Aspidosiphon gracilis (Baird, 1868)

(Fig. 5 D–H)

Material. Nha Trang Bay: Tre Island, Dombay Harbor, 10 m depth, tube of Cerianthus filiformis, 1 specimen.

Description. Trunk long and thin, 20 mm long, about 2 mm wide, yellow-grey or light brown, with minute papillae; introvert 1.5 X trunk length. Anal shield weakly developed, composed of noncontiguous brown papillae and dark platelets arranged in weakly defined longitudinal rows. Caudal shield light brown, conical, with distinct radial grooves. Unidentate hooks arranged in rings in the distal introvert; proximal part of the introvert with conical hooks. Base of unidentate hooks with bifurcated posterior edge. Retractor muscles originate close to the posterior trunk end, at 5–8% of trunk length from the caudal shield. Gut with 8–10 loops; spindle muscle attached posteriorly. Nephridia are free, 60% of trunk length.

Discussion. This species varies from other representatives of the subgenus Aspidosiphon (Aspidosiphon) in the structure of both the anal and caudal shields.

Aspidosiphon gracilis is a tropical species known from the Indian and Pacific Oceans. It has been found from Australia to the Philippines in the West Pacific. It inhabits coral rocks and mollusk shells in intertidal and shallow water.