26 A. C. OUDEMANS. 
possess a very primitive character. After a few efforts, I suc- 
ceeded in finding a very beautiful section through the longi- 
tudinal vessel, which section, being cut partly at an angle, 
furnished a very good picture of the two layers, which, as we 
see in fig. 74, have a thickness of only two or three cells, and 
consists of cells which have not yet lost their protoplasmatic 
character, and nevertheless give rise to a distinct fibre. Espe- 
cially an isolated cell on the right side of the figure shows 
which cells give rise to the outermost investment of the blood- 
vessels. A whole section through a vessel and vertical section 
I have figured in fig. 75. This section lay just before the 
nephridial region, to which we will pass immediately, after 
having mentioned that the two great vessels that take their 
course from the level of the mouth till far behind, are al- 
ways provided with a well-developed hyaline basal layer, and 
an epithelium with large nuclei (see figs. 74 and 75); and that 
in contracted portions of the vessels the basal layer forms 
protuberances towards the lumen, so regularly arranged that 
the whole has the appearance of a saw. These must be longi- 
tudinal folds lying side by side. 
Only a very few sections (four in this specimen) before we 
see a true nephridial canal, we see that the lower portion of 
the blood-vessel changes its structure. A portion of its wall 
now is lined by palisade-shaped cells, regularly placed by the 
side of each other. We soon see a section in which a furrow 
pierces this changed wall-portion, and in the following slice we 
see this furrow cut off from the vessel by similar cells, and 
the nephridial canal is ready. There is thus an open anterior 
communication between the nephridial and the vascular 
system. The blood-vessel retains this changed ventral wall from 
section 718 to section 768. The nephridial canal, once separate, 
remains small when compared to the vessel, and lies next to it. 
Here and there I distinctly saw cilia, which were so long that 
they touched the opposite wall of the canal. A little further 
backwards we see the nephridial canal again in connection with 
the blood-vessel (see figs. 5 and 57: the communication took 
place in slice No. 762). At first, behind this connection the 
