CIRCULATORY APPARATUS OF THE NEMERTEA. 37 
proboscidian sheath, as is the case in Valencinia. If the 
two lacunz become vessels, then we have a bidentate fork, 
a pitchfork, and we have the form that occurs in all the 
Hoplonemertea. The median dorsal vessel will, however, not 
go farther than the cesophageal region in the proboscidian 
sheath as in Carinella and Valencinia, and several higher 
forms mentioned hereafter; and suppose now that from the 
point where it finishes a groove is formed by the proboscidian 
sheath down its whole length, communicating with the vessel 
in the sheath, and is now cut off, then we have the median 
dorsal vessel of all the Schizonemertea and Hoplone- 
mertea. Simultaneously with the formation of diverticula 
on the intestine, there are formed transverse vessels over these 
diverticula between the longitudinal vessels. 
The reason that I do not treat the nephridial system at the 
same time with the vascular system is that I have nowhere 
seen open or direct communication between these two 
systems. 
The nephridial system is situated on the sides of the cso- 
phagus, in the lacunar spaces between cesophagus and longi- 
tudinal muscular layer, and against the latter. It consists 
(and this is very plainly visible on sagittal sections) of two 
longitudinal canals, which are united by transverse canals, just 
like a ladder. The foremost part was about one mm. from the 
hindmost point of the opening of the mouth, the hindmost part 
a little before the point where the median vessel leaves the 
proboscidian sheath. The whole length was fifteen mm. in a 
specimen of four cm. in length (in spirits). 
The nephridial system in Valencinia offers us an im- 
portant peculiarity, viz. there are more than one pair of 
excretory ducts. I have found in the specimen cut in trans- 
verse sections twenty-six pairs of excretory ducts, all lying 
above the nerve-trunks, nearly touching them. The slices in 
which I found them were the following : 
