The Atoll of Funafuti, Ellice group : its zoology, botany, ethnology and general structure based on collections made by Charles Hedley of the Australian Museum, Sydney, N.S.W.
Family Plumularidæ. — Aglaophenia clavicula, sp. nov. — (Plate xxiii., figs. 4, 5, 6.)
Aglaophenia clavicula, sp. nov.
(Plate xxiii., figs. 4, 5, 6.)
Trophosome: Hydrocaulus simple, monosiphonic, attaining to 3 cm. in height, the terminal 1·3 cm., consists of an undulate tubular extension indistinguishable from similar tubular growths which constitute the hydrorhiza. Hydrocladia alternate, one to each internode, arising from the front of the stem, from 2·5 to 7 mm. in length, and about 0′5 mm. apart. Hydrothecæ closely approximate, 0·25 mm. in height and about 0·14 mm. in diameter. The shape is urceolate with a slight constriction below the base of the teeth. Margin with seven erect teeth, the median one is evenly rounded at the apex, those at the sides are somewhat acute.
Intrathecal ridge distinct, extending transversely across the basal portion of the hydrotheca.
Lateral nematophores 0·1 mm. in length, 0·05 mm. in diameter, slightly projecting beyond the margin of the hydrotheca; aperture elongate, opening upwards and inwards.
Mesial nematophore 0·2 mm. in length, 0·05 mm. in its broadest diameter, adnate to the hydrotheca to within 0·1 mm. of the summit of the central tooth. Hydrothecal internode with a short ridge or fold opposite the basal constriction of the hydrotheca.
Gonosome: Corbula closed, 2′5 to 3 mm. in length, and 1·1 mm. in diameter; the first internode bears a normal hydrotheca. There are from 8 to 12 pairs of adnate costæ; each costa bears from 6 to 8 minute nematophores along its upper margin, and has a modified hydrotheca at its base. In a median longitudinal line on the upper surface are situated a series of from 8 to 10 elliptic or elongate apertures with broad, flat, thickened margins, similar to those figured by Allman in the Challenger Report.*
These species exhibit two characters which are of great interest from a morphological point of view.
* Allman—" Challenger " Report—Zool., vii., pl. xx., fig. 6.
The corbula is of the closed kind, and consists of a modified branch bearing an alternating series of short stumpy branchlets, each of which carries a hydrotheca differing from those on the ordinary pinnules in being longer, more cylindrical, and in having nine instead of seven marginal teeth.
The distal branches of the corbulæ exhibit the mode of origin of the costaæ and costal appendages from the mesial nematophore of the hydrotheca. The specimens at my command are very few, and their extreme transparency renders the outlines of the costal membranes difficult to trace. Three stages, however, can be distinctly discerned. In the earliest stage the mesial nematophore is seen projecting from the front and arising from the base of the hydrotheca, it assumes a fan-shaped outline, and consists of a wide membrane with an incipient micro-nematophore at its inner distal angle; in the next phase the membrane is larger and there is one fully formed micro-nematophore-and another incomplete one at the inner extremity; on the next older costa there are three fully formed micro-nematophores, and the membranous part is proportionately enlarged.
Prof. Allman, in his report on the Hydroida of the Gulf Stream,* describes two species—A. distans and A. bispinosa—in which there are modified hydrotheca at the base of each costa; both, however, are of the open corbulæ type, and the hydrothecæ appear to be more modified than in the species under notice.
* Mem. Mus. Comp, Zool., v., 2, pp. 44-46.