Ascidians from the tropical western Pacific Author Monniot, Françoise Author Monniot, Claude UPESA 8044, Laboratoire de Biologie des Invertébrés marins et Malacologie, Muséum national d’Histoire naturelle, 55 rue Buffon, F- 75005 Paris (France) monniot @ mnhn. fr. monniot@mnhn.fr text Zoosystema 2001 23 2 201 383 journal article 10.5281/zenodo.5391440 1638-9387 5391440 Trididemnum inermum n. sp. ( Figs 49 ; 119D) TYPE MATERIAL . — Papua New Guinea . Coral Sea, eastern Fields, 10°09.35’S , 145°33.92’E , 31 m , 15. VI .1998 ( MNHN A2 TRI 150). ETYMOLOGY. — From the Latin inermus : without arms. FIG. 48. — Trididemnum discrepans ( Sluiter, 1909 ) ; A , zooid; B , larva. Scale bar: 0.5 mm. DESCRIPTION The largest colony is 6 cm in extent and 2 mm thick. It is semi-transparent, colourless (Fig. 119D) and without any spicules. It is lobed, mammillated, with wide common cloacal openings. The colony’s consistency is fleshy and rather firm. The tunic is resistant, making the small zooids difficult to extract. There are large cloacal channels, some of them occupied by shrimp. A long waist elongates the zooid between the thorax and the abdomen ( Fig. 49A ). The oral aperture is small with six lobes. The cloacal siphon makes a short tube, directed backwards, encircled by a sphincter. The 24 oral tentacles are short and in three orders of size. There is a A wide unperforated area anteriorly ( Fig. 49A ). The branchial sac is large with up to 15 stigmata in the first half row. The second row is slightly shorter but the last row is long and protrudes slightly below the level of the cloacal siphon. The retractor muscle is very short ( Fig. 49A ), thread-like, arising from the upper part of the long oesophageal peduncle. The gut forms an elongated loop. The single testis follicle occupies its centre, covered with five to eight turns of the sperm duct ( Fig. 49B ). The first turn is set apart from the adjacent coils. The ovary, poorly developed in this specimen, is located against the testis. No larvae were found. FIG. 49. — Trididemnum inermum n. sp. ; A , zooid; B , testis. Scale bar: 0.5 mm. In some but not all zooids, very small spots can be seen on each side of the thorax near the cloacal siphon. They may be an indication of lateral thoracic organs, but no trace of spicules was found in the colonies. REMARKS This species differs from other Trididemnum in its total absence of spicules and its massive and lobed colonies. It lacks the pigment which is present in the black, almost spicule-less, species Trididemnum discrepans ( Sluiter, 1909 ) .