hydra body wall structure. educational material for lesson of zoology

anatomy animal background biology body capsule cnida cnidaria cnidoblast cnidocil cnidocyst cnidocyte cross diagram drawing ectoderma educational endoderma fresh hydra hydroid hydrozoa illustration internal isolated lesson material mesoglea mesogloea morphology nematocyst nematocyte organism polyp schema scheme section structure title vulgaris wall water white zoology