STRUCTURE OF NEPHRIDIA OF THE MEDICINAL LEECH. 301 
objective 10 immersion. These crystals are not stained 
by iodine ; they are soluble in nitric acid. I have not yet 
completed their chemical examination. These crystals are 
occasionally to be found in the central duct, but I have never 
found them in the ductules. On the other hand, in the 
ductules are found minute, structureless corpuscles (fig. 12), 
which on one occasion I saw exuded from the substance 
of a nephridial cell into the ductule. I have never found 
these corpuscles in the central duct nor in the vesicle. 
VIII. — Opening of the Systems of Ductules and Ducts 
INTO ONE another AND INTO THE BoDY-CaVITY. 
As I have before mentioned, I have failed to establish the 
existence of a communication between the central duct or its 
oflf-sets and the system of ductules ; but I am strongly inclined 
to regard the recurrent duct as the seat of this commu- 
nication. It seems to me not improbable, from the large 
size of the ductules of this portion, and from the way in 
which it enters the apical lobe on the one hand and the con- 
cavity of the main lobe on the other, that we shall even- 
tually find a connection through it of the two systems. 
I hope during the present summer, by the study of other 
genera of Leeches (Aulostomum, Nephelis, Clepsine, Pon- 
tobdella), to gain some light upon this question. 
The absence of cilia from the ducts and ductules of the 
Leeches nephridium, together with the absence of any large 
space in the body — comparable to a body-cavity — seems to 
be correlated with the absence of an open internal termina- 
tion to the duct and ductule systems. I am inclined to 
regard what I call the apical lobe as probably corresponding 
with the position in which an internal opening is found in 
the typical form of nephridium, such as occurs in Nephelis, 
Clepsine, and the Oligochaeta. 
This point also will be, I hope, decided by the investiga- 
tions which I am now commencing on those genera. 
The very wide difference between Hirudo and Clepsine 
(so far as descriptions of the latter genus enable me to judge) 
in the whole arrangement of duct and nephridial cells leads 
me to believe that the comparison of the series of Hirudinean 
genera will be a very interesting one. 
Summary. 
1. The Leech’s nephridium consists of gland and vesicle 
connected by the vesicle duct. 
2. The gland is a curved, horse-shoe shaped loop, present- 
