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If undisturbed, the body and tentacles may occasionally sway gently. If disturbed, a hydra usually shortens until it is a tiny ball.

Hydra

If undisturbed, the body and tentacles may occasionally sway gently. If disturbed, a hydra usually shortens…

Longitudinal section of a hydra.

Hydra

Longitudinal section of a hydra.

Cross section of a hydra.

Hydra

Cross section of a hydra.

Stinging cells of hydra, highly magnified. Cell containing thread capsule.

Hydra

Stinging cells of hydra, highly magnified. Cell containing thread capsule.

Stinging cells of hydra, highly magnified. Thread capsule partly discharged.

Hydra

Stinging cells of hydra, highly magnified. Thread capsule partly discharged.

Stinging cells of hydra, highly magnified. Thread capsule fully discharged.

Hydra

Stinging cells of hydra, highly magnified. Thread capsule fully discharged.