32 
CHARLES BADHAM. 
by the sphincters (PI. 1, fig. 2, VI .) , the dorsal vessel in the* 
testicular region is caused to contract by the pressure of 
lymph in the lacuna, so that its walls come into contact. In 
this manner the blood is forced forward and ultimately enters 
the paired branches ; these are not provided with valves. 
The blood then passes along the non-contractile lateral vessels 
into the non-contractile ventral vessel and so through the 
complicated branches in the posterior sucker, until it again 
reaches the intestinal siuus. The important relation of the 
contractions of the muscular walls of the intestinal sinus to 
the flow of the lymph and the contractions of the contractile 
lacuna has been dealt with in the section describing the 
circulation of the lymph. 
Nephridial System. 
There are eleven pairs of nepliridia arranged segment ally 
in the 13th to the 23rd somite. These form what is practi- 
cally a continuous network in this area. However, this is 
so arranged that the segmental character of the nepliridia is 
obvious. An inspection of PI. 2, fig. 7, will make this clear. 
The best developed parts of the nepliridia are two tubes 
( L.n.c .), which are placed ventral to that part of the lacuna 
formed by the fusion of the dorsal and veutral segmental 
lacunae. These tubes have a diameter of about 40 ju, while 
the diameter of their lumen is 5 p. They pursue a tortuous 
course and frequently branch, and the branches anastomose. 
They give off in each somite branches which run to open at 
the nepliridiopore (IVp.),and they receive the branches which 
run from the dorsal and ventral networks. 
The paired branches, which are given off in each somite ta 
open at the nepliridiopore, are of the same size as the chief 
branches of the lateral canals. They are given off from the 
lateral canals near the level of the first annulus of the segment, 
and run medially and posteriorly, curving sharply as they 
approach the ventral lacuna at an angle of 45° (Np. b.). They 
run laterally for about half the distance of their first course- 
