Hydrodendron dichotomum (Allman, 1880)

Colonies to 70 x 70 mm, hydrocaulus unfascicled, a tangled and anastomosing mass of dichotomous branches without main stem.

Branches and hydrorhiza of same diameter; cauline perisarc very thick proximally, thinning a little toward distal end of branches. Branches usually given off from distal part of internode below an oblique to transverse node; in younger branches sometimes an extra node or a series of imperfect internodal annulations, a hydrophore in each branch dichotomy.

Hydrophores alternate on younger branches, borne distally on internode on short, thickened apophyses, pedicel crumpled, walls of hydrophore above pedicel swollen, more tumid on adcauline side, walls thereafter smooth, expanding evenly to hydrothecal margin; diaphragm deep bowl-shaped, no desmocytes observed; margin circular, rim everted and somewhat outrolled.

Nematothecae borne on a small platform below apophysis, vase-shaped to almost tubiform, narrowest at base and slightly constricted just below margin, rim circular, sometimes weakly everted, perisarc thin.

Hydranths extensible, with c. 30 tentacles.

Colour: Colony pale yellow-brown, hydrothecae white.