159 
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 months. 
Vascutar 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 cesophageal 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 esophageal 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 
buceal muscles, prostomium, otocysts, &c. 
