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What is the chief difference between the...

What is the chief difference between the hydrozoans and the rest of the cnidarians regarding the germinal layer (s) in which its 'defensive structures or cells of defence' occur?

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The defencive structures Cnidocytes or Cnidoblasts are occur only in the ectoderm, in the hydrozoans in the rest of the Cnidarins the cnidocytes occur in both ectoderm and endoderm.
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