G54 THE MARINE ANNELIDES OF THE ORDER SERPULEA, 



the cai'diac sac ; from this it is driven at intervals forwards to 

 the common branchial vessels, from which it passes by the vessels 

 of the branchial stems to the extremity of the branchiae ; 

 returning by the same course it enters the lateral ventral trunks 

 and passes thus to the ventral vessel by which it is distributed to 

 the capillary networks of the collarette and lateral flaps and to 

 the body generally. That all the blood constantly and regularly 

 takes this course is not to be supposed ; the absence of valves, 

 the constant and irregular contractions of the body of the worm 

 interfere with any such regularity ; but the above may be taken 

 as an accurate general account of the course of the circulation in 

 this species. 



In Pomatoceros the arrangement of the vessels is much more 

 like that described by Claparede as occurring in Spirographs. In 

 the abdominal region the principal trunks are the peri-intestinal 

 vessel, an exceedingly minute ventral vessel, situated between 

 the widely separated nerve cords, and several other trunks, 

 mostly longitudinal in direction, which lie in the peri-intestinal 

 space and are covered by a layer of granular cells. In the front 

 of the thoracic region (pi. XXXV., fig. 2) the peri-intestinal 

 vessel becomes divided into a large dorsal vessel or cardiac sac 

 d. v.), and a series of about sixteen smaller vessels (I. v.), 

 which run along the wall of the alimentary canal. The 

 ventral vessel still retains its position. A little further forward 

 the peri -intestinal vessels join the dorsal trunk, so that we have 

 here two main vessels, a large dorsal and a very small ventral. 

 Very soon the dorsal vessel bifurcates into two branchial vessels. 

 The ventral vessel also bifurcates anteriorly ; but whether the 

 two ventral trunks thus formed communicate in front with the 

 dorsal branchial vessels as in Eupomatus I am uncertain : it 

 seems probable that they do, as they are traceable into the base 

 of the lophophore. 



All the vessels, as observed by Claparede, possess a muscular 

 wall ; in the smaller vessels this is only to be detected by the 

 presence at intervals of fusiform nuclei ; but in the larger trunks 

 the layer is more distinctly developed. In the structure of the 





