CIRCULATORY APPARATUS OF THE NEMERTEHA. 55 
In the same slice in which this aperture lay I also saw the first 
trace of the lacunar space above it. The circular muscular 
layer is at first only present as an upper arch, which gradually 
grows round and closes beneath the proboscidian sheath. This 
now lies against the lower arch of the circular muscular layer, 
Here again the proboscidian sheath has not to penetrate the 
circular layer. The lacuna is single and horseshoe shaped, 
with its legs downwards. Already quite anteriorly longitudinal 
fibres are visible, which are everywhere numerous. The sheath 
leaves its position, going upwards, the lacune thereby taking 
the shape of a circle in transverse sections. This is only appa- 
rently abnormal, for we may say here that the arch of the 
lacune above the sheath is very broad—so broad that it disap- 
pears when the lower arch is already visible. This lacunar 
ring is only really visible in three slices, for the proboscidian 
sheath reaches the upper arch, and the lacuna is again horse- 
shoe-shaped, with its legs upwards. Exactly above the lower 
brain-commissure the median vessel in the proboscidian sheath 
communicates with the lacuna. Nowhere did I see this pro- 
cess so distinctly as here. The vessel was swollen, filled with 
blood-corpuscles ; the opening was so large that it seemed that 
the wall of the proboscidian sheath was broken there. After 
this the horizontal band again becomes visible, dividing the 
lacuna into three—two on the sides of the proboscidian sheath, 
which gradually embrace the posterior lobes, and the third 
lying beneath the sheath. This third lacuna is soon divided by 
three vertical bands going from the sides of the proboscidian 
sheath downwards. Soon, however, these three lacune are 
pushed sideways by a mass of transverse and longitudinal 
fibres, and whilst the horizontal band disappears communicate 
with the two large lacune on the sides of the proboscidian 
sheath. The posterior lobes are still visible in these latter 
lacune. The mouth of Langia, as it seems to me, lies further 
backwards than in other genera. The two lacune have by this 
time coalesced with those two in which the posterior cerebral 
lobes bathe, and now surround the esophagus. As in Cere- 
bratulus in all the lacune, except in those ventrally from the 
