Hydroids (Cnidaria, Hydrozoa) of the Danish expedition to

218
Distribution
Indonesia, Philippines, Japan. Type locality: Indonesia,
1°42.5’S, 130°47.5’E, 32 m.
Family Aglaopheniidae
Gymnangium longicorne (Busk, 1852)
Fig. 65.
Plumularia longicornis Busk, 1852: 399.
Aglaophenia longicornis. – Bale 1884: 157, pl. 14: figs 7–8,
pl. 17: fig. 5.
Lytocarpus longicornis. – Allman 1883: 45, pl. 19: figs 4–6.
Macrorhynchia (?) longicornis. – Stechow & Müller 1923:
474.
Halicornaria longicornis var. sibogae Billard, 1913: 67, fig.
54, pl. 4: fig. 38.
Halicornaria intermedia Billard, 1913: 65, fig. 53, pl. 4: fig.
37.
Gymnangium longicorne. – Watson 2000: 60, fig. 48A–F.
Material examined:
Kei Islands Expedition stations: 67. – 68, 30 m and 50 m
depth. – 71. – 73, with gonothecae. – 104.
Description
Colonies up to 20 cm high, multi-pinnate,
branching in one plane; composed of a polysiphonic
main trunk and pinnately arranged sidebranches
bearing themselves pinnately arranged
hydrocladia. Main trunk either unbranched or
forked, strongly polysiphonic up to distal end,
composed of a superficial primary tube and numerous
auxiliary tubes. Primary tube with alternate
hydrocladia, these short (2–3 hydrothecae)
and often lost. No nematothecae on auxiliary
tubes. From auxiliary tubes of main trunk issue
pinnately arranged side-branches, these always
monosiphonic, 1.5–2 cm long, at base a deeply
cut, oblique hinge-joint rendering branch easily
movable, other nodes only visible in more distal
parts, each internode with an apophysis bearing a
hydrocladium. Below hinge-joint no hydrocladia,
only median nematothecae. Apophysis of
hydrocladia associated with two nematothecae,
one on abcauline end and one in upper axil, both
nematothecae with two apertures, one aperture
much larger than the other.
Hydrocladia thin and dense, curved, not in one
plane, planes forming an angle < 90°, hydrothecae
facing towards above, hydrocladia relatively
short, with up to 7 hydrothecae, with dis-
P. SCHUCHERT
tinct oblique nodes, internode length about 0.2
mm. Each segment with two well developed
internal ribs.
First or second hydrocladium of side-branch
often modified into long, very flexible pseudophylactocarp;
proximal 2–5 segments like in
normal hydrocladia, then up to 12 modified segments
with without hydrotheca but with three
long nematothecae.
Hydrotheca about 0.22 mm in total length, 1/5
of its upper end curved for nearly 90°, lower part
straight or slightly curved, slightly inclined towards
above, adcauline side adnate for ¾ of its
length, opening-plane tilted towards above or
parallel to internode axis, margin with two broad,
rounded cusps, one on each lateral side and one
frontal, the latter cusp shallow and appearing as
frontal tooth in side view. Distal part of abcauline
hydrothecal wall sharply bent, at site of bend
an abcauline perisarc thickening projecting into
hydrotheca, on adcauline side near base of hydrothecae
a second, horizontal intrathecal ridge,
length and thickness very variable, opening for
hydranth below this ridge, axis of extended
hydranth thus S-shaped within hydrotheca. Hydranth
with about 10 tentacles.
Median inferior nematotheca tubular, distal
end sometimes slightly swollen, mostly very
long, as long or longer as height of hydrotheca,
the first hydrothecae of a hydrocladium may
have much shorter median nematothecae. Median
nematotheca adnate up to the middle of the
hydrotheca, with three openings: one at distal
end, one on upper side where becoming free from
hydrotheca, and one leading into the lumen of the
hydrotheca. Lateral nematothecae thin and tubular,
straight, following free upper wall of hydrotheca,
reaching beyond margin of hydrotheca.
Gonothecae on upper axil of apophyses for
hydrocladia, 0.4 mm long, leaf-shaped, flattened,
rounded distal end without opening, aperture on
side, slit-like, short pedicel near base.
Remarks
With its feather-like side-branches, pivoting at
the slightest movement, the long nematothecae,
and the lash-like modified hydrocladia, Gymnangium
longicorne (Busk, 1852) is a very characteristic
species. The very flexible hinge joint