4. MUSCULATURE AND PAPILLAE 



45 



membrane of the girdle at a point where the conus of tonofibrillae of the 

 basal cell of a toothed bristle is attached. As may readily be conceived, the 

 contraction of these fibres must result in retraction of the girdle and thus 

 facilitate the disengagement of the toothed platelets from the walls of the 

 tube (Ivanov, 1960a). 



A proportion of the fibres of the longitudinal muscle layer turn inwards at 

 the boundary between the mesosoma and the metasoma to enter into the 

 structure of the transverse muscular diaphragm, which separates the cavities 

 of these segments. The diaphragm consists of radial and dorso-ventral 

 fibres. Some of the latter cross over between the dorsal and ventral blood 

 vessels in a sort of chiasma (Fig. 32). 



Fig. 32. Diagram of the arrangement of muscle fibres in the diaphragm of Siboglinum caulkryi. 

 ep - epidermis; vd - dorsal blood vessel; vv - ventral vessel. (After Ivanov, 1960a.) 



The adhesive papillae 



The papillae, whose disposition has already been described (Chapter 2), 

 are characteristic organs of the trunk. These outgrowths of the body may 

 contain a portion of the coelom or may be completely filled by muscle. 

 Several types of papillae may be distinguished, though intermediate forms 

 may be encountered. 



The papillae in the fore-part of the trunk in all Pogonophora, including 

 Siboglinum, possess their own proper part of the coelom, separated off from 

 the general body cavity by a basement membrane and by the layer of cutane- 

 ous musculature. In Polybrachia, Lamellisabella and Spirobrachia the coelom 

 of the papillae is voluminous and contains a liquid clearly different from the 

 coelomic fluid of the trunk (Fig. 68). In the papillae of Siboglinum the coelom 



