Cnidaria is a phylum within Kingdom Animalia which consists of hydras, jellies (or jellyfish), sea anemones, and corals. All cnidarians are radially symmetrical, and there are two main radially symmetrical body types.

The first of these types is known as a polyp. A polyp consists of a cylindrical body with tentacles projecting from one end, while the other end is anchored to something. The second type is known as a medusa, which consists of an umbrella-shaped body fringed by tentacles at the edge. Unlike the polyp form, the medusa floats freely in the water. Any given cnidarian species can exist in either the polyp form, or the medusa form, and some life cycles include both polyp and medusa forms.

In all cnidarians, the mouth leads directly into the gastrovascular cavity, a digestive cavity. Because there is no anus, excess food and waste products must exit through the mouth, so the digestive system is considered incomplete. This cavity is filled with fluid which services internal cells, provides body support, and help maintain structure.

A third distinguishing aspect of cnidarians is the fact that they are all carnivores which use tentacles to capture their prey, and then pull the prey into the mouth, but it the specialized cells on these tentacles which make them especially unique. These cells, called cnidocytes, are on the surface of the tentacles, and function both in the capture of prey and for defense. On the tip of each cnidocyte there is a section known as the "trigger". When this area of the cell is touched, a coiled thread within the cell is discharged. These threads can sting whatever triggered them, as well as entangle them, in either case causing the predator or prey to become unable to fight or escape.

Another aspect of cnidarians is that all cnidarians have two layers of living tissue. These two layers are always separated by a gelatinous layer called mesoglea. The first layer of living tissue is known as the dermis, and the second layer is know as the gastordermis. The dermis is the outer layer, and the gastordermis is the inner layer which forms the body cavity.

One final aspect of cnidarians is the fact that nearly all functions are performed by individual tissues, not organs, despite the fact that a very small number of them do contain organs.

Phylum Cnidaria includes the following classes:

..Class Schphozoa (The jellyfish, or jellies)
..Class Hydrozoa (Diverse polyps, including hydras)
..Class Cubozoa (A minor tropical class with cuboidal swimming cups)
..Class Anthozoa (Sea anemones, corals)

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