414 
EDWIN S. GOODRICH. 
of the growing ova. When an ovum reaches a certain size 
it drops off into the cavity of the ovisac, becomes surrounded 
with a thick porous membrane, and filled with a dense mass of 
yolk-granules (figs. 29-34). The ova grow to a very large 
size, become opaque and white, and subsequently make their 
way through the wall of the sac into the coelom of the neigh- 
bouring segments (figs. 1, 27, 32). Such ripe ova escape 
chiefly behind, but occasionally in front of the ovarian sacs. 
When in front they may bulge into segment 5, pushing septum 
5, 6, forwards. When behind, they may form a chain of 
egg-cells pushing the septa back until they reach the last 
segment of the body. As many as eighteen ripe ova may be 
found in one female. 
The disposition of the ovaries, ovisacs and ripe ova is 
always asymmetrical (figs. 1, 18) — a peculiar feature for an 
annelid. While the ovisac of the left side grows forwards, 
that of the right side grows backwards. The asymmetry of 
the ovisacs is accompanied, if not caused by, a twisting for- 
wards of the free end of the left ovary, so that the largest ova 
are pushed towards the head, while the free end of the right 
ovary grows more naturally backwards towards the anus. 
Thus the large ova are packed along the long axis of the 
body, and the intestine is forced to adopt a sinuous course 
between them. It is, I believe, for the sake of economising 
space, and more easily packing the ripe ova, which have to 
be retained for a considerable period in the coelom, that this 
asymmetrical structure has been adopted. 
On the inner lower surface of each ovisac is developed a 
little pocket, always filled with a darkly staining mass (fig's. 
18, 28, 31). The pocket lies a little farther forward on the 
right than on the left side. Probably these are the vesicles 
described by Miss Pereyaslawzewa as being filled with sper- 
matozoa and connected with ducts. They are, however, 
quite independent of the oviducts, though situated near them. 
That fertilisation in Nerilla is internal seems very probable, 
considering that the male is provided with what are doubtless 
copulatory appendages (fig. 6), and that internal fertilisation 
