CIRCULATORY APPARATUS OF THE NEMERTEA. 45 
In all sections through the precerebral region the lacuna is 
only one, horse-shoe shaped, having between its legs the pro- 
boscidian sheath, which become visible at the same time with 
this lacune. The circular muscular layer, the lacuna and the 
proboscidian sheath, I saw together in the sixth slice (j; mm. 
from the tip of the snout). The former was present only on 
the dorsal side, and the fibres diverge on the sides, extending 
and diverging through the surrounding tissue nearly to the 
basal membrane. At first in the eighteenth slice the lower 
arch of the circular muscular layer presents itself, also diverg- 
ing in the sides and thus crossing with its fibres those of the 
upper arch. So the proboscidian sheath in Lineus has not 
penetrated through the circular muscular layer, as in the other 
species, for it was already present when the lower arch of this 
layer formed itself. Already from the beginning I saw longi- 
tudinal fibres occurring within the circular muscular layer, as 
in Valencinia (see, however, Polia). Just before the upper 
brain commissure the lacune united beneath the proboscidian 
sheath, whilst they parted above it. The horseshoe is now 
turned, with its legs upwards. Now also it breaks beneath the 
sheath, forming thus two lacunz, one on each side of the 
proboscidian sheath. The left of these (in my specimen) (fig. 
54) communicates with the median vessel in the proboscidian 
sheath. But this I regard as absolutely abnormal. (In the 
two other specimens I examined, the proboscis much interfered 
with my observations of this process.) My specimen was 
greatly contracted, but it seems to me, that in general the 
region that follows closely resembles that same region in 
Valencinia, both with regard to the form and extent of the 
circular muscular layer, the longitudinal fibres, the transverse 
bands, lacunz, proboscidian sheath, &c. Also with regard to 
the manner of embracing the posterior lobes and the ceso- 
phagus. 
Better than in the two former species, the lacunze on the 
sides of the proboscidian sheath remain visible. In transverse 
sections they are round, their hyaline basal layer well coloured, 
their epithelium well developed. They are separated from the 
