the sessile, polypoid stage of Hydrozoans.

Small invertebrates whose colonies can take many growth forms including flower-like, tree-like or feathery. Hydroids are best represented in cool temperate southern Australian seas.

Marine organisms within the phylum Cnidaria that have life cycles in which they alternate between a plant-like, polyp stage and a medusa stage. During their polyp phase, individual hydroids tend to come together to form colonies. These colonies are commonly seen in bottom photographs from Georges Bank.

A hydroid is a type of cell contained in many mosses. When it dies, it leaves a tiny channel which water can travel through. The hydroid may be the progenitor of the tracheid, the characteristic water-conducting cell of the tracheophytes.