Phylum Cnidaria

This page is part of a series introducing various animal phyla. For the first day of animal phyla, you should look at these pages:

On other lab days, we'll look at some other animal phyla:

This phylum includes jellyfish, sea anemones, corals, and their relatives.


Hydra with bud

Hydra is a small freshwater cnidarian that normally exists as a single polyp. Examine the live specimens in lab. The polyp uses its stinging tentacles to grab small animals and bring them into the gastrovascular cavity for digestion and absorption.

This picture shows a young polyp (at bottom) budding off from a larger polyp. This is asexual reproduction; the new polyp simply forms as an extension of the original body and eventually separates itself and moves away to an independent life. This picture shows a whole mount, but you can see the two tissue layers that make up the polyp.

Hydra, cross section

In cross section, you can more clearly see the two tissue layers that make up this cnidarian body:

  • Epidermis lines the outside, and
  • Gastrodermis lines the gastrovascular cavity.

Between these two tissue layers is the mesoglea, which, in Hydra is a thin sheet of protein and does not contain cells. In some cnidarians, such as larger jellyfish, the mesoglea is a thick jellylike layer and contains cells.

Note the simplicity of the body plan. In this view you don't see any organs, just two layers of cells. There is no circulatory system or excretory system, and every cell is in fairly close contact with the environment.

Hydra with testes

Hydra can also reproduce sexually. This picture shows the testes, or sperm-producing structures, of a male Hydra. This species releases sperm and eggs into the water for fertilization.


Obelia hydroid

Obelia has a more complex life cycle. This species has two very different stages in its life: a colonial polyp and a medusa. The colonial polyp stage, shown here is called a hydroid. It consists of multiple genetically identical polyps joined together by continuous tissues. As with other cnidarians, there is no specialized circulatory or excretory system, but the gastrovascular cavity is continuous among all the polyps. The hydroid stage is sessile, meaning it remains attached to one spot.

Obelia hydroid with medusa buds

The hydroid stage of the Obelia life cycle can produce swimming medusae. In this picture you can see one polyp that is filled with medusa buds. When these mature, they will turn into medusae that swim away from the hydroid. The medusae are either male or female; eventually, they release sperm or eggs into the water for fertilization. If fertilization is successful, the zygote develops through its embryonic stage and eventually becomes a planula larva. The planula can swim a short distance, settle on the bottom, and begin to grow into a new colonial hydroid.

Obelia medusa

Here are a few of examples of the swimming medusa stage.



This picture shows the planula larva of Aurelia, also known as the moon jelly. The larva is small (about 0.3 mm) and simple, having two tissue layers like the adult cnidarians. The planula can settle on the bottom and metamorphose into a polyp, which eventually produces medusae.

References & further reading

Hydra on Invertebrate Anatomy Online; see also Obelia, Aurelia aurita and Cassiopeia xamachana.

Working with Hydra, a lab handout from Ward's Science.

Coral Colors on Vimeo. Just for fun, an amazing time-lapse video of coral colors and behavior.

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