STRUCTURE OF NEPHRIDIA OF THE MEDICINAL LEECH. 293 
DUCT. This recurrent duct takes an axial course in that 
delicate cord of gland-cells which depends from between the 
two limbs of the arch of the main lobe, and which, as an inspec- 
tion of the diagram (fig. 1) will show, is nothing more than 
a free or detached continuation of the apical lobe beyond the 
point where it meets the main lobe. The recurrent duct thus 
reaches the middle point of the concavity of the main lobe, 
when it turns downwards again, running among the cells of 
the anterior limb of the main lobe parallel with the first part 
of the central duct. It crosses without any junction the 
origin of the central duct from the vesicle duct, and runs up 
the somewhat narrow portion of the gland which connects 
the main lobe with the base of the testis lobe and apex of 
the apical lobe. 
Here (and I must beg the reader to follow my description 
with the aid of the figure) the recurrent duct branches, just 
in the same way as the central duct branched in giving origin 
to it. One branch runs up into the testis lobe for a short 
distance, and there appears to terminate — how I have not 
been able to determine — but possibly this is a point at which 
the duct and the ductules of the gland are in communi- 
cation. 
The other branch runs into the apical lobe and joins the 
central duct, as shown in the diagram (fig. 1), and in fig. 2, 
which is a careful drawing from a fresh preparation of this 
part. Thus we find the recurrent duct, which originated as 
a branch of the central duct at the other end of the apical 
lobe, coming again into full continuity with the central duct 
after a long course. 
The interpretation of the duct of the Leech’s nephridium, 
with its central and recurrent portions, is exceedingly diffi- 
cult. It is totally unlike the duct or central lumen of the 
nephridium of the earthworm. Possibly the recurved apex 
of the apical lobe represents the funnel-like extremity of the 
nephridium of Nephelis and Oligochseta. But in the latter 
the funnel-like aperture leads into a passage which is not 
intercellular as is that of Hirudo, but ^V^^m-cellular, as are the 
ductules of the latter. It is at this apical region — where the 
recurrent duct rejoins the central duct — that I have looked 
most carefully for rudiments of the funnel-like aperture, or 
for a small opening to the body cavity ; but I am able to 
state definitely that none such exists. The relation, or rather 
the difficulty of explaining the relation, between the nephri- 
dium of the Leech and, that of the Oligocbrnta will become 
more apparent when we have examined the ductules. 
