70 A. 0. OUDEMANS. 
distinct and separate from the rest. In the remaining portion 
of the body, three longitudinal vessels, connected by trans- 
verse vessels, are present. The communication in the tail lies 
above the intestine. 
In the cesophageal region we find only one single vessel in 
the proboscidian sheath. Anteriorly, this is connected with 
the lacunar communication beneath the proboscidian sheath, 
piercing its lower wall. Posteriorly, it also pierces the lower 
wall of the sheath, and is continued into the median vessel 
lying under the proboscidian sheath. 
In the lower forms (Valenciniide) the two lacune on 
the sides of the proboscidian sheath continue further back- 
wards. The transverse vessels, which otherwise only connect 
the three longitudinal vessels, in that case also enter into com- 
munication with these lacune. 
The third type, the Hoplo-type, is especially characterised 
by the absence of lacunar spaces. Here there is a closed 
vascular system. Here we find (excl. Amphiporus 
hastatus, McInt.) in the head two vessels which communi- 
cate in front, forming a vascular loop, above the proboscidian 
sheath. These vessels also communicate within the cerebral 
ring, but now beneath it. From this point down to the tail 
three longitudinal vessels occur, of which the median vessel 
in the cesophageal region often lies partly in the proboscidian 
sheath. In the intestinal region generally the three longi- 
tudinal vessels are united by transverse vessels. The com- 
munication in the tail les above the intestine. 
The cephalic loop may form branches (fig. 8, Malacob- 
della). The portion of the median vessel which lies in the 
proboscidian sheath, in this type gradually becomes shorter 
and shorter. There are even forms in which it no more enters 
the sheath, but remains beneath it along its whole length. 
(Nemertes, Tetrastemma, Malacobdella.) In the in- 
testinal region, along the whole length, or only in the posterior 
portion, transverse vessels occur. In some cases, branches 
arise from the longitudinal vessels (more especially from the 
lateral ones) during sexual maturity (fig. 3, Malacobdella). 
