Cnidaria is a phylum containing some 9,000 species of animals found exclusively in aquatic, mostly marine, environments. Despite their early appearance in the evolutionary history of animals and their simple morphology, the modern forms are genetically sophisticated and bio-chemically complex. The unifying characteristic of the Cnidarians is the presence of cnidocytes, specialized cells that carry organelles called nematocysts. It is widely accepted that all cnidarians inherited cnidocytes fro… (More on Cnidaria)