SIBOGLINUM PELLUCIDUM 233 



tube is unsegmented, colourless and transparent, with weak transverse 

 fibres, except in one individual which had no fibres at all. Then just discern- 

 ible yellowish rings gradually begin to appear, separated by narrow, colour- 

 less, transparent intervals. The length of the rings is about a quarter the 

 diameter of the tube in this region, and the fibrous structure is distinct. In 

 the middle of the tube the rings gain a light muddy-brown colour and a 

 lustrous surface, their boundaries become clearer and the fibres coarser 

 (Fig. 118/)). The longest fragment of tube is 140 mm long, and the fragments 

 are up to 0-34 mm in diameter near the front end. 



S. meridiale may be distinguished from other species by the long fore-part 

 of the body, by the almost colourless, ventrally fused, delicate keels of the 

 bridle, by the slender tentacle, and by the very small cephalic lobe. In this 

 last respect S. meridiale recalls S. microcephalum (p. 227), but it is easily 

 distinguished from it by the nature of the tube. 



Material : seven broken-off tubes, three of which contained badly pre- 

 served animals. 



Locality: the Indian Sector of the Antarctic Ocean. 



Depth: 200-1200 m. 



Substratum: silty clay, sometimes with an admixture of pebbles, fine 

 gravel or sand. 



75. Siboglinum pellucidum Ivanov (Figs. 20, 119) 



Ivanov, 1957a: 449-51, 458, 459, 491, 495-7, Figs. 17-19, 59; Ivanov, 1960a: 1529, 

 1541, 1616; Ivanov, 1960c: 5, 10, 12, 27, 96, 105, 128, 167-70, 180, 185, 197, 265, Figs. 

 20,87, 119. 



The small S. pellucidum, whose delicate tubes were found in the south- 

 western part of the Bering Sea, is evidently one of the species characteristic 

 of the abyssal zone of that sea, and is usually found in company with S. 

 minutum, S.fedotovi, Heptabrachia gracilis and Polybrachia annulata. 



The protosoma and the mesosoma are equal in length (Fig. 119.1, B). The 

 transverse groove between them curves backwards on the ventral side and is 

 interrupted on the dorsal side. Tapering gradually towards the front end, the 

 protosoma merges imperceptibly into the small conical cephalic lobe, while 

 the very long and slender tentacle lacks pinnules. The cylindrical mesosoma is 

 scored by weak median furrows on both dorsal and ventral sides (Fig. 

 119v4, В). The delicate keels of the weakly developed bridle, lying at the 

 front end of the mesosoma, are usually interrupted by a series of gaps, 

 making them look like lines of dashes. As in larger species of Siboglinum, 

 their ventral ends are fused together but their dorsal ends do not meet. 



