318 PART II. SYSTEMATIC ACCOUNT 



them are clear and creased (Fig. WW133y4). In the next part of the tube the 

 rings are very irregular, "chinks" appear in them and they anastomose with 

 one another (Fig. WW1332?). Then the rings regain their regularity but be- 

 come much shorter. Their edges are a little darker in colour (Fig. WW133C). 

 In the hind part of the tube the rings gradually fade and disappear, and finally 

 the tube becomes glassily transparent (Fig. WW133D). The fragment of tube 

 is 115 mm long, with an anterior diameter of 0-32 mm and a posterior of 

 0-27 mm. 



S. zanzibaricum is distinguished by several unique features. Usually when 

 the pinnules form two longitudinal rows they are thick-set, while a single 

 row of pinnules is sparse. In S. zanzibaricum the thick-set pinnules are 

 uniserial. Other peculiarities of the species appear to be the short fore-part, 

 the extraordinarily small protosoma and the fused keels of the bridle. 



Material: one individual in a tube. 



Locality: Vityaz* station 4686 (31st cruise): Indian Ocean to the east of 

 the Islands of Zanzibar and Pemba. 



Substratum: sandy mud. 



Depth: 2172 m.] 



[38. Siboglinum sergeevi sp. nov. Ivanov (Figs. XX133-ZZ133) 



A new species of Siboglinum was found in a dredge haul taken by R.V. 

 Vitya£ on 17 October, 1959, in the Ceram Sea at a depth of 4774 m. 

 Unfortunately only a single specimen was found, but it was excellently pre- 

 served. I have named the new species after Captain I. V. Sergeev, command- 

 ing officer of R.V. Vityaz\ to whom Soviet oceanology owes so many of 

 its achievements and who was unfailing in his courteous assistance of my 

 work on board R.V. Vityaz\ 



The very short fore-part of the body is slightly compressed from side to 

 side. Its length to breadth ratio is about 3 : 1 and the bridle lies at about its 

 middle. An annular groove in front of the bridle, sharply defined ventrally 

 and disappearing dorsally, represents the boundary between the protosoma 

 and mesosoma (Fig. YY133^4, B). The tentacle is inserted a little nearer 

 to this groove than to the tip of the cephalic lobe. There is a post-tentacular 

 groove, equally well defined on both sides of the body. Anteriorly the 

 protosoma runs imperceptibly into the short conical cephalic lobe (Fig. 

 YY1335). The long thin tentacle has no pinnules (Fig. XX133). 



The mesosoma is no more than twice as long as it is broad. There is a 

 short mid-ventral furrow in front of the bridle, and behind it a transverse 

 fold or groove. The bridle is very like that of S. zanzibaricum sp. nov., but 



