OLIGOCHATA OF TROPICAL EASTERN AFRICA. 233 
sac. The sperm-sacs are paired, and extend, as in Poly- 
toreutus magilensis, through a large number of segments. 
Their commencement is in the 11th segment, and here they are 
somewhat dilated. The upper wall of the sperm-sac curves 
round, and is attached to the dilated part of the sperm-duct, 
the testes lying just in the angle formed by it and the wall of 
the sperm-duct dilatation. The lower wall of the sperm-sac is 
formed by the septum separating Segments x1/x11. The sperm- 
sac then perforates this septum, and becomes a very narrow 
tube not more than a quarter or less of its dimensions at first. 
These two fine tubes pass along the dorsal surface of the gut 
side by side, and in contact with each other. They are 
partially concealed from view by the bulky atria, which also 
lie—at least for the greater part of their course—upon the 
dorsal lateral aspect of the intestine. At about the 30th seg- 
ment of the body the two elongated and narrow sacs appear to 
fuse together and form a much wider sac, which extends back 
for about ten segments, and is deeply constricted at the points 
where it passes through the intersegmental septa. The atria 
are circular in section. The lining epithelium is, as usual, 
composed of two kinds of cells; it is much folded. The 
muscular layers are excessively thin. On the lower surface of 
the atrium, but within the muscular coats, run two blood- 
vessels, between which is a distinct thickening of the longi- 
tudinal muscular coat. The two atria fuse together before 
opening on to the exterior. The exact point where the sperm- 
duct enters the atrium I have not discovered, but in any case 
it is not at a very great distance from the external pore. 
The illustration (fig. 7) will give some idea of the very re- 
markable character of the spermatothecal sac. It extends 
through five segments—from its beginning in the 14th to its 
external aperture on the border line of Segments xvrt1/x1x, 
and for two segments beyond this point—to the 21st segment, 
in fact. The spermatothecal sac is single, but shows unmis- 
takable indications of being the result of a fusion between 
two originally separate sacs. Anteriorly it is in contact with 
the wall between Segments x111/x1v. There are two diverging 
