Dr. J. Anderson on the Anatomy of Sacculina. 15 
of water may be seen to pass into and out of the cavity of the 
body, the sac at the same time alternately distending and con- 
tracting. The ova, when fully developed, are extruded by this 
orifice, the structure of which confines the water to the sae which 
contains them. The orifice is situated upon the posterior 
margin of the body, and is slightly raised above the level of the 
sac. There is a constriction at its base, and a thickened portion 
of the sac plays the part of a sphincter muscle. The inner 
margin of the orifice is thrown into folds, usually eleven in 
number, sometimes of a delicate and pellucid appearance, By 
this arrangement the orifice is capable of great distention, 
The corium (fig. 2 e).—On reflexion of the external skin we 
expose the underlying corium, which invests nearly the whole 
inner surface of the sac. I have succeeded, in one or two in- 
stances, in separating this membrane 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 ex- 
tremity to the horny ring of the peduncle, and posteriorly to the 
ovario-branchial orifice. I think it probable, 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 sae, 
where it becomes reflected on to the anterior portion of the pes 
duncle, and can be traced no further as a separate structure, 
At the left margin of the peduncle the corium is attached by a 
septum (fig. 3g) to a pulpy body embraced in the folds of the 
ovigerous lamelle. 
Organs of reproduction.—On opening Sacculina by an incision 
extending through the sae and continued from the ovario- 
branchial orifice to the peduncle, we expose a pellucid sac (fig, 
4 2) filled with ova. This sac is found on both sides of the pulpy 
body above referred to (fig. 2), which it embraces within 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 plugged 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 membrane 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 ther development, to be cast off like its 
predecessor (fig.5). The ovigerous sac appears 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 
