2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 



21 



first of all that in the most primitive known pogonophore, Oligobrachia 

 dogieli (Fig. 15), there is no metameric arrangement of the adhesive papillae 

 (Ivanov, 1957a, 1959f ). This fact alone is sufficient to show that the metameric 

 papillae of Pogonophora are a secondary acquisition. It also strongly con- 

 traverts the recent conclusion of Hartman (1954) that the metameric papillae 

 of Pogonophora may be considered as a manifestation of true segmentation 

 and that the adhesive papillae with their cuticular plaques are homologous 

 with the parapodia of Polychaeta. In the rest of the Athecanephria [with the 

 exception of the almost equally primitive Nereilinum murmanicum] we 

 already find a rather large number of metameric papillae — e.g. 50-55 pairs 

 in Birsteinia vitjasi, 50-75 pairs in Siboglinum caulleryi (Fig. 8). In Birsteinia 

 and Siboglinum, however, representing independent lines of development 

 (p. 141), the metameric arrangement of the papillae has been independently 

 acquired (Ivanov, 1957a). In the Thecanephria we see the same picture of 

 gradual metamerization of the papillae. Thus in Heptabrachia gracilis there 

 are always с 17-18, in H. abyssicola 14-20 and in H. beringensis 24-26 pairs 

 of metameric papillae. In representatives of other genera the metamerism 

 affects more and more papillae, as may be seen in Table 1, until finally in 

 Spirobrachia the transformation involves the majority of these organs in the 

 anterior section of the trunk (Fig. 16). 



Table 1 



Number 



of pairs 



of metameric 



papillae 



Approximate 



number of 



nonmetameric 



papillae 



Ratio of lengths 



of the metameric 



to the nonmetameric 



parts of the trunk 



if 



Oligobrachia dogieli 

 \ Birsteinia vitjasi 

 Siboglinum caulleryi 





 50-55 

 50-75 



Very many 

 Many 

 Many 



? 

 1 :5 



3 I Heptabrachia gracilis 17-18 



J3 I Heptabrachia abyssicola 14—20 



8* J Polybrachia annulata 22-28 

 g \ Zenkevitchiana longissima 38* 



л I Spirobrachia beklemischevi 160-180 

 t -1 \Spirobrachia grandis 220* 



Very many 



Very many 



250-360 



Many 



15-50 



? 



? 

 1 :10 



1 :10 



? 



5 :1 



? 



* Counted in a single specimen only. 



