The Cnidarian Skeletal System:
Cnidarians, unlike the other phylums, don't really use a traditional sense of a skeleton. Instead, they have a three-layered body wall; the outer epidermis layer, the middle mesoglea, and the inner gastrodermis layer. There are grouped into 4 major classes: the anthozoa, cubozoa, hydrozoa, and scyphozoa classes.
The sea pen, shown above, belongs to the anthozoa class, which has the tentacles positioned around the mouth and the body wall surrounding all vital organs.
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Chironex fleckeri, otherwise known as the box jellyfish, is part of the cubozoa class, whose members, as indicated by the name, have cube-shaped body walls and four tentacles found on each side of the organism. Their soft walls make it very rare for fossils to form.
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One of the two forms used by members of the hydrozoa class, the fire coral forms as a colony of polyps that altogether create the current shape. Other hydrozoans have a free moving medusa form, while others still move from polyp to medusa form.
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Chrysaora colorata, the above scyphozoan, is part of the "true jellyfish" class. They go through two phases of growth, as small medusae that than grow into the adult jellyfish stage. They are usually symmetrical into 4 parts.
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