The order Hydroida has members in the marine environment that belong to two suborders.
In the Anthomedusa the skeletal covering, when present, does not surround the hydranth (Athecate (athecate)).
In the Caribbean the Solitary gorgonian hydroid (Ralpharia gorgoniae) and the Solitary sponge hydroid (Zyzzyzus warreni) can be found, but they are quite rare.
In the Leptomedusa the hydrant is surrounded by a little cup in which the polyp can withdraw (Thecate (thecate)).
The Feather hydroid (Gymnangium longicauda), the Slender feather hydroid (Gymnangium speciosum), the Feather plume hydroid (Aglaophenia latecarinata), the Branching hydroid (Sertularella speciosa), the Christmas tree hydroid (Halocordyle disticha) and the Algae hydroid (Thyroscyphus ramosus) branch in one plane, while the Feather bush hydroid (Dentitheca dendritica) forms bush-like colonies.