SIBOGLINUM PLUMOSUM 197 



doublets (Fig. 102/)). In the intervals between the rings the walls of the tube 

 are colourless and transparent. Then, in the hindmost part of the tube, the 

 pattern of rings gradually breaks down and the walls become structureless, 

 thin and transparent. The tube may be up to 25 cm long and 0-25 mm in 

 diameter. 



From related species, which also possess a bipectinate tentacle and a pre- 

 tentacular groove, S. fedotovi is distinguished by the shortness of the 

 protosoma, by the very long mesosoma, by the delicate pinnules, by the 

 comparatively large toothed platelets of the girdles and by the detailed 

 structure of the tube. The tube is like those of S. ekmani and S. pinnulatum 

 in the segmentation, and like that of S. plumosum in the splitting of the rings 

 to form doublets. 



Material : many tubes, some containing animals. 



Locality: the southwestern part of the Bering Sea. 



Depth: 2995-3875 m. 



5. Siboglinum plumosum Ivanov (Figs. 19B, 103-4) 



Ivanov, 1957a: 456-61, 491, 495-7, Figs. 28-30, 59; Ivanov, 1960a: 1607, 1612, 1613; 

 Ivanov, 1960c: 5, 12, 14, 27, 92, 96, 105, 128, 134, 136, 140-2, 145, 196, 265, Figs. 19, 

 87, 103^1. 



A near relative of the preceding species, to all appearances, is S. plumosum, 

 one of the very largest species of this genus, which was taken in a trawl in the 

 Sea of Okhotsk to the east of the coast of Sakhalin. Unfortunately, the only 

 known specimen lacks the girdles and the postannular region. 



The fore-part of the body is comparatively shorter than in S. fedotovi, 

 with a length to breadth ratio of 5 : 1. The fore-part, however, and also the 

 tentacle, is evidently strongly contracted, as may be seen from the numerous 

 cross wrinkles, and also from the way the keels of the bridle lie in a trans- 

 verse line. The protosoma is broader than it is long (Fig. 103у4, В) and is 

 divided by annular grooves from the mesosoma and from the cephalic lobe, 

 which last is narrower than the protosoma and somewhat irregularly 

 rounded. The rather stout, long tentacle bears two rows of comparatively 

 large pinnules, which are absent only from its proximal part, and they are 

 arranged alternately in the right and left rows (Fig. 103^). The cylindrical 

 mesosoma is divided from the trunk by a simple transverse groove. The bridle 

 lies near the front of the mesosoma and its keels are not fused on the ventral 

 side and are wide apart on the dorsal side. Behind the bridle lies a pair of 

 lateral glandular belts, more strongly developed than in S. fedotovi. 



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