considering
that _S. rostratum_ and _S. Peronii_ are ordinarily attached, in a
certain definite position, to horny corallines, and considering that the
exact points of attachment in these three parasites, (of which I have
seen no other instance amongst common Cirripedes,) namely, between the
scuta, would inevitably cause their early destruction, either directly
or indirectly, by their living supports being destroyed. Nevertheless, I
carefully examined a young specimen of _S. rostratum_ only thrice as
large as the parasite; and not having very young specimens of _S.
Peronii_ and _villosum_, I procured the young of closely-allied forms,
namely, of _S. vulgare_, (with a capitulum only 4/100th of an inch in
length,) and of _Pollicipes polymerus_, (with a capitulum of less size
than that of one of the parasites,) and there was not the least sign of
anything abnormal in the development of the valves. In _S. vulgare_, at
a period when the calcified scuta could have been only 1/100th of an
inch in length, (and therefore considerably less than the scuta in the
parasites,) the upper latera must have been as much as 4/1000ths of an
inch in length, and the valves of the lower whorl certainly
distinguishable.

To sum up the evidence on the sex of the parasites, I was not able to
discover a vestige of ova or ovaria in the two male Iblas; and I can
venture to affirm positively, that the parasites of _S. Peronii_ and _S.
villosum_ are not female. On the other hand, in the two male Iblas, I
was enabled to demonstrate all the male organs, and I most distinctly
saw spermatozoa. In the parasitic complemental male of _S. vulgare_, I
also most plainly saw spermatozoa. In the parasites of _S. rostratum_,
_S. Peronii_, and _S. villosum_, the external male organs were present.
I may here just allude to the facts given in detail under Ibla, showing
that it was hardly possible that I could be mistaken regarding the
exclusively female sex of the ordinary form of _I. Cumingii_, seeing how
immediately I perceived all the male organs in the hermaphrodite _I.
quadrivalvis_; and as the parasite contained spermatozoa and no ova, the
only possible way to escape from the conclusion that it was the male and
_I. Cumingii_ the female of the same species, was to invent two
hypothetical creatures, of opposite sexes to the Ibla and its parasite,
and which, though Cirripedes, would have to be locomotive! I insisted
upon this alternative, because if the parasite