Organs in certain Pulmog aster opo da, 91 
ing the actual development and course of exit of the zoosperms 
in the order Pulmogasteropoda ; for this purpose it is neces- 
sary to give a short sketch of the anatomy of two or three 
species taken from two different families in the order; I have 
selected P. corneus, L. stagnalis, and H. aspersa, as being 
distinctive species and also very common. 
Fig. 1, PI. VII. — The dichogamic gland in P. corneus occu- 
pies the first whorl of the shell ; it is not imbedded in the liver_, 
as is the case in H. aspersa and L. stagnalis, but simply overlaps 
it by its anterior extremity ; it is covered by an extension of 
the mantle^, which forms a muscular envelope, separating it 
from the internal surface of the shell. The gland is formed 
by an immense number of elongated blind sacs^ which are 
closely packed together ; the walls of the sacs are composed 
of a fine membrane^ lined by a ciliated epithelium ; they all 
open into a longitudinal duct, which runs along the concave 
surface of the gland from one end to the other. The csecal ex- 
, tremities are filled with ova ; the riper ones are of a yellowish 
colour; this_, with the orange-coloured debris which also 
abounds in this part of the sac^ gives a yellowish spotted 
appearance to the outside of the gland. In the development 
of the ovum the germinal vesicle and spot are first found free^ 
the yolk being developed afterwards; the smallest ovum 
which was found that had acquired a yolk measured -001 1''^ 
the germinal vesicle of the same measured •0005^^_, and the 
spot '0002^^ The fact of the germinal vesicle being first 
formed, and afterwards developing a yolk around itself, agrees 
with what is found by other observers in other classes of 
animals. The remaining portion of the sac towards its 
mouth is occupied by sperm-cells and zoosperms in various 
stages of growth. The sperm-cells, on being examined under 
the microscope with the simple addition of the fluids of the 
animal, appear as clusters of cells, with finely granular con- 
tents, the individual cells varying in size from -0003'' to 
'0008''. I was totally unable to find any cell-wall enveloping 
these clusters, such as y^ould be found if they were developed 
from a parent ceil, as is described by some writers. On the 
addition of dilute acetic acid a nucleus is to be observed 
occupying nearly the whole of the cell in the smaller ones, 
but in the larger ones being more restricted in its position ; 
this nucleus is composed of coarse granules, but does not 
appear to be separated from the rest of the cell contents by 
any distinct partition, but, on the contrary, the coarser gra- 
nules of the nucleus gradually merge into the finer granules 
of the remainder of the cell. The course of the develop- 
ment of the zoosperms from these cells appears to be as 
