34 A. OC. OUDEMANS. 
median vessel in the proboscidian sheath has, as I suggested 
from the two vessels in the sheath of Carinella and Cari- 
noma, some important bearing on the material interchange of 
the fluid in the proboscidian sheath. In accordance with this 
view the hyaline basal layer of the dorsal half is thin, of the 
ventral one thick. I further saw (see above) that in Cari- 
nella they were sometimes swollen and spongy. ‘Their 
surface was thus enlarged. In Carinoma I have never seen 
them so swollen, but here too their surface was on all sides 
surrounded by the fluid of the sheath, because their attachment 
on the inner surface of the sheath only takes place along a very 
small base; whilst in Valencinia the vessel is not only lifted 
up and supported by little bands, but is also washed on its 
ventral surface by the fluid of the proboscidian sheath. The 
preparations, however, were not sufficiently preserved to give 
a representation of it. In sagittal sections I have also seen 
the median vessel leave the proboscidian sheath. 
In this region the lacunz, as we have mentioned above, 
diminish in number, especially ventrally. However, not all 
the ventral lacunz disappear. The intestine shows here, on the 
sides of the median line, an inward fold, in which a lacuna 
remains visible, which further backwards obtains a layer of 
circular muscular fibres, so that finally these lacune have 
become vessels. Not so the proboscidian lacunz, with which 
all the remaining lacune have gradually coalesced. They 
remain running, on the sides of the proboscidian sheath, 
without muscular or fibrillar walls, through the whole foremost 
half of the intestinal region. Now and then they seem to be 
lied with fibres, but on closer examination these prove to be 
only transverse bands, which temporarily only limit them on 
their upper or lower side. They disappear as soon as they 
have come. 
In the foremost half of the intestinal region thus there are 
five longitudinal blood-spaces. These are united together by 
transverse vessels, which are extremely thin, and run regularly 
over each diverticulum (figs. 14, 15, 52). These transverse 
vessels I have seen both in transverse and in sagittal sections, 
