Fig. 141. Heptabrachia subtilis: Tube: 

 A - part of the front half of the tube 

 (external layer partly removed); В - 

 hind part. 



e - outer fibrous layer; i - inner layer. 

 (After Ivanov, 1957я.) 



slightly raised up on the surface of the tube, giving it a delicately wrinkled 

 or ridged appearance. An analogous structure, to all appearances, is en- 

 countered in Birsteinia vitjasi, in which, however, the similar peripheral 

 fibrous layer is distinguished by its noticeably greater thickness and fre- 

 quently by its black tint (Ivanov, 1952). The very front part of the tube of 

 Heptabrachia subtilis is transparent, with membranous, limp walls. In the 

 hind half of the tube the external fibrous layer vanishes (Fig. 1415), and the 

 hindmost end of the tube has homogeneous, transparent walls, muddy- 

 brown and rather firm in consistency. To judge by the available fragments 

 the overall length of the tube cannot be less than 25 cm, with a diameter of 

 0-5 mm near the front end and 0-17 mm near the back end. 



The diagnostic features of H. subtilis are as follows : the absence of any 

 secondary grooves of the mesosoma (a feature shared by H. abyssicola), the 

 fusion both dorsally and ventrally of the keels of the bridle, the higgledy- 

 piggledy papillae of the zone of thickening, the possession of 5-7 papillae 

 in each transverse row of the postannular section, the oval shape of the 

 adhesive plaques (as in H. gracilis, H. beringensis and H. talboti), and the 

 development of the peculiar external fibrous layer of the tube. The characters 

 of the seven species of Heptabrachia are compared in Table 3, (facing p. 368). 



Material : two females and one empty tube. 



Locality: the Japan Trench to the northeast of Ogasawara Gunto (or the 

 Bonin Islands) (29°N 143°E). 



Depth: 9715-9735 m. 



