CIRCULATORY APPARATUS OF THE NEMERTEA. 41 
the cesophagus. There is a tendency to form five longitudinal 
blood-spaces. Behind the cesophageal region there are three 
vessels, united by transverse vessels. Communication of the 
three vessels occurs in the tail before the anal commissure, and 
like this, above the intestine. In the foremost half of the ceso- 
phageal region the median vessel lies in the proboscidian sheath. 
Its foremost communication is with the lacunes above the lower 
brain-commissure. 
In the head the vascular system becomes visible not at the 
tip of the snout but a little distance from it; in the specimen 
I examined, in the twenty-first slice, that is, about two-fifths 
mm. from the tip, the specimen had (in spirits) a width of 
one and a half, and a length of twenty-three mm. Quite in 
front there is no communication, two lacunz becoming visible 
at once. They are flat (fig. 43). The circular muscular layer 
is already well formed dorsally and ventrally from the 
lacunz. On the sides, however, the fibres cross one another 
and extend divergingly in the surrounding tissue to the skin. 
A similar crossing of transverse fibres, but in a higher degree 
and number, also takes place in the middle of the area between 
the two layers of circular fibres, so that it appears as if the 
lacune lie in a transverse muscular band in the shape of an 
8 ( c© ) lying sideways. 
As the upper lobes protrude forwards and have on their 
foremost part the upper brain-commissure, which is very thick, 
we obtain a figure as if the lacune with the circular layer lie 
beneath the brain. 
Behind the upper brain-commissure the brain on both 
sides increases in size, and the proboscidian sheath, which in 
the meantime has become visible, has penetrated the circular 
muscular layer, and taken up a position between the two 
lacune, and is gradually placed, with the lacune and the 
circular muscular layer, between the two brain-masses. Here 
is found the lower brain - commissure. Just before this 
becomes visible the lacunz coalesce above the proboscidian 
sheath (fig. 44), and gradually spread over the brain. On 
both sides of the sheath two small lacune remain; these go 
