Hydroid are primitive relatives of Corals

Hydroids live in feathery colonies. Each polyp feeds. They produce a medusa when the reproduce which floats off into the plankton . That is there are two phases to their lifestyle. Feeding polyps and reproductive jelly meduase.

 

 
hydroid Stylaster
This giant feathery Hydroid from Grubb Reef off Townsville hs scores of tiny polyps on each branch of its feathery support structure. There are hundreds of branches and hundreds of feathers. This colony will have several hundred thousand identical polyps which are all clones of each other. Pink Stylaster coral likes to grow on the ceilings of caves and overhangs. It is a primitive polyp living in a cement tree.
Solanderia fire coral
Solanderia is a Hydroid colony that resembles Gorgonian Fan Corals. However, Gorgonian fan corals are Octocorals. Solanderae is a very primitive Hydroid colony. Fire coral lives in a cement structure. However, it is much more primitive than coral. Fire Coral branching colonies can dominate the underwater landscape in some reefs. Here a pair of Regal Angelfish are near a fire coral colony.