Phylum III: Cnidaria
This phylum is also called Coelenterata. So far about 9,000 species have been described. These are considered to be the first true multicellular animals (metazoa). The cells are organised into tissues which are coordinated by nerve cells into a unified body.
- They are all marine with a few exceptions which live in fresh water.
- Radially symmetrical, diploblastic metazoa.
- The mouth usually surrounded by tentacles.
- Gastrovascular cavity present but no anus.
- Special stinging cells (nematocytes) are unique to this group.
- Sessile or free floating (pelagic).
- Solitary or colonial or polymorphic.
- Metagenesis (alternation of generation) occurs. Reproduction asexual in the polyp and sexual in the medusa stages.
Examples of Phylum Coelenterata: A. Hydra; B. Aurelia; C. Adamsia (Sea-anemone); D. Obelia; E. Pennatula; F. Physalia (Portuguese man-of-war)
Corals and Coral Reefs
Corals are minute coelenterates which secrete deposits of lime and calcium carbonate that help in the building of coral reefs and islands. Because of this corals have been very popular and of scientific interest. The reef buildings corals require warm, shallow waters. They are therefore confined to continental and island shores in the tropical and subtropical zones. Coral reefs in the East Indies, and the Great Barrier reef of north eastern Australia are examples of coral reefs. The coral reefs provide a comfortable living place for a variety of animal species and they are one of the most beautiful sights in the world. They are of utmost importance to ecologists because coral reefs provide a unique opportunity to study many types of animal relationships.