122 PART I. GENERAL ACCOUNT 



the top of the tube serve a protective function. In Lamellisabella and Spiro- 

 brachia, on the other hand, the thick-walled rigid anterior part of the tube 

 ends in a broad funnel (Fig 84Z)). 



The wall of the tube is always laminated (Fig. 86). The surface layer is 

 often much the thickest. In Birsteinia and Heptabrachia subtilis a considerable 



Fig. 86. Longitudinal section of the wall of the tube of Siboglinum ekmani. The lamellar structure of 

 the tube is conspicuous (After Jagersten, 1956.) 



part of the tube is covered with a special external fibrous or felt-like layer 

 (Ivanov, 1957a). The walls of the tube in many species of Siboglinum and in 

 Diplobrachia japonica frequently contain thicker or thinner yellowish or 

 brownish annular fibres. The tube grows in thickness from the internal 

 surface, thanks to the production of new layers. Such thickening of the walls 

 is undoubtedly accompanied by considerable increase in length. 



Chemical investigations of the tubes of pogonophores have given interest- 

 ing results. The biochemist N. V. Fishenko, at the Institute of Cytology of 

 the Academy of Sciences of the U.S.S.R., found that the material of the 

 tubes appeared to be polysaccharide, yielding a reducing sugar upon acid 

 hydrolysis. The chemical nature of this polysaccharide was clarified by 

 Brunet & Carlisle (1958), who reached the conclusion that the tubes of three 

 species of Siboglinum and Zenkevitchiana longissima contained chitin. [The 

 tubes gave a clear chitosan reaction and acid hydrolysis yielded glucosamine 

 and several amino acids, but no glucose or any other monosaccharide; 

 enzymic hydrolysis yielded N-acetylglucosamine. Fishenko's "reducing 

 sugar" must be glucosamine. The tube, then, must consist of chitin and 

 protein. No sulphur-containing amino acids were found, so the protein can 

 only be hardened by tanning, not by sulphur cross-links; that is to say it is a 

 schlerotin, not a keratin. This is confirmed by the aroma given off by burning 

 — the tube of Zenkevitchiana smells like the wings of a locust when burnt, 

 not like burning hair. Rudall (1962) has confirmed the presence of chitin 

 in pogonophore tubes by X-ray diffraction studies. It is perhaps worth 

 mentioning that the organic remains of graptolites have yielded glucosamine 

 upon acid hydrolysis and N-acetylglucosamine upon enzymic hydrolysis 

 — D.B.C.]. 



