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Cnidaria Classes

The Phylum Cnidaria can be further categorized into classes. Cnidarians have stinging cells called nematocysts, which they use to sting and catch prey. These nematocysts are the reason why jellyfish sting.

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Anthozoa (Corals, Sea Anemones, Sea Pens)

Anthozoa species have a polyp body. Hard corals (also called stony corals) have a calcium carbonate skeleton, and they form coral reefs. Most corals form colonies. The large coral structure you see is actually a colony of many individual polyps. Soft corals do not form coral reefs. Some soft corals resemble anemones. Like other Cnidarians, corals and anemones have nematocysts which they use to sting and catch prey.

Corals and anemones both form a symbiotic relationship with photosynthetic algae that lives inside of them. The algae absorbs light and makes food for their animal host. Corals and anemones also catch and eat prey, which sometimes includes jellyfish. 

Scyphozoa (Most Jellyfish)

Jellyfish have a polyp stage and a medusa stage. The polyp stage stays connected to the ground, and it looks similar to an anemone. The polyp reproduces asexually to make more polyps. Eventually, the polyp grows into the medusa stage, which is the jellyfish you see swimming in the water. The medusa stage reproduces sexually.

Cubozoa (Box Jellyfish)

Box Jellyfish have a polyp stage and a medusa stage. Unlike other jellyfish with simple eyes, Box Jellyfish have complex eyes that can see blurry images. These eyes combined with their fast swimming ability enables them to be active hunters. Box Jellyfish swim after their prey and carefully dodge obstacles.

Hydrozoa (Hydrozoans)

Hydrozoa includes siphonophores, hydra, and Fire Corals (which are not corals). 

The Man o’ War is a siphonophore (not a jellyfish). The Order Siphonophora is within the Class Hydrozoa. Siphonophores are colonial organisms. This means the Man o’ War’s body is made up of many genetically identical multicellular individuals.