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Cnidaria
Cnidaria is classified under the domain Eukaryota, kingdom Animalia. Sometimes referred to by an older group name, Coelenterata (coelenterates), Cnidaria is divided among four major classes of animals: Scyphozoa, Cubozoa, Hydrozoa and Anthozoa.
Scyphozoa and Cubozoa
Scyphozoa is considered to be the class of "true jellyfish," whereas Cubozoa members are called box jellyfish. Among their differences, scyphozoans have a rounded shape, while cubozoans have a more cube-like appearance. Cubozoan tentacles are more evenly spaced apart, allowing it to swim significantly faster than scyphozoans. Also, while most scyphozoan tentacle stings can cause mild to moderate discomfort to humans, cubozoans are notoriously venomous and are known to cause human fatalities.
Hydrozoa
The cnidarian class, Hydrozoa, consists of more than 3,000 species distributed worldwide. Most hydrozoans live in colonies, are carnivorous predators and are predominantly marine. Like all cnidarians, members of Hydrozoa have varied stages of development depending on the species, including a polyp stage and/or medusa stage. Portuguese man-o-wars (Physalia physalis) and by-the-wind sailors (Velella velella) are two of the many varieties of hydrozoans. One of the most common hydrozoans is the hydra.
Hydra
Hydra is the common name for members of the hydrozoan genus Hydra. Usually growing up to 1 inch in length, hydras have a stalk-like body with a mouth encircled with tentacles. They feed off of plankton organisms, such as microcrustaceans, and are among the few hydrozoans that live in fresh water.
Anthozoa
Anthozoa is the cnidarian class that includes corals and sea anemones. Unlike many members of Cnidaria, anthozoans live exclusively in the form of a polyp, and do not go through a medusa stage during development. Highly diverse anthozoans represent the largest class under Cnidaria, with more than 5,000 species.
Sea Anemone
Sea anemones are anthozoans that are distributed worldwide, but are most abundant in the waters of coastal regions. Growing up to 3 feet in diameter, they have the appearance of "living flowers" due to their form and vivid colors. Even though sea anemones are closely related to corals, they do not have skeletons and only temporarily anchor themselves to underwater objects.
Coral
Corals consist of a polyp -- which is the marine animal; and a skeleton, which is also called coral in a global context. The polyps are elongated and sac-like, with tentacles encircled around the mouth. The skeleton is created by the polyps as they secrete calcium-carbonate (CaCO3). Coral comes in a wide color array, and grows up to 1/8 inch high. Although there are some solitary coral, most thrive in colonies that grow 10 inches to thousands of miles in diameter.