SIBOGLINO'lDES 



329 



tube in this region are very thin and homogeneous (Fig. Z133^). Then dark 

 rings appear rather rapidly, three per segment. The middle ring of the three 

 is often a little shorter than the others. The colour in this region is a pale 

 brown, or rather brownish, and glistening (Fig. Z133.5). Then the walls 

 become thicker and darker, taking on a characteristic lustre and a golden 

 brown tint. The interspaces between the rings are narrow and pale gold in 

 colour (Fig. Z133C, D). In the hind part of this region of the tube the rings 

 often become doublets (Fig. Z133£). Still farther back the outlines of the 

 rings begin to break down and become very irregular, and neighbouring rings 



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Fig. Z133. Siboglinoides dibrachia : Tube: A-G - successive parts starting at the front end. (After 

 Ivanov, 1961a.) 



