OIRCULATORY APPARATUS OF THE NEMERTEA. 43 
held up by ligatures, of which the remotest on the left and 
right sides are the most solid; and also here from time to 
time lumina are distinctly visible beneath the vessel, and it 
appears that the proboscidian sheath fluid also bathes its 
ventral surface. In another specimen I saw a part of this 
vessel, quite at its commencement, still before the mouth, 
where it seems not te be as described above. I have repre- 
sented it in fig. 68. The hyaline basal layer everywhere is the 
same as in Valencinia, but the vessels of Polia I have not 
observed to be surrounded by circular muscular fibres. In the 
median vessel beneath the proboscidian sheath I have seen out 
of the hyaline basal layer large nuclei in the gelatinous stroma, 
heaped up against the hyaline basal layer, just as is seen in 
fig. 65 of a transverse vessel in Cerebratulus marginatus. 
The two ventral vessels fill the whole space of the longi- 
tudinal grooves in the intestine and between it and the longi- 
tudinal muscular layer, so that the radially developed stroma, 
visible in Valencinia and Langia (figs. 62—64), was absent 
here, at least in the specimens examined by me. The dorsal 
vessel beneath the proboscidian sheath was pressed against the 
intestine, but not against the wall of the"proboscidian sheath. 
There was a little stroma between them. 
The nephridial system here is also situated in the cesopha- 
geal region. It commences near the mouth, and finishes when 
the first transverse vessel is seen; thus in a region where 
three longitudinal vessels are already present without lacune 
it penetrates even into the intestinal region. 
I cannot say how it appears when viewed obliquely. 
Openings and ducts of this system I found in the following 
slices : 
