CIRCULATORY APPARATUS OF THE NEMERTEA, 19 
muscular wall of the proboscidian sheath in a direction slanting 
downwards, the lacunz doing so in the opposite direction, so that 
a connection between these two spaces may exist. In a slice 
which was very thick, the connection was complete, but the 
section was so thick that the whole canal, if it was one, lay in 
it. I have distinctly seen a lumen in it. The walls of this 
narrow canal were limited by fibres which were directed from 
the wall of the proboscidian sheath towards the walls of the 
blood-lacuna. Fig. 24 shows us a section through the ceso- 
phageal region of the second form. 
In regard to the histological structure of these vessels I 
must be very brief. Sometimes both the epithelium layer and 
the hyaline basal layer of the proboscidian sheath, the latter 
being tolerably thick, covered them, so that the vessel cannot 
be said to lie in the lumen of the proboscidian sheath. Some- 
times they were considerably swollen, and I saw neither hyaline 
basal layer nor epithelium layer covering them and separating 
them from the lumen of the proboscidian sheath. In such 
cases their wall was very thin, and their lumen divided into little 
compartments, more or less resembling certain portions of the 
nephridial system to be mentioned below. In both cases they 
lay upon the hyaline basal layer of the proboscidian sheath. 
Sometimes they were in no way of a peculiar colouring, some- 
times they distinctly showed two colours, one part being ex- 
tensively yellow. Of their cellular structure I can say nothing. 
If nevertheless I were to make a hypothesis as to their 
possible physiological signification, I should be inclined to ask: 
Could they perhaps have any significance with respect to nutri- 
tion, oxygenation, &c., of the fluid of the proboscidian sheath ? 
In this opinion I was strengthened when I noticed their 
probable connection with the vascular system, which, as we 
will soon see, communicates with the exterior by means of the 
nephridial system. This very provisional hypothesis will be 
further discussed in the description of the arrangements which 
we find in the following species. 
Gradually going backwards we have now approached a 
region where the singular arches formed by the lacune 
