Hydroid may refer to: Marine Biology Colonial, plant-like animals closely related to jellyfish, with stinging cells any member of the invertebrate order Hydroida (class Hydrozoa, phylum Cnidaria). Hydroids have three basic life-cycle stages: (1) a tiny free-swimming planula larva about 1 mm (0.04 inch) long, which settles and grows into (2) a sessile (attached), usually colonial hydroid stage, which in turn liberates (3) medusae. This cycle's exemplified by the genus Obelia, whose members are w… (More on Hydroid)