26 PART I. GENERAL ACCOUNT 



by a single nerve, the left tentacles receiving nerves from the left side and the 

 right from the right side of the brain. In Siboglinum the nerve of the single 

 tentacle arises asymmetrically on the right side of the brain (Fig. 36). This 

 suggests that, in this genus, only a single right tentacle remains of the whole 

 tentacular apparatus. 



[Other genera in both orders of Pogonophora also show signs of a reduction 

 in the number of tentacles, though no other genus has been found in which 

 there is only a single tentacle. But in two genera of the Thecanephria and in 

 two of the Athecanephria we find species with only a pair of tentacles. In the 

 latter order Nereilinum and Siboglinoides, belonging to the Oligobrachiidae 

 and Siboglinidae respectively, occupy an intermediate position between 

 Oligobrachia and Siboglinum. The right tentacle of the pair in these two genera 

 corresponds to the single tentacle of Siboglinum and to the tentacle which 

 appears first in the ontogeny of Oligobrachia ; the left pair is homologous with 

 the second tentacle of Oligobrachia, i.e. with the left anterior tentacle. 

 Amongst the Thecanephria Diplobrachia and Sclerolinum each possess only 

 two tentacles and we may presume that they are homologous with the anterior 

 pair of tentacles of the Oligobrachiidae. Ivanov (1961a) is of the opinion that 

 in Diplobrachia they represent all that remains of a complete tentacular 

 crown found in its ancestors, and it is of course necessary to add that not all 

 species of this genus possess only two tentacles, despite the name. It seems 

 not improbable that future investigation may reveal the existence of species 

 of the Thecanephria in which the process of oligomerization of the tentacles 

 has been carried to extremes as it has in the unitentaculate Athecanephrian 

 Siboglinum, and we must beware of automatically ascribing any form with 

 but a single tentacle to this genus — D.B.C.] 



[Thus in several parallel lines of evolution of the Pogonophora a reduction 

 of tentacles has occurred.] On the other hand, in a series of genera {Poly- 

 brachia, Lamellisabella and Spirobrachia) we see a picture of successive 

 multiplication of tentacles accompanied by increasing complexity of organi- 

 zation (Ivanov, 1952) — a typical example of the polymerization of organs 

 in the sense of Dogel' (1954). 



In forms with a few or a moderate number of tentacles, such as Oligo- 

 brachia, most species of Heptabrachia and Birsteinia, the area of their attach- 

 ment to the body is horseshoe-shaped, with the ends of the horseshoe 

 turning in on the ventral side (Fig. НА, B), and the whole crown is bilater- 

 ally symmetrical. In some forms, however, the ventral ends approach in the 

 mid-line to form a complete circle of tentacles, e.g. in Cyclobrachia, Hepta- 

 brachia beringensis, Zenkevitchiana and Lamellisabella (Fig. ME). 



