168 PART II. SYSTEMATIC ACCOUNT 



Thecanephria. Nereilinum murtnanicum possesses metameric dorsal glan- 

 dular shields and isolated ventral papillae in the postannular region, and 

 also fusiform spermatophores. These are the principal external characters of 

 the Athecanephria and in the absence of any study of the internal anatomy 

 one can only conclude that it must be ascribed to this order. It should further 

 be noted that it differs from Diplobrachia in the structure of the bridle, which 

 lacks the cuticular rodlets attached behind the keels, and in the absence of 

 cuticular plaques. 



When Nereilinum murtnanicum is compared with the other species of 

 Athecanephria it is easily seen that it shares certain features with Siboglinum 

 and Oligobrachia. But it differs from Siboglinum in possessing two tentacles 

 instead of one, and in lacking adhesive papillae and any trace of metamerism 

 in the front part of the metasoma. Particular importance must be attached to 

 the latter trait which it shares with Oligobrachia, but whereas in Oligobrachia 

 the pyriform glands are scattered at hazard throughout the latero-ventral 

 ridges of the metasoma, without any trace of regularity, in Nereilinum 

 murmanicum they are more regularly arranged and take on a metameric 

 disposition in the posterior part of this region. The shape of the protosoma, 

 and in particular the way in which its posterior border sweeps back on the 

 dorsal side to meet the keels of the bridle, closely recalls the condition in 

 Oligobrachia dogieli and in the more primitive species of Siboglinum (e.g. 

 S. caulleryi and S.japonicum), so that we may consider this a primitive feature. 

 The short oval eggs of Nereilinum closely resemble those of Oligobrachia and 

 are very different from the long eggs of Siboglinum. In other ways Nereilinum 

 approaches Siboglinum. The form of the cephalic lobe, the glandular belt of 

 the mesosoma, the zone of thickened papillae which lack cuticular plaques, 

 the structure of the girdle region and of the toothed platelets, the post- 

 annular region with its metameric papillae and dorsal glandular shields — 

 all these are found in various species of Siboglinum. One can attach no im- 

 portance to the absence of pinnules, since some species of Siboglinum possess 

 them while others do not. The metamerism in the postannular region sharply 

 demarcates the species under consideration from Oligobrachia, but the tubes 

 are very similar. 



In summary, one may say that the main features of Nereilinum mur- 

 manicum are all primitive and that it occupies a place intermediate between 

 Oligobrachia and Siboglinum. From the absence of adhesive papillae in the 

 anterior part of the trunk it must be placed in the Oligobrachiidae. 



Material: about 100 fragments of tubes, 28 containing whole animals and 

 one containing embryos. 



