Hydroids are colonial polyps that form upright branching, stem like structures. They attach themselves to substrates using a stolon. The stolon bears shoots that protrude into the water at regular intervals. The ends of these shoots are either occupied with zooids that have feeding and hunting parts or growing tips that serve to elongate the tubes and shape the new zooids (hydranths). The colony body is covered, except for the hydranths, with a rigid chitinous skeleton called perisarc (Kosevich, 2006 Zoology 109 pp. 244-259).
Attaching themselves to various substrates such as seaweeds (like Fucus, as in the example above), rocks, mollusc shells, cnidarians, sponges and even other hydriods (Genzano et al., 2008 Marine Ecology 30 pp. 33-46), they can be found in intertidal rock pools to the the deep sea (+700 m) (Henry et al., 2008 Deep Sea Research Part I: Oceanographic Research Papers 55 pp. 788-800). Some species have even developed symbiotic relationships with other organisms. Brooks and Mariscal (1985, Journal of Experimental Marine Biology and Ecology 87 pp. 111-118) report on one such relationship in the gulf of Mexico. The hydroids Hydractinia and Podocoryne colonise the shells of the hermit crab Pagurus pollicaris in the Gulf of Mexico. The movement of the crab gives these bottom feeding hydroids access to greater amounts of food, which it obtains by catching zooplankton using its gastrozooids. In turn, the hydroids provide the crab with protection from predators by repelling them with stings from their cnidae, capsules containing a colied tubule and venom used for hunting. The stings of some hydroids such as Lytocarpus phillippinus found in the Southern Pacific can cause itchy painful weals (Rifkin et al., 1993 Journal of Wilderness Medicine 4, pp 252-260).
Morphologically, hydroids bear a superficial resemblance to plants (and are often named as such like the Sea Fir, Dynamena pumila). This shape is achieved when the larva settles and undergoes metamorphosis to form the primary hydranth. During further colony development the shoot organisation of the hydroid transforms slowly, until large shoots emerge which have characteristic species specific features (Kosevich, 2006 Zoology 109 pp. 244-259).