ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 459 



has correctly shown the heart and its relations to the neigh- 

 bouring vessels, and has also figured the blood-vessels of the 

 alimentary canal and the nephridia. Jaquet (1886) has 

 described the vascular system of A. Clap are dii, but through- 

 out his memoir he refers to this species as A. marina^ 

 although the distinction between these two species had been 

 pointed out some time before by Levinsen (1883). In our 

 paper on A. marina (1898)^ we have fully described and 

 figured the vascular system of this species^ and still more 

 recently, since our account and figures for the present memoir 

 were completed, Fauvel (1899) has published a short account 

 of the circulatory system of A. ecaudata. 



The vascular system of A. Grubii (except a figure of the 

 vessels of a single segment by Claparede, 1868, pi. xix, fig. 2) 

 and A. cristata has not hitherto been described or figured. 



The circulatory system is similar in its general arrange- 

 ment throughout the Arenicolid^. 



The dorsal vessel arises near the anus and runs forwards 

 on the alimentary canal to the anterior end, where it breaks 

 up into small vessels and capillaries. It contracts fairly 

 regularly from behind forwards. In its course forward the 

 dorsal vessel gives off intestinal vessels, each of which runs 

 round the intestine and opens into the ventral vessel. In 

 the gill-bearing region, in A. Grubii and A. ecaudata, the 

 dorsal vessel receives an efferent vessel from each gill, while 

 in A. cristata and A. Claparedii (as in A. marina) only 

 seven pairs of the posterior gills send efferent branches to the 

 dorsal vessel. Between consecutive efferent branchial vessels 

 there are two or three pairs of intestinal vessels. From the 

 point at which the dorsal vessel receives the most anterior 

 efferent branchial vessel to the oesophageal pouches, the 

 dorsal vessel receives no segmental vessels, but numerous 

 branches of the gastric plexus open into it. 



The dorsal vessel has no direct communication with the 

 heart. 



In front of the heart the dorsal vessel receives on each 

 ' See foot-note on p. 519. 



