What is Cnideria?
Cnidaria is a large phylum composed of some of the most beautiful of all the salt and freshwater organisms: the true jellyfish, box jellyfish, coral and sea anemones, and hydra. Although Cnidaria is an incredibly diverse group of animals, there are several traits that link them together.
Coral Reef
Coral reefs are important in determining the amount of carbon dioxide in the atmosphere. The zooxanthellae algae, through photosynthesis, remove carbon dioxide from the air and make carbohydrates available as food for both the zooxanthellae and the coral polyps.
Cnidarians such as corals have no circulatory organs or respiratory system. Therefore, the only way they can get CO2 out and O2 in is the same way that all unicellular life does it: diffusion. Oxygen diffuses into the coral's cells and CO2 diffuses out. Corals can get away with this technique (where as higher organisms need a circulatory system) because they have only two cell layers, ectoderm (outside) and endoderm (inside the ecotderm and the gastrovascular cavity), a very thin layer of non-living matrix called the mesoglea in between, and a very close ratio of body volume to surface area exposed to seawater.
Cnidarians such as corals have no circulatory organs or respiratory system. Therefore, the only way they can get CO2 out and O2 in is the same way that all unicellular life does it: diffusion. Oxygen diffuses into the coral's cells and CO2 diffuses out. Corals can get away with this technique (where as higher organisms need a circulatory system) because they have only two cell layers, ectoderm (outside) and endoderm (inside the ecotderm and the gastrovascular cavity), a very thin layer of non-living matrix called the mesoglea in between, and a very close ratio of body volume to surface area exposed to seawater.
Jelly Fish
Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive.
Fire Coral
During the daylight hours the symbiotic algae produce more oxygen than the coral polyp can use for its respiration, and some of the carbon dioxide produced by the respiratory process is started again by the algae into new organic matter.