general appearance
and structure, I found the peculiar, pointed, hoof-like discs, which are
confined, I believe, to the genera Ibla and Scalpellum. In the
hermaphrodite forms of Scalpellum, I was enabled to examine the antennae
only in two species, _S. vulgare_ and _S. Peronii_, (belonging,
fortunately, to the two most distinct sections of the genus,) and after
the most careful measurements of every part, I can affirm that, in _S.
vulgare_, the antennae of the male and of the hermaphrodite are
identical; but that they differ slightly in the proportional lengths of
their segments, and in no other respect, from these same organs in _S.
Peronii_,--in which again the antennae of the male and of the
hermaphrodite are identical. The importance of this agreement will be
more fully appreciated, if the reader will consider the following table,
in which the generic and specific differences of the antennae in the
Lepadidae, as far as known to me, are given. These organs are of high
functional importance; they serve the larva for crawling, and being
furnished with long, sometimes plumose spines, they serve apparently as
organs of touch; and lastly, they are indispensable as a means of
permanent attachment, being adapted to the different objects, to which
the larva adheres. Hence the antennae might, _a priori_, have been deemed
of high importance for classification. They are, moreover, embryonic in
their nature; and embryonic parts, as is well known, possess the highest
classificatory value. From these considerations, and looking to the
actual facts as exhibited in the following table, the improbability that
the parasites of _S. vulgare_ and _S. Peronii_, so utterly different in
external structure and habits one from the other, and from the
Cirripedes to which they are attached, should yet have absolutely
similar prehensile antennae with these Cirripedes, appears to me, on the
supposition of the parasites being really independent creatures, and
not, as I fully believe, merely in a different state of sexual
development, insurmountably great.

The parasites of _S. vulgare_ take advantage of a pre-existing fold on
the edge of the scutum, where the chitine border is thicker; and in this
respect there is nothing different from what would naturally happen with
an independent parasite; but in _S. ornatum_ the case is very different,
for here the two scuta are specially modified, _before the attachment of
the parasites_, in a manner which