The hydrothecal diaphragm of the hydroid is usually transverse, but in some hydrothecae it is oblique. The diaphragm forms the junction between the pedicel and hydrotheca and is a point of weakness at which hydrothecae often break away.
Hydroid:
Colonies both stolonal and erect.
Hydrorhiza tubular, loosely adherent to the substrate. Stems small, unfascicled, corrugated throughout, rarely with smooth patches, perisarc thicker proximally, becoming thinner distally. Proximal stem region unbranched, branching thereafter pseudo-sympodial, but often two to four hydrothecae given off from the same stem internode. Pedicel of hydrotheca corrugated, perisarc thinning distally and widening gradually to merge into base of the hydrotheca.
Hydrotheca almost cylindrical distally, widening slightly to margin, a transverse to oblique diaphragm near the base and above it a circle of desmocytes at the point of attachment of the hydranth. Young hydrothecae with a conical operculum of about eight very delicate wedge-shaped segments with blunt apex, the opercular flaps opening to form the margin.
Hydranth with about 16 long moniliform tentacles armed with the same nematocysts as in the medusa.
Gonothecae borne singly on hydrorhiza or hydrocauli, pedicel short, with one or two annulations, variable in shape from almost cylindrical to elongate top-shaped, widening distally, summit truncated, perisarc thin and smooth, containing one medusa and sometimes one medusa bud. The distal perisarc ruptures at release of the medusa leaving a loose sheath of tissue protruding.
Medusa:
Medusa at liberation dome-shaped, jelly thick, deeper than wide, velum broad, with four perradial tentacles with oval to triangular bulbs, tentacles long, moniliform when extended, with clusters of nematocysts. Four thin radial canals each with a rudimentary gonad in the distal third; four undeveloped interradial tentacle bulbs and eight adradial statocysts each with two statoliths on the ring canal. Manubrium saccate, mouth quadrate with four short lips and a few nematocysts. Four dark markings in the interradial position at base of the stomach. No umbilical canal and no exumbrellar nematocysts. Tentacular nematocysts microbasic mastigophores, shaft thick, with long spines, thread thin, spinous. Preserved specimens at the eight-tentacle stage with elongate stomach and manubrium with slightly fluted lips; four characteristic interradial brown-black spots at the junction of peduncle with exumbrella. At this stage the gonads are hollow canoe-shaped structures occupying the distal third of the radial canals; some contain ripe ova.
Colour: Colonies white to colourless. Medusa at liberation transparent, stomach and perradial tentacle bulbs yellow to pale brown. Older medusa (preserved material) clear, gonads white to creamy pink.
Cosmopolitan.
Subtidal, on algal and invertebrate substrates.