that the diagnostic differences between them in the
several genera are briefly given, for a special purpose, in a discussion
on the sexes of Scalpellum at the end of that genus. We have seen in the
larva, that the cement-ducts, with their opaque cellular contents, can
be traced from within the discs of the antennae to the anterior or lower
ends of the two gut-formed bodies, which it can be demonstrated are the
incipient ovaria.

In mature Cirripedes these ducts can be followed, in a slightly sinuous
course, along the muscles on each side within the peduncle, till they
expand into two small organs, which I have called cement-glands. These
glands are found with great difficulty, except in _Conchoderma aurita_,
where they are placed on each side under the inner layer of corium, at
the bottom of the sack, so as to be just above the top of the peduncle;
they resemble in shape a retort, (Pl. IX, fig. 3.). In _Pollicipes
mitella_ and _polymerus_ they lie half way down the peduncle, close
together, and apparently enclosed within a common membrane; in these two
species the broad end of the gland is bent towards the neck of the
retort. In Scalpellum the position is the same, but the shape is more
globular. In Ibla the structure is more simple, namely, a tube slightly
enlarged, running downwards, bent a little upwards, and then resuming
its former downward course, the lower portion forming the duct. The
gland contains a strongly coherent, pulpy, opaque, cellular mass, like
that in the cement-ducts; but in some instances, presently to be
mentioned, this cellular mass becomes converted within either the ducts
or gland, or within both, into transparent, yellow, tough cement.
Generally in Conchoderma, Pollicipes, and Scalpellum, two ovarian tubes,
but in one specimen of _Conchoderma aurita_, three tubes, and in Ibla
one tube could be seen running into or forming the gland; of the nature
of the tubes there could not be the least doubt, for at a little
distance from the glands they gave out branches (Pl. IX, fig. 3),
containing ova in every state of development. In some specimens as in
that figured of _Conchoderma aurita_, the ovarian tube on one side of
the gland is larger than on the other, and has rather the appearance of
being deeply embedded in the gland than of forming it; but, in other
specimens, the two ovarian tubes first formed a little pouch, into which
their cellular contents could be clearly seen to enter; and then this
po