154 ZOOLOGY 



lumen of the alimentary canal is usually distended with sand, 

 which is eaten in large quantities by the worm for the sake 

 of the small amount of vegetable debris which may be mixed 

 with it. At the commencement of the tail the intestine passes 

 into the rectum, which is supported by the numerous septa of 

 this region, and ends in the terminal anus. 



The blood-vessels consist of (i.) a dorsal vessel (Fig. 97), 

 which at the anterior end anastomoses with the ventral vessel 

 — the blood flows forward in this ; (ii.) a ventral vessel under- 

 neath the alimentary canal, in which the blood flows backward ; 

 (iii.) a subintestinal vessel which lies in the wall of the 

 intestine parallel and dorsal to (ii.). In the first six of the 

 gill-bearing segments this vessel receives the efferent vessels 

 from the gills. There are also a pair of small lateral vessels 

 which end anteriorly in the heart. 



The heart consists of a pair of enlarged, muscular, con- 

 tractile transverse vessels, which lie in the sixth segment. 

 They receive blood from the dorsal, subintestinal, and lateral 

 vessels, and by their contraction force it iuto the ventral 

 vessel. There are numerous capillaries given off from the 

 chief vessels to supply the various organs of the body ; the 

 blood is red. 



The blood in the ventral vessel is mainly venous, in each 

 of the thirteen segments which carry giUs this vessel gives off 

 a pair of afferent branchial vessels, one of which passes to 

 each gill. The gill consists of a number of branching fila- 

 ments, into each of which the body-cavity is prolonged. Up 

 one side of the filament runs the afferent vessel ; down the 

 other side courses the efferent vessel to open in the seventh to 

 the tweKth segments into the subintestinal vessel, and in the 

 thirteenth to the nineteenth segments into the dorsal vessel. 



The nephridia are twelve in number, a pair being found 

 in each of the last four segments of the neck and the first two of 

 the gill-bearing region. They consist of the usual funnel-shaped 

 opening into the body-cavity, of a large vesicle which opens to 

 the exterior, and of a glandular swelling which opens iato the 

 vesicle, and is probably the secreting portion of the apparatus. 

 In the breeding season the whole organ is crowded with ova 

 or spermatozoa. 



