680mya Cnidarians

Cnidarians, also called coelenterates are any member of the phylum Cnidaria (Coelenterata), a group made up of more than 9,000 living species. Mostly marine animals, the cnidarians include the corals, hydras, jellyfish, Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans.

They are radially symmetrical, aquatic, invertebrate animals that have a hollow digestive cavity opening to the outside by a single opening surrounded by one or more nematocyst-studded whorls of tentacles, that occur as single or colonial sessile, typically columnar polyps or usually free-floating with an organized locomotory system

The five main characteristics of cnidarians are:

Radial symmetry.

Diploblastic animals.

Tissue level of organisation.

Presence of cnidoblasts with stinging nematocysts on the tentacles.

Polymorphism and have two body forms, i.e. polyp and medusa.

Climate change and ocean temperature warming is threatening reefs world wide as ocean levels rise and warmer temperatures interfere with the viability of the cnidarians that build corals.  Coral bleaching is when the cnidarian population dies off and leaves the stark, calcareous scaffolding to disintegrate.  As the oceans acidify, precipitating calcium to create these structures also suffer.

For more information see:

http://www.wildsingapore.com/wildfacts/cnidaria/cnidaria.htm