ent, removed from the outside of a Cirripede, consists of a thin
layer of very tough, bright-brown, transparent, laminated substance,
exhibiting no structure under the highest powers, or at most a very fine
dotted appearance, like a mezzotinto drawing. It is of the nature of
chitine; but boiling caustic potash has rather more effect on it than on
true chitine; and I think boiling nitric acid rather less effect. In one
single instance, namely, in Coronula, the cement comes out of the four
orifices of the two bifurcating ducts, in the shape of distinct cells,
which, between the whale's skin and the basal membrane, arrange
themselves so as to make a circular, continuous slip of cement; then the
cells blend together, and are converted into transparent, structureless
cement. Cementing tissue or membrane would, perhaps, have been a more
correct title than cement; but, in ordinary cases, its appearance is so
little like that of an organised tissue, that I have for this reason,
and for brevity-sake, preferred the simple term of Cement.

In the larva the cement always escapes through the prehensile antennae;
and it thus continues to do throughout life in most or all of the
species of Lepas, Conchoderma, Dichelaspis and Ibla. In the first two of
these genera, the cement escapes from the borders of the lower side of
the disc or penultimate segment of the antennae, and can be there seen
radiating out like spokes, which at their ends divide into finer and
finer branches, till a uniform sheet of cement is formed, fastening the
antennae and the adjoining part of the peduncle down to the surface of
attachment. In _Dichelaspis Warwickii_ and _Scalpellum Peronii_, the
cement, or part at least, comes out of the ultimate segment of the
antennae, in the shape of one tube, within another tube of considerable
diameter and length. In _Scalpellum vulgare_, and probably in some of
the other species, which live attached to corallines, the cement soon
ceases to debouch from the antennae, but instead, bursts through a row of
orifices on the rostral margin of the peduncle (Pl. IX, fig. 7), by
which means this margin is symmetrically fastened down to the delicate,
horny branches of the zoophyte. In Pollicipes, the two cement-ducts,
either together or separately (Pl. IX, fig. 2, 2 _a'_), wind about the
bottom of the peduncle in the most tortuous course, at each bend pouring
out cement through a hole in the membrane of the peduncle. In Ibla the
lower pa