By Joseph Enea
bACKGROUNd
Some of the members of this large phylum include jellyfish, sea anemones, corals, etc. Cnidarians are biradially symmetrical and are made up of mostly tissues. Cnidarians usually have two layers that make up their bodies, but some can have three layers. Cnidarians have a coelenteron, which is an exterior opening that serves as both a mouth and an anus. Tentacles usually surround the opening. Cnidarians have two nerve nets that help coordinate its muscles and sensors-one epidermal and one gastrodermal. Two basic body forms exist in this phylum: medusa and polyp. The biggest differnce between the two is that medusae are free-swimming and polyps are attached to something, or sessile.
Jellyfish
Jellyfish breathe by directly absorbing dissolved oxygen from water. Jellyfish do not have lungs or gills. Instead, jellyfish form hydrogen bonds with the oxygen. Where does the hydrogen come from? The water of course! When breathing, jellyfish maximize their surface ratio to make use of the constantly moving water. The entire body of the jellyfish is used to create oxygen. This method of breathing helps support the small percentage of the jellyfish that is not in direct contact with the water. This process happens in their internal cavity, which is called the coelenteron.
Sea Anemones
Sea anemones spent most of their life in one place. They are meat-eating animals that come in many different colors and sizes. These mysterious creatures breathe by breathing in the constant flow of water through their pores and their main opening, which functions as both a mouth and an anus. They take in dissolved oxygen like jellyfish. Sea anemones are polyps. Few fossils of sea anemones exist today.
Soft Coral
Soft coral, as opposed to hard coral, is alive. Soft coral does not need air, but it needs oxygen. Oxygen is an essential part of keeping coral reefs alive and flourishing. How does coral get oxygen? The answer is photosynthesis from other plants. Marine plants takes in carbon dioxide that is emitted from marine animals around it and from the sun. The oxygen that they release goes to the coral. Zooplankton that live in coral assist in this process. Coral grows in shallow water to allow plankton to live inside them.