72 



PART I. GENERAL ACCOUNT 



vd 



Fig. 49. Groove-like invaginations of the ventral wall of the dorsal vessel in Polybrachia annulata, 

 in transverse section. 



fm - muscle fibre; int- intima of the vessel; inv - invaginations of the dorsal vessel; mes- mesentery ; 

 my - myocytes ; per - peritoneum; vd - dorsal vessel; vv - ventral vessel. (After Ivanov, 1960a.) 



considerably taller in the sagittal plane, though still flattened from side to 

 side (Figs. 33, 45, 64, 67). 



In the female, paired longitudinal lateral vessels branch off from the dorsal 

 vessel immediately behind the diaphragm to run in the walls of the ovary 

 (Johansson, 1939). They are very long and extend without branching to the 

 hind end of the ovary. Their number varies, according to the size of the 

 animal, from two to five pairs [Siboglinum : two pairs (Fig. 33) ; Polybrachia 

 (Fig. 68) and Lamellisabella 2д<г/ш: three pairs; Spirobrachia grandis : five pairs]. 

 In the male the dorsal vessel gives off numerous, not especially long, irregu- 

 lar lateral branches in the front part of the trunk, running along the internal 

 walls of the spermducts (Fig. 64). 



In the posterior part of the trunk, beginning at the level of the zone of 

 thickened papillae, the dorsal vessel becomes considerably smaller in 

 diameter and comes to lie in the dorsal body wall. The number of lateral 

 branches running from it increases and some of them are quite large. They 

 are suspended from folds of peritoneum and extend backwards for a con- 

 siderable distance, frequently branching irregularly (Fig. 45). In the posterior 



