ing, Pl. V, fig. 15, which is three times the natural size. I
carelessly dissected one or two specimens, and concluded that they
belonged to some new class or order amongst the Articulata; but did not
at that time even conjecture, that they were Cirripedes. Many months
afterwards, when I had seen in Ibla, that an hermaphrodite could have a
complemental male, I remembered that I had been surprised at the small
size of the vesiculae seminales in the hermaphrodite _S. vulgare_, so
that I resolved to look with care at these parasites; on doing so, I
soon discovered that they were Cirripedes, for I found that they adhered
by cement, and were furnished with prehensile antennae, which latter, I
observed with astonishment, agreed in every minute character, and in
size, with those of _S. vulgare_: the importance of this agreement will
not at present be fully appreciated. I also found, that these parasites
were destitute of a mouth and stomach; that consequently they were
short-lived, but that they reached maturity; and that all were males.
Subsequently the five other species of the genus Scalpellum were found
to present more or less closely analogous phenomena. These facts,
together with those given under Ibla (and had it not been for this
latter genus, I never probably should have even struck on the right
track in my investigation,) appear sufficient to justify me, in
provisionally considering the truly wonderful parasites of the several
species of Scalpellum, as Males and Complemental Males. When these
parasites are fully described, will be the proper time to discuss and
weigh the evidence on their sexual relations and nature. I will now
describe the parasite of _S. vulgare_.

_General Appearance._--Shape, flask-like, compressed (Pl. V, fig. 9,
magnified 36 times), with a short neck: the outline is usually
symmetrical, but sometimes is a little distorted on the under side. The
creature is imbedded more than half its length or depth in the
transparent, spine-bearing chitine border of the scutum of the
hermaphrodite. Its length, or longer axis, varies from 10 to 11/400ths;
its breadth, or transverse axis, is 6 to 7/400ths; and its thickness,
for it is much flattened, is only 4/400ths of an inch. On the summit,
there is a fimbriated orifice (_a_), the size of which can rarely be
made out quite distinctly, owing to the extreme thinness of the
membranous edges. A little way beneath the orifice, there are four
little blunt, bristly