Hydractiniida
Hydractiniida is an order of hydrozoans, a class of marine invertebrates belonging to the phylum Cnidaria. These colonial organisms are characterized by their polymorphic colonies, meaning individual polyps within a colony can have specialized structures and functions. A defining feature of Hydractiniida is the presence of a chitinous exoskeleton, or perisarc, that covers the stolons, which are the root-like structures connecting the individual polyps. This perisarc often encrusts hard substrates like shells, rocks, or even the backs of living animals, such as sea turtles and hermit crabs.
The colonies typically consist of feeding polyps (gastrozooids) and reproductive polyps (gonozooids). Some species also possess