Gymnangium longicorne (Busk, 1852)

Hydrorhiza a mass of tubular stolons entwining substrate, stolonal filaments passing up into strong cauline fascicular tubes.

Stems to 3 mm in diameter at base and 190 mm long, plumose, prosegment ahydrocladiate, unbranched. Branches of up to three orders given off in almost same plane from frontal tube of stem, ultimate branches monosiphonic, a long, blade-shaped hinge joint separating monosiphonic branches from preceding polysiphonic region, two short nematothecae on base of branch above joint. Branch internodes short, visible only in distal region, nodes transverse, distinct.

Hydrocladium short , with up to six hydrothecae, borne on short apophysis on front of branch, hydrocladia alternate, crowded, recurved, two on internode; internodes short, node almost transverse, two strong, complete intranodal septa crossing internode, proximal one passing into hydrotheca as a partial intrathecal septum, distal node adjoining lateral nematotheca.

Hydrotheca long, saccate, occupying entire internode, adcauline wall adnate to internode, abcauline wall convex posteriorly, free half of wall saddle-shaped and tilted upwards to margin, a deep wedge of perisarc passing into hydrotheca below margin. Margin parallel with or inclined slightly to internode, a short mesial abcauline cusp connecting with perisarcal flange; margin circular with a pair of broad, centrally placed erect lateral lobes.

Mesial nematotheca very variable in length from very long, considerably overtopping hydrothecal margin to shorter than hydrothecal margin, length increasing distally along hydrocladium, longest nematothecae thin, tubular, sometimes a little swollen in distal third, terminal orifice small, circular, a larger secondary orifice on a short neck above hydrotheca and a small internal foramen connecting with hydrotheca. Shorter nematothecae with same terminal and secondary orifices as on longer nematothecae. Lateral nematotheca long, thin, tubular, sinuous, adnate to hydrotheca and extending beyond margin, terminal orifice circular. Two cauline nematothecae at hydrocladial apophysis, short, narrowing from base to small terminal orifice, proximal one pointing along hydrocladium, the other facing across branch.

Gonotheca inserted without pedicel in axil of hydrocladium, large, leaf-shaped at maturity, perisarc very thin, no terminal orifice, mature female gonophore with many small ova, not filling cavity. Stem and branches bearing " pseudophylactocarps", flexible structures resembling phylactocarps, but not bearing hydrothecae or gonothecae, usually replacing the first or second hydrocladium on a branch; pseudophylactocarp long, lax, proximal two or three hydrothecae normal, followed by up to eight internodes each bearing two or three long, digitate nematothecae.

Colour: Live colonies dark grey-brown; preserved stems shining golden-brown, branches and hydrocladia paler brown.