Question

Corals are:

 A. Poriferans attached to some solid support 

B. Cnidarians, that are solitary living 

C. Poriferans present at the seabed 

D. Cnidarians that live in colonies

Answer 1

Reefs are formed of colonies of coral polyps (a sessile form of members of phylum Cnidaria) held together by calcium carbonate. They are as hard as a rock. 

The Great Barrier Reef is the world's largest coral reef system. The reef is located in the Coral Sea, off the coast of Queensland, Australia.