142 PART I. GENERAL ACCOUNT 



Within the limits of the one genus Siboglinum somewhat different phyletic 

 lines have diverged. Thus it is possible that species with a bipectinate pinnu- 

 lar arrangement — species which are also characterized by a pretentacular 

 groove on the protosoma and a tendency for the rings of the tentacle to form 

 doublets (S. plumosum, S. fedotovi, S. ekmani, S. pinnulatum) may form a 

 separate group of species. And S. atlanticum and S. taeniaphorum are un- 

 doubtedly related and sharply distinguished from the remaining species. 



Some species of Siboglinum are distinguished by their lack of pinnules 

 (S. inerme, S. pellucidum, S. hyperboreum, S. bogorovi, etc.). The organiza- 

 tion of animals of the genus Siboglinum is in general somewhat simplified in 

 comparison with other forms and this is to some extent bound up with the 

 small size of the body. It is interesting that all the species we have just 

 mentioned are amongst the smallest in the genus and the lack of pinnules may 

 be a result of this further reduction in size. The job of absorbing food sub- 

 stances in these species must be taken over by the tentacles themselves. 

 Analogous reduction of physiologically important organs associated with 

 small body size is well known in a variety of animals. Thus some small 

 crustaceans (copepods and others) lack gills and a blood system. Bryozoa 

 have lost their blood system, undoubtedly in association with their small 

 body size which in its turn is associated with the formation of colonies. 

 Whether species of Siboglinum which lack pinnules may be accounted near 

 relatives is a moot point. It is possible that reduction of the pinnules has 

 taken place independently in various small forms. 



Amongst the Thecanephria the most primitive family is undoubtedly the 

 Polybrachiidae in which the protosoma and mesosoma are divided (except in 

 Zenkevitchiana), the tentacles are free and their bases are arranged in the 

 primitive horse-shoe. [If we except Diplobrachia and Sclerolinum] the smallest 

 number of tentacles (five to eight) is found in Heptabrachia, which in this 

 regard is like Oligobrachia. In other respects also Heptabrachia is primitive. 

 The cephalic lobe is not divided from the protosoma by a secondary ring 

 and the number of metameric papillae in the preannular region is still small 

 (14-20 pairs). Thus we may take Heptabrachia as lying near the common stem 

 of the Thecanephria. It is doubtful whether this genus is derived from primi- 

 tive Athecanephria of the type of Oligobrachia ; it is more likely that both 

 orders had a very ancient origin and sprang from a common distant ancestor. 



Diplobrachia [and Sclerolinum] with two tentacles hold the same place 

 amongst the Thecanephria as Siboglinum, [Siboglinoides and Nereilinum] 

 amongst the Athecanephria. They are distinguished by their small body 

 size, and, apparently in association with this, by secondary oligomerization 



