structure of hydra. cross-section of hydra polyp. educational material for lesson of zoology

anatomy animal biology capsule cnida cnidaria cnidoblast cnidocil cnidocyst cnidocyte cross diagram drawing ectoderma educational endoderma fresh hydra hydroid hydrozoa hypostome illustration internal material mesoglea mesogloea morphology nematocyst nematocyte organism polyp schema scheme section structure tentacle vector vulgaris water zoology