300 
A. G. BOURNE. 
nephridial cells is perfectly distinct. These are not mere 
intercellular passages but have their distinct membranous 
walls like those of the finer blood-vessels m other parts 
of the Leech. In these thin- walled vessels, both in the 
nephridium and elsewhere, I have not succeeded in tracing 
any cell-structure by the use of various histological 
methods. It appears propable from what Professor Lan- 
kester has ascertained with regard to the relations of the 
vaso-fibrous tissue and the blood-vessels, that the nuclei 
are discharged at an early period of its development from 
the wall of the blood-vessel into the lumen of the vessel, and 
accordingly we find only the metamorphosed cell-substance 
left to form the wall which is accordingly structureless. 
VII. — The Vesicle and the Secretion of the Gland. 
The vesicle is remarkable for its great dilatability, and 
for possessing a muscular coat which the glandular portion 
of the nephridium does not possess. On this latter point I 
am at variance with Professor Leuckart, who has ascribed 
a muscular coat to the glandular portion. The vesicle may 
be regarded as an expansion of the central duct to which it 
is joined by the vesicle duct,’’ but it differs greatly from 
any part of the glandular portion vof the nephridium, not 
only in the fact that it has a muscular tunic, but also in the 
fact that the cells lining it have not a cuticle but a ciliated 
surface. Cilia are not found in any other part of the nephri- 
dium, which is a remarkable fact, when we compare the 
structure of other nephridia such as those of the earthworm 
and the molluscan organ of Bojanus. The cilia on the epi- 
thelium of the vesicle are exceedingly short, the cells them- 
selves being small and short, and thrown into ridges when 
the vesicle is in a state of contraction. The vesicle is seen 
in section in the drawing (fig. 14), which represents accurately 
an actual preparation. 
The vesicle does not open directly to the exterior, but is 
placed in communication with the body-surface by a short 
duct, which is lined near its external opening by an involu- 
tion of the epidermis of the integument. 
The wall of the vesicle is supplied with a very regular 
capillary plexus of the haemal system (fig. 1), and it seems 
probable that secretion may be carried on from its walls as 
well as from the more distinctly glandular portion of the 
nephridium. 
The vesicle contains a liquid' in which bunches of needle- 
like crystals are found, which are drawn in fig. 11, as seen 
when magnified 3500 diameters by means of Hartnack’s 
