The Phylum Cnidaria:
Within the phylum Cnidarian there are over 10,000 different species, which are primarily found in marine environments. This phylum consists of marine life, such as the jellyfish, sea anemone, and coral. The structure of Cnidarians are very similar to those of a sponge, but are far more complex. Cnidarians have two main layers of cells, the exoderm, being the outer layer, and the gastroderm, which is the inner layer. In jellyfish and anemones, between these two layers is a jelly-like substance called mesoglea, which acts as their skeletal structure in order to keep maintain their shape. Corals form a protective external calcium carbonate skeletal structure. Because species in the cnidarian phylum are typically very thin with only two layers, they have a gastrovascular system, in which the digestive system and the cardiovascular system are combined into one. This is why the inner layer, the gastroderm, is called that, and also it is where all the necessary nutrients and gases needed to sustain itself are absorbed into. Cnidarians are typically carnivorous animals, but can also filter feed from the water around them.
This is the basic structure in which the Cnidarian phylum follows. Jellyfish and hydra would fall under the medusa category, and sea enemones fall under the polyp. The main difference between these two forms is that medusa can move throughout bodies of water, but polyps have an adhesive bottom to keep them stuck in place. Coral's anatomy would differ slightly, mainly because of the hardened calcium shell around it, which has the same function.