AP Biology Evolution Project

Cnidarians are grouped based on the presence of a special type of stinging cell called cnidocytes, which are used for defense and sometimes for capturing food. While almost all of these aquatic organisms live in saltwater environments, there are a few species that can be found in freshwater.

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Members of the phylum Cnidaria include coral, sea anemones, jellyfish, and other similar aquatic creatures like sea fans and hydras.

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Cnidarians do not have special breathing organs such as lungs or gills.  Therefore, respiration occurs by individual cells obtaining oxygen directly from water through diffusion. The walls of the body and tentacles are so thin that the oxygen molecules are able to pass directly from the water into the internal organs. Carbon dioxide molecules are also able to pass out of the body in the same manner. Depending on the species, water either enters through a mouth or through surface cells, and exits through either location (Cnidarian Respiratory System).

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One of the biggest problems facing cnidarians in terms of respiration is ending up in an area where water is stagnant, or not adequetely circulating. In these conditions, many organisms will die because it is hard for them to acquire the amount of oxygen needed from the surrounding water as it is not constantly being refreshed.

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Cnidarians when viewed through the evolutionary lens for respiration are considered near equal to the phylum Platyhelminthes. They respire in nearly identical ways through diffusion of oxygen directly into their cells. The only advantage the Cnidarian phylum has is that the organisms are on average larger in total volume, which allows them to be slightly more efficient in their ability to respire. Thus, both phyla have similar evolutionary tracks and are relatively close in terms of the evolutionary timeline, when viewed through the evolutionary lense for respiration.