2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 17 



The metasoma changes considerably from one end to the other. Two 

 main regions can be distinguished — in front the preannular region reaches to 

 the peculiar girdles [or annuli], behind which is situated the postannular 

 region (Figs. 2, 8, 16) (Ivanov, 1952). 



The preannular region is furnished in its anterior part with a rather wide 

 longitudinal ventral sulcus which gradually opens out backwards (Figs. 2, 

 3, 8, 16, etc). The sulcus is bounded at the sides by lateral ridges, each 

 bearing a single row of adhesive papillae which are round, oval or conical 

 protuberances furnished with small cuticular plaques (Figs. 5, 16). In the 

 primitive Oligobrachia dogieli, however, [and in Nereilinum murmankum] 

 adhesive papillae are absent from this region (Fig. 15), and in Siboglinum, 

 Siboglinoides and Zenkevitchiana they lack plaques (Figs. 3, 8, 9). By 

 means of these papillae and their plaques the animal, in all probability, clings 

 to the inner surface of the tube as it moves up and down inside it (Johansson, 

 1939; Ivanov, 1952). Thus the papillae play the same role as the parapodia of 

 the tubicolous polychaetes and deserve the appellation of locomotory, 

 adhesive or attachment organs. 



On the front part of the metasoma, in all pogonophores except Oligo- 

 brachia [and Nereilinum], the arrangement of the adhesive papillae is dis- 

 tinguished (though somewhat imperfectly) by a definite metamerism, which 

 it is impossible to consider as other than secondary (Ivanov, 1952, 1957a). 

 Farther back, however, the metamerism for the most part breaks down and 

 shortly thereafter the adhesive organs are found in scattered disorder, though 

 in the majority of species they are usually restricted to the ventral side. In 

 almost all the Pogonophora the greater part of the preannular region of the 

 trunk is characterized by just such a higgledy-piggledy scattering of the 

 adhesive organs. Thus the anterior metameric part of the metasoma is 

 followed by a posterior non-metameric part (Figs. 2, 8, 16). Amongst the 

 haphazard adhesive organs of the non-metameric part one may single out, 

 however, a group of particularly large papillae, lying ventrally a little in front 

 of the girdles. These papillae, usually conical and furnished with cuticular 

 plaques (except in a few species of Siboglinum which lack them also from 

 other parts of the body), are arranged in a single dense mid-ventral row 

 (e.g. in Siboglinum caulleryi, Fig. 10; and Heptabrachia gracilis) or without 

 any definite order (in Siboglinum vinculatum, Heptabrachia subtilis, H. 

 abyssicola, Polybrachia annulata — Fig. 11, etc). They constitute the "zone of 

 thickened papillae". The number of these papillae varies from three (Sibo- 

 glinum taeniaphorum), or five (S. caulleryi) to several dozen (Polybrachia 

 annulata, etc). In Spirobrachia there is no zone of thickened papillae. 



