On the Anatomy o/Sacculina. 
309 
expose the underlying corium, which invests nearly the 
whole inner surface of the sac. I have succeeded, in one or 
two instances, in separating this memhrane into two well- 
marked layers. The external layer is a very thin membrane 
investing the whole inner surface of the sac, attached at its 
anterior extremity to the horny ring of the peduncle, and 
posteriorly to the ovario-branchial orifice. I think it pro- 
bable, when the external skin is moulted, that its place is 
supplied by this structure. The inner layer, following it 
from the ovario-branchial orifice to which it is attached, 
passes forwards, closely applied to the outer layer, till it 
nearly reaches the anterior margin of the sac, where it be- 
comes reflected on to the anterior portion of the peduncle, 
and can be traced no farther as a separate structure. At 
the left margin of the peduncle the corium is attached by a 
septum (fig. 3 g) to a pulpy body embraced in the folds of 
the ovigerous lamellse. 
Organs of reproduction. — On o^Qumg Sacculina by an inci- 
sion extending through the sac and continued from the 
ovario-branchial orifice to the peduncle, we expose a pellucid 
sac (fig. 4 i) filled with ova. This sac is found on both sides 
of the pulpy body above referred to (fig. 2 li), which it em- 
braces wdthin its folds. The sac is merely a temporary 
structure including the ova till their full development ; and 
at this period I have seen the ovario-branchial orifice plug- 
ged up by the extruded sac and its contents, and in other 
cases I have found it lying quite loose in the general cavity 
of the parasite. In specimens like these, a delicate mem- 
brane may be separated, by gentle manipulation, from the 
inner surface of the corium and from the surface of the 
pulpy body or internal ovaries. This membrane appears to 
be an ovigerous sac, in the process of growth, destined to 
receive a brood of ova, but, after their development, to be 
cast off like its predecessor (fig. 5). The ovigerous sac ap- 
pears to be continually present in one stage or another of 
its development, so that the water which passes in at the 
ovario-branchial orifice is never in contact with any other 
structure. In a large specimen of Sacculina carcini, I found 
two small mussels living in and attached to the inner sur- 
