326 LExcretory Organs kc. of Tetrastemma graecense. 
cilia, although it was only now and then that I could perceive 
this distinctly in prepared sections. 
The knobbed terminal organs, the outer surface of which 
is smooth, either rest with a relatively broad base upon the 
end-canalicules, which not infrequently ramify, or else the 
portion turned towards the canalicule is drawn out like a 
stalk; in the former case they are more bulky in shape, in 
the latter more elongate. Each terminal organ is closed at 
its free end by two flame-cells (more rarely one), from which 
delicate strands of plasma radiate into the mesenchyma; in 
the formation of its wall several—as it seems to me from three 
to five—cells take part, which in structure resemble those of 
the end-canalicule. In general it may be said that the end- 
canals open into the main canals not directly, but by means 
of the connecting portions; yet I have also seen a direct 
communication between the two. 
The number of the excretory pores could not be determined 
with certainty in the living animal, but on examining the 
series of transverse sections from two individuals it was tound 
that in the first case there were five pores on each side and 
in the second six on the one side and three on the other. 
The pores are always situated dorsally and at irregular 
distances one from another ; in the case of the specimen, too, 
which possessed five pores on each side, those of the right 
side did not correspond with those of the left. 
At the points at which excretory pores occur the excretory 
organ is applied closely to the dermo-muscular tube and the 
short efferent duct bores its way straight through dermo- 
muscular tube, basement membrane, and epithelium. 
A more intimate relation between the nephridia and the 
blood-vessels, such as has been demonstrated by Biirger in 
the case of marine Metanemertines, especially Drepanophorus, 
does not exist in Tetrastemma graecense. 
The blood-vascular system consists of three trunks, two 
lateral vessels and a dorsal vessel. The latter opens in front 
into the right Jateral vessel close behind the brain, behind into 
the anal commissure between the two lateral vessels. 
The wall of all these vessels is formed by an internal endo- 
thelium, a muscular sheath, and an outer layer of mesenchyma 
cells arranged in epithelial fashion. Now between the endo- 
thelium and muscular sheath large cells of hemispherical 
form and peculiar structure thrust their way ; in the condition 
of diastole these cells stand out like knobs from the wali of 
the vessel, but in that of systole they project into its lumen. 
Since two cells of this kind always lie directly opposite to 
one another, or nearly so, they are able in systole to close the 
lumen almost completely and prevent a backward flow of the 
blood. 
