296 
A. G. BOURNE. 
ing from -—-o of an inch to of an inch in diameter. They 
are sub-spherical, tending to the form of polyhedrons, owing 
to mutual adpressure. The protoplasm is fairly transparent 
in the living state. By the use of reagents it is seen to be 
very finely granular, the granular matter being more abun- 
dant peripherally, so as to form a cortical layer, which is 
strongly marked after maceration in potassium bichromate. 
In cells which have undergone prolonged maceration in this 
reagent. Professor Lankester observed a rod-like structure 
or striation of the cortical layer, the rods being set at right 
angles to the lumen of the ductules. 
By the use of silver nitrate I have succeeded in demon- 
strating a similar structure in the cortical substance of the 
nephridial cells from the apical lobe, that is to say, in cells 
wuth a single large traversing ductule (see PL XXV, 
fig. 9). 
A delicate membranous cuticle exists on the surface of 
each cell, and is continuous with the cuticle of the ductules 
and with the abundant cuticular deposit which forms the 
lining of the central duct (figs. 7 and 13, c. d.). The 
cuticle of the ductules is well seen in fig. 10. After pro- 
longed maceration it is possible to obtain the ductules in an 
isolated condition owdng to the resistance of their cuticle to 
the disintegrating process which 'affects the surrounding 
protoplasm. 
Each cell possesses a large well-marked spherical nucleus 
varying in diameter from^Vir inch to -oVoo- inch. The nucleus 
has a strongly-marked capsule, and its contents have, after 
maceration in w^eak chromic acid, the appearance shown in 
fig. 10 a, viz. a clear substance in which a nucleolar net-* 
work and nucleolus can be distinguished. 
The protoplasm of the nephridial cells of all regions ap- 
pears to be distensible and contractile. Whether it is 
actively contractile is uncertain, but there is no doubt that 
the volume of the ductules varies periodically. There is no 
muscular coat to the nephridium itself, as there is to the 
vesicle, and the change of volume is probably due to the 
physiologically changed condition of the secreting cells. 
Gegenbaur found the nephridia of Tubifex to be actively 
contractile. 
The cells are set around the central duct of the main lobe 
radially, as many as eight being thus shown in some sec- 
tions taken at right angles to the duct. Superficially to 
those which thus abut upon the central duct, other cells 
similar to the deeper ones may be seen (fig. 13), forming 
portions of a second or even of a third row. 
