Family Plumulariidae McCrady, 1859

Diagnosis (modified from Calder 1997; Bouillon et al. 2006). Hydroid colonies erect, arising from a creeping, root-like, or disc-shaped hydrorhiza. Stem usually a monosiphonic or polysiphonic, branched or unbranched hydrocaulus, stem infrequently a branched or unbranched main axis giving off hydrocauli laterally. Hydrocladia alternate, opposite, or in verticils, with those on polysiphonic hydrocauli arising from a single primary axial tube. Hydrothecae uniseriate, usually small and partly adnate, occurring only on hydrocladia, with or without marginal cusps. Nematophores protected by nematothecae rather than occurring as naked sarcostyles. Nematothecae welldeveloped, usually bithalamic and movable, not fused to hydrothecae; at least three of them (with two lateral nematothecae and one median inferior nematotheca) adjacent to each hydrotheca.

Gonophores fixed sporosacs or infrequently as free but short-lived medusoids. Gonothecae solitary, with or without phylactocarps, seldom with nematothecae.

Remarks. The diagnosis of the family Plumulariidae is modified slightly herein to accommodate Schizoplumularia n. gen. and its unique colony morphology, especially the nature of the main axis and the origins and locations of the hydrocauli.