SEA-FISHERIES LABORATORY. 239 



the castings in measured areas, he. estimates that the 

 amount of sand brought up by the worms in a year is 

 1,911 tons per acre, which, if evenly spread, would form 

 a layer thirteen inches in thickness. Taking two feet as 

 the average depth to which the worms descend, the layer 

 of sand in which they burrow passes through their bodies 

 once in every twenty-two mouths (see also p. 301). 



Vascular System (See Plate III.). 



The dorsal blood-vessel arises near the anus, and runs 

 on the alimentary canal to the anterior end, when it 

 breaks up into small vessels and capillaries. It contracts 

 moderately regularly from behind forwards. Connected 

 with it, in the tail, are intestinal vessels (a pair in each 

 segment), each of which runs round the intestine and 

 opens into the ventral vessel. The last seven pairs of gills 

 send efferent vessels to the dorsal vessel, and between con- 

 secutive efferent branchial vessels there are two or three 

 pairs of intestinal vessels. After receiving the most 

 anterior of these efferent branchial vessels at the level 

 of the thirteenth seta the dorsal vessel receives no 

 segmental vessels until it reaches the oesophageal pouches, 

 but it has numerous connections with the gastric plexus 

 or sinus. 



The dorsal vessel has no direct connection with the 

 heart. In front of the heart the dorsal vessel receives on 

 each side (1) vessels from the first three nephridia and 

 neighbouring body wall, (2) from the oesophageal pouches, 

 (3) from the. second diaphragm and the body wall near the 

 second setal sacs, and (4) from the body wall near the first 

 setal sacs. It then runs forward, pierces the first 

 diaphragm, and breaks up into capillaries supplying the 

 buccal muscles, prostomium, otocysts, &c. 



