Hydroids (Cnidaria, Hydrozoa) of the Danish expedition to

HYDROIDS OF THE DANISH EXPEDITION TO THE KEI ISLANDS
tapering into pointed horn-like process with a
nematotheca, sometimes on top of this secondary
hydrocladium an additional terminal segment
bearing a hydrotheca and nematothecae (Fig.
62C).
Internodes with internal ribs, variably developed:
one at each end, two at rear wall of hydrotheca,
one curved between hydrotheca and
median inferior nematotheca.
Hydrotheca cup-shaped, depth 0.13 mm, centrally
placed on internode, adcauline side adnate,
abcauline wall weakly s-shaped, adcauline wall
straight, hydrothecal margin with two broad,
rounded lateral lobes.
All nematothecae two-chambered and movable,
conical, walls straight, rim not incurved,
adcauline wall of upper chamber lowered, lower
chamber longer than upper one. Median inferior
nematotheca immediately below hydrotheca,
reaching beyond middle of hydrotheca, sometimes
on indistinct prominence of internode. Lateral
nematothecae inserted near rim of hydrotheca,
about 80 µm high.
Gonotheca inserted in upper axil of apophyses,
0.45 mm, diameter 0.3 mm, conical, walls
quite straight, distal end planar, circular.
Remarks
If present, the characteristically modified secondary
hydrocladium (Fig. 62B) renders Polyplumaria
cornuta (Bale, 1884) easy to recognize.
Unfortunately, these horn-like processes can be
quite rare and some younger colonies may lack
them entirely. Because the microscopic structure
of the hydrocladia of Polyplumaria cornuta and
Plumularia scabra are virtually indistinguishable
(cf. Figs 61C and 62D), other characteristics
Table 1. Differences between Polyplumaria cornuta and Plumularia scabra.
215
must be used to identify such material. The important
differences are summarized in Table 1.
The easiest way to distinguish them is by comparing
the colony form. While P. scabra has
colonies comprising usually several stems that
are branched up to the third order, P. cornuta
colonies form very elongate, solitary stems with
simple, short branches. The hydrocladia are also
generally shorter, but the length is quite variable
(cf. Figs 61A and 62A). The longest hydrocladia
were observed in a sample from deeper waters
(250 m). This sample also regularly had secondary
hydrocladia, while in colonies with shorter
hydrocladia they can be scarce or absent.
The hydrocladia of the stem of Polyplumaria
cornuta can be arranged in verticels and the
coenosarc of the stem can be canaliculated, both
characteristic for the genus Nemertesia, a fact
already noted by Billard (1913). The limits of the
genera Plumularia, Polyplumaria and Nemertesia
are thus somewhat diffuse.
Distribution
Indonesia, tropical coasts of Australia. Type locality:
Holborn Island, Queensland, Australia.
Nemertesia indivisa (Allman, 1883)
Fig. 63.
Sciurella indivisa Allman, 1883: 26, pl. 5: figs 1–4. –
Kirkpatrick 1890: 609.
Antennularia cylindricala Bale, 1884: 146, pl. 10: fig. 7.
Nemertesia indivisa. – Billard 1910: 38. – Billard 1913: 60,
fig. 50.
Material examined:
Kei Islands Expedition stations: 67. – 71. – Kei Islands
Expedition, Samalon Island, Ujungpandang, Sulawesi, 25
m, 29 Jun 1922.
Character Polyplumaria cornuta Plumularia scabra
Hydrocladia may be branched, secondary branch modified never branched
Terminal apophyses never modified some drawn into horn-like process
Branching (exclusive hydrocladia) 1st order up to 3rd order
Stem soft stiff
Hydrocladia on stem two rows or in verticels two rows
Hydrocladia often recurved towards branch end mostly straight
Gonotheca conical, circular cross-section ovoid, flattened, distal end truncated
Hydrotheca, abcauline wall shallow double-curvature strong double-curvature