126 UNITED STATES NATIONAL MUSEUM BULLETIN 251 



56. Hypodistoma vastum (Millar) 



FiGUKE 45 



Sigillina vasta Millar, 1962, p. 153, fig. 10. 



MATERIAL EXAMINED 



Paiau IstANDs: Iwayama Bay, on east side of Kaki-suido (Oyster Pass) be- 

 tween Island XXIX and east end of Koror Island: T^IS'ST" N., 134°30'09" E. 

 (HO chart 6076, 2nd ed.) ; 3-20 ft. limestone with living and dead coral. GVF 

 sta. 220A, October 22, 1955. On Dromidia undentata (Riippell) (USNM 11414). 



Description. — A single large colony is described, 80 mm. X 75 mm. 

 in extent and 50 mm. in height, dark purplish brown and roughly 

 hemispherical. It was carried on the carapace of a dromid crab, 

 Dromidia undentata (Riippell) (fide Fenner A. Chace), about 30 

 mm. in length. 



Colony surface generally smooth; many branchial apertures scat- 

 tered over surface; about 20 common cloacal apertures, roundish or 

 elliptical in outline and up to 8 mm. in long diameter, distributed 

 along periphery on underside or on posterior part (held on posterior 

 side of crab) of colony. Well-developed hypothoracic lacunae under 

 thoracic layer about 2 mm. in thiclmess. Lacunae may be 3 mm. in 

 height; inner surface very smooth and glistening like soft blackish 

 leather, wall rather hard in consistency. Atrial apertures of zooids 

 open on side wall or on ceiling of lacunae. Abdommal and bottom 

 layers very thick. Test somewhat coarse in consistency, purplish 

 black. No system found in arrangement of zooids. "VVlien alive colony 

 pale brown with slightly purplish tint and test very soft to touch, 

 as lacunae were fully expanded. Largest examined zooid 9 mm. in 

 length, 2.5 mm. consisted of thorax. Branchial aperture 6-lobed, 

 located at anterior end of thorax. Atrial aperture also 6-lobed and 

 opens at tip of atrial siphon which issues from dorsoposterior comer 

 of thorax at level of third stigmatal row and usually reaches anteriorly 

 to level of middle of first stigmatal row, at maximum even to level 

 of dorsal ganglion. From 15-20 longitudinal muscles on each side 

 of thorax continued onto abdomen as two pairs of longitudinal muscu- 

 lar bands running along ventral side. About 35 transverse muscles 

 confined to stigmatal area, also 5 others near middle of spacious in- 

 termediate area between peripharyngeal band and first stigmatal row ; 

 this area as wide as first stigmatal row. Atrial siphon encircled with 

 a number of fine circular muscles, but quite devoid of longitudinal 

 muscles. Approximately 25 elongated stigmata in each of three rows. 

 Horizontal membrane well developed along transverse vessel. Ten- 

 tacles 12, larger and smaller ones alternate regularly. Ciliated groove 

 a small oval opening. Dorsal languets displaced from dorsomedian 

 line to left side for the distance of about four stigmata. Anus 

 bilobed, opens at level of second transverse vessel. Outline of in- 

 cubatory chamber seen clearly at dorsoposterior portion of thorax 



