The Cnidarians are rather bizarre looking creatures. Most members of this group possess very little mobility. The jellyfish for example are at the whims of the sea. All Cnidarians undergo two stages in their life cycle. In the case of the jellyfish, the larvae starts out as a sedentary polyp (resembling a sea anemone) later evolves to a free floating medusa - the jellyfish itself. 

One of the main characteristics of the Cnidarians are the "stinging cells" they carry about in their tentacles to act both as defense as well as for feeding purposes. Some of the infamously dangerous creature of the sea - Portuguese man-of-war and Box Jelly - both belongs to this group. However, the most beautiful underwater scenery - i.e. the coral reefs of tropical regions - also belongs to this phylum.