144 PART I. GENERAL ACCOUNT 



development. Thus Polybrachia is distinguished by the secondary separation 

 of the cephalic lobe, Galathealinum by the formation of ventral, and Cyclo- 

 brachia by that of dorsal mesosomal outgrowths. It is interesting that 

 among the Polybrachiidae a circular arrangement of the tentacles is already 

 developed in some genera — a feature which is especially pronounced in the 

 Lamellisabellidae. The tentacle base forms a circle in Heptabrachia beringen- 

 sis, Cyclobrachia and Zenkevitchiana. 



The two higher families of the Thecanephria — the Lamellisabellidae and 

 the Spirobrachiidae — have from the very beginning apparently followed a 

 separate phyletic path from the more primitive forms exemplified by 

 Heptabrachia. Both these families have the protosoma united with the 

 mesosoma to form a unified fore-part of the body, and each tentacle fused 

 firmly to the next. The evolution of the tentacular apparatus in these two 

 families has, however, followed different paths. A tentacular cylinder is 

 formed in the Lamellisabellidae, while in the Spirobrachiidae the asymme- 

 trical spirally coiled tentacular plate is supported on a special corkscrew- 

 shaped lophophore (p. 27). 



My current ideas on the phylogenetic relationships within the Pogonophora 

 are set out in Fig. 88. 



