11. THE TUBE 



121 



and many others), they may have irregular edges {Siboglinum minutum for 

 example), or sometimes they may be quite irregular in outline {Heptabrachia 

 subtilis). The tubes of a few species of Siboglinum are distinguished by each 

 ring splitting to form a doublet (e.g. S. fedotovi, Fig. 84v4) or neighbouring 

 rings may anastomose or fuse (S. ekmani and S. plumosum, for example). In 

 some forms irregular rings are found 

 only in the hind part of the tube 

 {Polybrachia annulata) or they may be 

 discovered only after stripping off an 

 external fibrous layer (Birsteinia, 

 Lamellisabella zachsi). 



Transverse or annular lines or nar- 

 row striations repeated at rather long 

 but regular intervals, may sometimes 

 be noticed on the surface of the tube 

 as well as the rings {Siboglinum 

 ekmani, according to Jagersten, 1956; 

 S. fedotvi, Diplobrachia japonica and 

 others). They are also found on tubes 

 which lack rings (perhaps more fre- 

 quently indeed) and are said to divide 

 the tube into segments. Segmented 

 unringed tubes are found in Zenkevit- 

 chiana, Polybrachia, Lamellisabella 

 johanssoni (Fig. 845) and the poste- 

 rior part of the tube of Spirobrachia 

 beklemischevi. The front edge of each 

 segment of the tube of many Poly- 

 brachiidae is furnished with a mem- 

 branous funnel-like frill (Fig. 48 C). 



In the majority of Pogonophora the 

 long anterior part of the tube is 

 different from the rest. The walls are 

 thin and soft, they are usually trans- 

 parent and too flimsy to support their 

 own weight (Fig. 85). It is possible 



that when the animal has retreated Fig. 85. Tube of Siboglinum caulleryi. The 



into the depths of the tube the walls of J™ 1 P art ° f the tube Jacks /ings and is 



r flimsy and transparent, with walls which 



this region collapse and by closing collapse easily. (After Ivanov, 1957a.) 



