Welcome to the World of Cnidarians
There are many different organisms in Phylum Cnidaria. These include jellyfishes, corals, sea anemones, sea fans, sea whips, and hydras. Many Cnidarians go through two stages in their lives--polyps and medusas.
Polyps are usually the first stage of Cnidarians' life cycles. They have a tubular shape with tentacles branching out at the top. Polyps are usually sessile. In contrast, Medusas are motile, meaning that they can move. They have an upside-down-bowl shaped body, and their tentacles hang downward. These Cnidarians are fully mature.
Watch Out For Cnidocytes
Cnidocytes are the reason Cnidarians are called Cnidarians! They are stinging cells that are found on Cnidarians' tentacles. Cnidocytes are used to protect Cnidarians from predators, and to make prey easier to catch. In each of these cells lie a poisonous structure call nematocysts. When an unlucky animal brushes against a trigger, the nematocyst shoots a poison filled barb into it. With all of these nematocysts working together, Cnidarians can paralyze or even kill other animals. 
Types of Cnidarians
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| Class Scyphozoa |
Class Hydrozoa includes hydras and their relatives. Most hydrozoans are made up of specialized polyp colonies. Hydras are special because they do not have a medusa stage in their life cycles. There are almost 3,200 species in Class Hydrozoa!
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| Class Hydrozoa: Portuguese man-of-war |
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| Class Anthozoa: Sea Anemone |
Class Anthozoa are made up of organisms like corals, sea anemones, sea whips, and sea pens. They only have the poly stage in their life cycle. There are around 6,100 species!
Diversity of Cnidarians
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| Sea Anemone |
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| Coral Reef |
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| Ultra Coral Australia |
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