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"Portion of the body-wall of Hydra, showing ectoderm cells above, separated by "structureless lamelia" from three flagellate endoderm cells below. The latter are vacnolated, and contain each a nucleus and several dark granules. In the middle ectoderm cell are seen a nucleus and three nematocysts, with trigger hairs projecting beyond the cuticle. A large nematocyst with everted thread, is seen in the right-hand ectodermal cell." — Encyclopedia Britannica, 1893

Body-wall

"Portion of the body-wall of Hydra, showing ectoderm cells above, separated by "structureless lamelia"…

"Bougainvillea ramosa. A, entire colony, natural size; B, portion of the same magnified; C, immature medusa; cir. c, circular canal; cu, cuticle or perisarc; ent. cav, enteric cavity; hyd. polype or hydranth; hyp, hypostome or manubrium; med, medusa; mnb, manubrium; rad. c, radial canal; t, tentacle; v, velum." -Parker, 1900

Bougainvillea Ramosa

"Bougainvillea ramosa. A, entire colony, natural size; B, portion of the same magnified; C, immature…

"Epidermo-muscular cells of Hydra m, muscular-fiber processes." — Encyclopedia Britannica, 1893

Epiderm Cels

"Epidermo-muscular cells of Hydra m, muscular-fiber processes." — Encyclopedia Britannica, 1893

"Stages in the development of two zoophytes; I-M, fixation of the planula and development of the hydrula." -Parker, 1900

Eudendrium

"Stages in the development of two zoophytes; I-M, fixation of the planula and development of the hydrula."…

"Heracles and the Hydra" — Gayley, 1893

Heracles

"Heracles and the Hydra" — Gayley, 1893

Hercules slaying the Hydra, from sculpture at Florence

Hercules

Hercules slaying the Hydra, from sculpture at Florence

Hydra are in the Jellyfish family, class Hydrozoa.

Hydra

Hydra are in the Jellyfish family, class Hydrozoa.

Hydra are in the Jellyfish family, class Hydrozoa. Cross-section of the body showing the two layers of cells of the body wall.

Hydra

Hydra are in the Jellyfish family, class Hydrozoa. Cross-section of the body showing the two layers…

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.

"Nettling cells of Hydra. A, unexploded; B, exploded; b, barbs; c, the nettling cell in which the nettling organ is developed; cn., the cnidocil or "trigger"; cp., the capsule or nettling organ; f, the nettling filament or lasso; n, neck of the capsule; nu., nucleus of the cell." — Galloway

Hydra

"Nettling cells of Hydra. A, unexploded; B, exploded; b, barbs; c, the nettling cell in which the nettling…

"Greek Hydra, Good Style." — Encyclopedia Britanica, 1893

Hydra

"Greek Hydra, Good Style." — Encyclopedia Britanica, 1893

"Hydra hanging from water-weed." -Thomson, 1916

Hydra

"Hydra hanging from water-weed." -Thomson, 1916

In Radiata the digestive cavity is a pouch with a single opening, into which the food is passed and from which the residuum is ejected, as in the Hydra.

The Digestive Apparatus of a Hydra or Freshwater Polyp

In Radiata the digestive cavity is a pouch with a single opening, into which the food is passed and…

"Hydra vividis. ov, ovary; te, testis." — Encyclopedia Britannica, 1893

Hydra Vividis

"Hydra vividis. ov, ovary; te, testis." — Encyclopedia Britannica, 1893

A freshwater hydra, magnified. Example of an animal with no skeleton. There are animals without a skeleton that are not able to accomplish very powerful motions. Mot of them live in water where they are partly or wholly held up by the water and have only slow motions. Most are also very small and generally have no muscles.

Freshwater Hydra

A freshwater hydra, magnified. Example of an animal with no skeleton. There are animals without a skeleton…

"Longitudinal section of a Hydra; <em>b</em>, bud which will form a young one; <em>ba</em>, base by which it is attached when not creeping." &mdash; Davison, 1906

Section of a hydra

"Longitudinal section of a Hydra; b, bud which will form a young one; ba, base by…

"Hydrozoon is a name given to the great class of the sub-kingdom C&oelig;lenterata, 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."&mdash;(Charles Leonard-Stuart, 1911)

Hydrozoon

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

Fresh water Hydra, a jellyfish, is a many celled animal. Shown enlarged 25 times, is the entire animal.

Hydra Jellyfish

Fresh water Hydra, a jellyfish, is a many celled animal. Shown enlarged 25 times, is the entire animal.

Fresh water Hydra, a jellyfish, is a many celled animal. Shown is a Hydra nettle cells.

Hydra Jellyfish

Fresh water Hydra, a jellyfish, is a many celled animal. Shown is a Hydra nettle cells.

Fresh water Hydra, a jellyfish, is a many celled animal. The organs of defence, nettle cells or lasso cells , in the event of disturbance, the sac will discharge coiled thread an barbs into the body of the intruder.

Hydra Jellyfish

Fresh water Hydra, a jellyfish, is a many celled animal. The organs of defence, nettle cells or lasso…

Fresh water Hydra, a jellyfish, is a many celled animal. The organs of defence, nettle cells or lasso cells , in the event of disturbance, the sac will discharge coiled thread an barbs into the body of the intruder.

Hydra Jellyfish

Fresh water Hydra, a jellyfish, is a many celled animal. The organs of defence, nettle cells or lasso…

"Nematocyst of Hydra, showing cell-substance and nucleus, cyat, trigger hair, and everted thread." &mdash; Encyclopedia Britannica, 1893

Nematocyst

"Nematocyst of Hydra, showing cell-substance and nucleus, cyat, trigger hair, and everted thread." —…

Sertularia are members of the Protista kingdom. It is an example of another group of the hyroid zoophytes, almost all the members of which are marine.

Sertularia

Sertularia are members of the Protista kingdom. It is an example of another group of the hyroid zoophytes,…

Sertularia are members of the Protista kingdom. It is an example of another group of the hyroid zoophytes, almost all the members of which are marine.

Sertularia

Sertularia are members of the Protista kingdom. It is an example of another group of the hyroid zoophytes,…

Sertularia are members of the Protista kingdom. This is an example of the egg-producing gonophore.

Sertularia

Sertularia are members of the Protista kingdom. This is an example of the egg-producing gonophore.

Tubularia are members of the Protista kingdom. The pipe-coralline (Tubularia) is an example of another group of the hyroid zoophytes, almost all the members of which are marine.

Tubularia

Tubularia are members of the Protista kingdom. The pipe-coralline (Tubularia) is an example of another…