CIRCULATORY APPARATUS OF THE NEMERTEA. 51 
10. Cerebratulus urticans (J. Mill), Hubr., fig. 48. 
Quite at the tip of the snout there is only one lacuna. The 
proboscidian sheath lies beneath it. A circular muscular layer 
surrounds them. This layer is already formed on its dorsal 
part, when the lacuna and the proboscidian sheath become 
visible. So here, too, the proboscidian sheath does not pene- 
trate a circular layer. Already, at the foremost part, longi- 
tudinal fibres are visible within the circular muscular coat, 
which in the meanwhile is closed. The proboscidian sheath 
rests on the ventral side of the circular layer. In the medial 
line dorsally a bundle of longitudinal fibres increase in 
number, and finally touch the probiscidian sheath, so that the 
lacuna is divided into two, one on each side of the probos- 
cidian sheath. But we may as well say there are in the head 
of C. urticans two lacunz, which communicate anteriorly by 
a broad arch above the proboscidian sheath, as occurs in so 
many Nemertea. A Jittle further backwards the wall of the 
proboscidian sheath increases so that it pushes away the 
longitudinal bundle on its two sides, and touches the dorsal 
arch of the circular muscular layer. Now the lacune are each 
surrounded by longitudinal fibres. Immediately before the 
cerebral ring both the circular layers greatly diminish; one 
would not recognise them, especially that of the proboscidian 
sheath. The two lacune approach each other ventrally, finally 
unite, lying beneath the sheath. They communicate directly 
with the dorsal vessel in the sheath, but remain one single 
lacuna beneath the preboscidian sheath, between this and the 
lower brain-commissure. The proboscidian sheath again 
acquires a thicker circular layer. The upper arch of the 
medial vessel is again different from the lower. It is held up 
by a spongy tissue. 
As soon as the lower brain-commissure has disappeared, the 
lacuna beneath the proboscidian sheath goes down still further, 
and acquires a trapezoidal figure, limited above by the sheath ; 
to the left and right by the lower lobes (from which the lateral 
nerves arise), and beneath by the outer longitudinal muscular 
