellular digestion.

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Digestion / Excretion

• CO2 and wastes diffuse directly out, since
  cnidarian body is only two cells thick.
• Wastes also pass back out through the mouth
  (incomplete digestive system)

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Reproduction

• Sexual
• External fertilization occurs when male and
  female medusas release gametes into the water.
• Zygotes form swimming larvae.
• Asexual
• Larvae settle and grow into a polyp that bud
  medusae.

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Circulation

• No system needed, since oxygen can diffuse
  directly into every cell.
• Nutrients, when broken down by extracellular
  digestion, can also diffuse into every cell.

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Nervous System

• No CNS (central nervous system)
• Nerve net carries signals throughout the body,
  coordinating activities of the cnidocytes and
  tentacles.

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The Myth
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August 1st, 2007

• Scientists create a many headed jellyfish by
  turning off a Cnox gene. However, a decapitated
  head only regrows another head, not two.