366 PART II. SYSTEMATIC ACCOUNT 



с 8000 m in the region of the New Britain Trench to the south-west of 

 Bougainville Island (6°S, 155°E) in the British Solomon Islands. The genus 

 is characterized by the following features: the protosoma, sharply divided 

 from the mesosoma, is very small and possesses a post-tentacular groove; 

 there are only a few tentacles, which are always free, but the base of the 

 tentacular crown forms a closed circle; the metameric section of the pre- 

 annular region of the trunk supports a large number of paired papillae; the 

 cuticular adhesive plaques are kidney-shaped and their front, thickened 

 edges are bowed with rounded ends; the toothed platelets of the girdles are 

 arranged in a few rows; paired epidermal, auricular lappets are developed 

 on the dorsal side of the mesosoma; the tube is simple, without either 

 segmentation or rings ; the spermatophores are like those of Heptabrachia 

 and Polybrachia. Almost all the features enumerated suffice to distinguish 

 the genus from Heptabrachia, and almost the only feature they appear to 

 have in common (apart from the common characters of the family) is the small 

 number of tentacles. Important distinctions are the following: the circular 

 area of attachment of the tentacles, the numerous paired metameric papillae, 

 the multi-serial disposition of the toothed platelets of the girdles, and the 

 structure of the tube, which in Heptabrachia is always ringed. Cyclobrachia is 

 distinguished from Polybrachia and from other related genera not only by 

 the small number of tentacles but also by the circular base of the tentacular 

 crown, by the post-tentacular groove, and by the structure of the tube. The 

 circular base of the tentacular crown appears to be a feature shared by 

 Heptabrachia beringensis, Zenkevitchiana and Lamellisabella. Particular 

 interest attaches to the length of the secondary metamerism of the pre- 

 annular region of the trunk in Cyclobrachia. This feature is not general 

 amongst the Polybrachiidae as a whole. Indeed, in number of metameric 

 papillae (more than 100 pairs) Cyclobrachia is equalled only by Spirobrachia. 

 Thus, in Cyclobrachia a primitive feature — the small number of tentacles, is 

 coexistent with an advanced character. It is also note-worthy that the cuti- 

 cular plaques of the papillae in Cyclobrachia are very similar to those of the 

 Spirobrachiidae and distinguished from those of the other Polybrachiidae in 

 which they are never bordered at the front by thick bows with rounded ends. 

 As regards the dorsal auricular lappets of the mesosoma, something similar 

 has been described only in Galathealinum where they lie on the ventral side 

 in front of the bridle (Kirkegaard, 1956a). 

 Type of genus : Cyclobrachia auriculata Ivanov. 



