330 PART II. SYSTEMATIC ACCOUNT 



may anastomose (Fig. Z133.F). The hindmost part of the tube has thin walls. 

 It is pale brown with dark rings some distance apart (Fig. Z133G). The longest 

 tube we have had at our disposal was 217 mm long. Its anterior diameter 

 varied from 120 to 135//, and its posterior was 105//,. 



It is natural to enquire if the two athecanephrian species with two tentacles 

 — Nereilinum murmanicum and Siboglino'ides dibrachia — could perhaps be 

 placed in the same genus. This is, however, impossible. S. dibrachia is very- 

 close to the genus Siboglinum and if it did not possess two tentacles it would 

 be almost impossible to find any serious reason against its inclusion there. It 

 is true that the possession of four or five girdles is not characteristic of the 

 genus Siboglinum, but the disposition of the girdles into an anterior and a 

 posterior group is a feature most characteristic of this genus and not en- 

 countered anywhere else. A more fundamental difference is the development 

 of flask-shaped glands along the whole length of the mesosoma. This has not 

 been observed in other pogonophores, in which, so far as we know, they are 

 not developed in front of the bridle. All the other features of Siboglino'ides 

 turn up sporadically in different species of Siboglinum. Above all, the strong 

 development of the anterior group of teeth on the toothed platelets is other- 

 wise encountered only in a few species of Siboglinum, notably in S. pinnu- 

 latum, S. fedotovi, S. buccelliferum, S. hyperboreum and S. robustum. The 

 postannular dorsal glandular shields of Siboglino'ides dibrachia recall of those 

 of Siboglinum ekmani (Fig. 97 D, E). The form of the cephalic lobe, the nature 

 of the bridle, and the differentiation and arrangement of the anterior papillae 

 of the trunk are what one expects to find in species of Siboglinum. In brief, 

 one could, almost without hesitation, class this species as a Siboglinum with 

 two tentacles. 



What can be argued against this idea? From what we know about the 

 ontogeny of the tentacular apparatus we can be certain that the single 

 tentacle of Siboglinum is the end point of a process of reduction of the ten- 

 tacular crown, which has left these animals with only the first right tentacle. 

 It is inconceivable that the left tentacle of Siboglino'ides has been added to the 

 right to complete a pair, for this would imply that the direction of evolution 

 had been reversed. We must conclude that in comparison with Siboglinum the 

 presence of two tentacles in Siboglino'ides is a primitive character marking a 

 stage in the reduction of the tentacular crown. We cannot derive Sibo- 

 glino'ides from Siboglinum by the addition of another tentacle, but must derive 

 it from a multi-tentaculate ancestor by the reduction of tentacles. From the 

 systematic point of view then, Siboglinum and Siboglino'ides must be regarded 

 as separate genera. 



