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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.

"Hydrozoon is a name given to the great class of the sub-kingdom Cœlenterata, of which hydra is the type. They exhibit a definite histological structure, their tissues having a cellular organization. These tissues are two, an outer or ectoderm, and an inner or endoderm. In most the prey is seized by tentacles surrounding the mouth and furnished with offensive weapons called thread cells, The hydrozoa are all aquatic, and nearly all marine. Their distribution is world-wide. [Pictured] Hydra fusca, with a young bud at b, and a more advanced bud at c."—(Charles Leonard-Stuart, 1911)

Hydrozoon

"Hydrozoon is a name given to the great class of the sub-kingdom Cœlenterata, of which hydra is…