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Hydra (whole mount)

tentacle tentacle tentacle tentacle

Hydra with buds

tentacle tentacle bud bud bud tentacle

Hydra with ovary

tentacle ovary basal disk hypostome (with mouth)

Hydra with testes

testes testes tentacle

Hydra (cross-section) gastrovascular cavity gastrodermis mesoglea epidermis gastrovascular cavity gastrodermis epidermis


The Cnidarian animals are radially_symmetrical, aquatic and mostly marine. They are at the tissue level of organization and have special stinging cells called cnidocytes that contain stingers termed nematocysts. They range in motility from slow-moving to sessile. They are efficient predators and they display various forms of symbiosis. They have two basic body plans: (1) the polyp and (2) medusa.

These organisms have two tissue layers: the epidermis and the gastrodermis, with a jelly-like mesoglea sandwiched between the two layers. They all have either two or three possible germ_layers: all of them have ectoderm and endoderm and some also have mesoderm. Their larva is called the planula_larva. They lack a coelom but they do have a nerve_net as well as a gastrovascular_cavity.

(1) Hydrozoa: Hydra, Obelia, and Physalia. Solitary or colonial forms. Polyp or medusa.

(2) Scyphozoa: Aurelia. Medusa stage dominates.

(3) Cubozoa: Medusa stage dominates. Medusa is square-shaped when seen in cross section.

(4) Anthozoa: Sea anemones and corals. The polyp stage predominates.

Hydra are Cnidarian polyps. They move by using their tentacles or by somersaulting from their base to their tentacles.  You may need to review several slides in order to see all of the Hydra structures we are required to learn.

The tentacles and basal_body with expanded basal_disk (for attachment to a substrate) are visible on some of the slides. To view the various reproductive structures, you will need to scan various different slides. One microslide may have a Hydra with a bud, another with an ovary, another with testes and yet another with an embryo.

The process of sexual reproduction requires gonads, which will appear during the autumn months. Gonad development is stimulated by the lower environmental temperatures and the reduced aeration of stagnant water. Hydras are dioecious, which means one Hydra body will have either ovaries or testes, but not both. They are “separate sex” organisms. Eggs in the ovary mature one at a time, and are fertilized by sperm released from the testes of a different Hydra.

The cross-section micrograph of the Hydra shows its gastrovascular_cavity, gastrodermis (cellular lining of the gastrovascular cavity), mesoglea, and epidermis.