Cnidaria are primarily found in marine environments. This phylum consists of marine life, such as the jellyfish, sea anemone, and coral. Within the phylum Cnidarian there are over 10,000 different species. Cnidarians have two main layers of cells: the exoderm, and the gastroderm, which are the outer and inner layers. In jellyfish and anemones, between these two layers there mesoglea, or a jell-like substance, which acts as their skeletal structure in order to keep maintain their shape. Corals form a protective external carbonate skeletal structure. Being that Cnidaria are fairly thin organisms, they have one central gastrovascular system, meaning that their digestive and circulatory system are combined. The inner layer, the gastroderm, is where all the necessary nutrients and gases needed to sustain bodily functions are taken in. Cnidarians are typically carnivorous animals, but can also filter feed from the water around them.