Alleurythoe tenuichaeta

Description. Type specimens preserved alcohol pale, without pigmentation. Body quadrangular in cross section, middle region enlarged, tapering posteriorly (Fig. 1 a). Prostomium rectangular, divided into two parts by transverse groove. Anterior lobe prominent, slightly bilobed anteriorly, with two palps and two lateral antennae, palps on ventrolateral part (Fig. 2 a, b), lateral antennae subulate, emerging on posterior edge of anterior lobe, similar in shape and size to palps. Posterior lobe rectangular, slightly smaller than anterior one, with median antenna, digitiform, emerging in front of caruncle, short, extending back only to first chaetiger (Figs 1 b; 2 a). Two pairs of eyes present. Buccal opening occupying two chaetigers (Figs 1 f; 2 b). Caruncle trilobed, conspicuous, about 2 times as long as prostomium, attached to and confluent with posterior prostomial lobe, free from body wall, extremity tapering, extending back to middle of fourth chaetiger, median keel broadly sinusoidal, lateral lobes plicate each with approximately 6 - 7 folds (Figs 1 b-e; 2 a), located slightly behind posterior prostomial lobe, supported by a basal plate. Pharynx unarmed, sac-like (Figs 1 f; 2 b). All parapodia biramous, with thickened collars encompassing noto- and neuropodial fascicular lobes. Chaetiger 1 greatly reduced, incomplete dorsally and ventrally. Chaetiger 2 surrounding mouth posteriorly, represents first complete segmental ring (Figs 1 f; 2 b), with distinctly separated notopodia and neuropodia (Figs 1 g, 2 c); dorsal and ventral cirri conical and digitiform, respectively, both with stout basal cirrophores and slender distal cirrostyles; cirri of anterior 2 chaetigers longer than those of following chaetigers. Branchiae present from chaetiger 4, dendritically branched, filaments densely ciliated (Figs 1 h, 2 a). First branchia with eight terminal filaments, best developed branchiae with 43 - 46 terminal filaments in 21 - 53 chaetigers, reducing posteriorly to four or five filaments, the last three chaetigers without branchiae. Notochaetae coarser and shorter than neurochaetae. Notochaetae of three kinds: simple chaetae (harpoon chaetae without barbs, Fig. 2 d), stout harpoon chaetae, greatly reduced in anterior chaetigers (Fig. 2 e-g), well developed on following chaetigers (Fig. 2 h); and slender capillary chaetae. Notoaciculae spinose, numbering 4 - 6 per fascicle, (Fig. 2 i), arranged in row immediately in front of dorsal cirri, extending beyond chaetal lobe. Chaetiger 5 with 20 simple chaetae and few capillary chaetae; chaetiger 14 with six harpoon chaetae (with 8 - 9 barbs), 12 harpoon chaetae (without barbs) and 12 capillary chaetae; middle and posterior chaetigers each with 19 - 22 harpoon chaetae (each with about 23 coarse barbs) and 21 - 23 capillary chaetae. Neurochaetae of one basic kind: capillaries with or without spurs (Fig. 2 j), the former with smooth long prongs 3 - 4 times length of short prongs (Fig. 2 k-m). Neuroaciculae spinose, numbering 7 - 9, extending beyond neuropodial lobe, arranged along dorsal superior region of fascicle (Fig. 2 n). Paratype: Pygidium with dorsal anus opening on last three chaetigers (Fig. 1 i), pygidial cirrus with rounded anal papilla.

Measurements. Holotype incomplete, with anterior fragment and posterior fragment, without posterior end. Anterior fragment with 62 chaetigers, 71 mm long, and 10 mm maximum width, posterior fragment with 50 chaetigers, 60 mm long. Paratype complete, broken into two fragments. Anterior fragment with 60 chaetigers, 75 mm long, and 8 mm maximum width, posterior fragment with 61 chaetigers, 57 mm long.

Diagnosis. Body elongate, quadrangular. Caruncle trilobed, conspicuous, attached to and confluent with posterior prostomial lobe, free from body wall, median lobe broadly sinusoidal, each lateral lobe with 6 - 7 folds, supported by a basal plate. Parapodia biramous, with thickened collars encompassing noto- and neuropodial fascicular lobes; chaetiger 2 first complete anteriormost annular ring. Branchiae present from chaetiger 4, continuing almost to end of body, dendritically branched. Notochaetae coarser and shorter than neurochaetae, include harpoon chaetae and capillaries; barbs of harpoon chaetae on anteriormost chaetigers absent to few in number, better developed in following chaetigers. Bifurcate neurochaetae capillary.

Distribution. Presently known only from the type location, Beibu Gulf, South China Sea.

Etymology. The name of this species refers to the slender form of its capillary neurochaetae.

Material examined. Holotype, MBM 200146, Beibu Gulf, 20 ° 15 ' N, 109 ° 15 ' E, 38 meters, mud, coll. Ruiping Sun, 27 August 1962. Paratype: MBM 010006, Beibu Gulf, 19 ° 30 ' N, 108 ° 30 ' E, 66 meters, mud, coll. Zhengang Fan, 14 May 1960.