ABSTRACT
Cnidarians comprise a varied collection of organisms united by their possession of nematocysts, and containing many difficult groups. The five classes are generally grouped into two subphyla, the Medusozoa containing the Hydrozoa, Scyphozoa and Cubozoa, and the Anthozoa containing the Alcyonaria and Zoantharia. The life cycle typically contains both a benthic and a pelagic phase. Whereas in anthozoans the larval phase is usually a simple planula larva and the medusa stage lost from the life cycle, in medusozoans the planktonic phase can be dominant (as in jellyfish). This can make the decision as to what constitutes a benthic taxon somewhat arbitrary. Within the Hydrozoa, the dominant stage in the hydroid life cycle is typically benthic; for hydromedusans or leptomedusans, the distinction between truly benthic or plankton taxa is more difficult. The Scyphozoa are predominantly pelagic and are not considered further here. The Cubozoa (box-jellyfish) are pelagic and exclusively tropical.
