STRUCTURE OF NEPHRIDlA OF THE MEDICINAL LEECH. 295 
time it is not by any means always exhibited by nephridia, 
e.g. those of adult Mollusca (organs of Bojanus) and those 
of Vertebrata. I am not aware that a branched duct has 
been described as excavating the cells of any nephridium, or, 
indeed, of any gland previously to this. The cells from the 
nephridium of Hsemopis, figured by Leydig (Joe. cit.^y are 
simple cylindrical cells with unbranched passages passing 
through them, and probably come from the walls of the re- 
current duct. In the salivary glands of some Insects and 
Crustaceans the nearest approach to the kind of relation now 
established between cell and duct in the nephridia is to be 
found,^ but I believe that hitherto only an excavation of the 
gland-cell by the ductules of origin has been observed in 
these structures, and not an actual thorough perforation of 
the gland-cell. 
I have been able tx) satisfy myself,^by repeated observa- 
tions, that the whole system of ductules in the nephridium 
of the Leech is a continuous network of passages, but I 
have not been able to determine any aperture in that net- 
work by which it communicates either with spaces in the 
general substance of the body or with the central duct of 
the nephridium, or its branch the recurrent duct. It seems 
in the highest degree probable that these ductules do com- 
municate with the central duct and so with the vesicle and 
exterior, yet I have watched them in various conditions of 
distension, and by pressure have caused certain small cor- 
puscles (fig. 12) which float in the colourless liquid, which 
more or less distends them, to move along from one ductule 
to another, without gaining any indication of a communica- 
tion with the adjacent central duct. 
The ductules do not exist in the cells which line the 
vesicle or its duct. These cells are no doubt homologous 
with the nephridial cells, yet it is not until the horse-shoe 
shaped glandular mass is reached that the ductules make 
their appearance. It is difficult to separate any description 
of the ductules from that of the nephridial cells themselves, 
and I shall therefore describe together the appearances pre- 
sented by these structures in the different regions of the gland. 
Cells and ductules of the main lobe (figs. 5, 7, 10, lo). — 
As in all parts of the gland, the cells are of large size, vary- 
gland which has undergone a repeated cell-division transverse to the axis 
of its duct. 
* And, it may be noted, that the more archaic exam))les of salivary 
glands consist of bundles of unicellular glands, each with its own ductule, 
which is so far in favour of the hypothesis that the nephridial cells were 
originally unicellular glands. 
VOL. XX. NEW SER. 
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