hydra cross section.

aboral anatomy animal animals aquatic biology body cavity cells class cnidaria coelenterates cross design drawing ectoderm educational endoderm epidermis epithelial epitheliomuscular freshwater gastrodermis gastrovascular gland hydra hydrangea hydrozoa internal interstitial isolated lesson material mesoglea morphology nematocyte nerve oral organisms phylum polyp science stinging structure transverse wall white zoology