hould not, however, be overlooked, that the two main groups of
parasites differ from each other, far more than do the two corresponding
groups of species to which they are attached; and, on the other hand,
that the parasitic males of _S. Peronii_ and _S. villosum_ resemble each
other more closely, than do the two hermaphrodite forms;--but it is very
difficult to weigh the value of the differences in the different parts
of species.

Besides these general, there are some closer relations between the
parasites and the animals to which they are attached; thus the most
conspicuous internal character by which _Ibla quadrivalvis_ is
distinguished from _I. Cumingii_, is the length of the caudal appendages
and the greater size of the parts of the mouth; in the parasites, we
have exactly corresponding differences. Out of the six species of
Scalpellum in their ordinary state, _S. ornatum_ is alone quite
destitute of spines on the membrane connecting the valves; and had it
not been for this circumstance, I should even have used the presence of
spines as a generic character; on the other hand, _S. villosum_, in
accordance with its specific name, has larger and more conspicuous
spines than any other species. In the parasites we have an exactly
parallel case; the parasite of _S. ornatum_ being the only one without
spines, and the spines on the parasite of _S. villosum_ being much the
largest! This latter species is highly singular in having no caudal
appendages, and the parasite is destitute of these same organs, though
present inn the parasites of _S. rostratum_ and _S. Peronii_. Again, _S.
villosum_ approaches, in all its characters, very closely to the genus
Pollicipes, and the parasite in having prehensile antennae, with the disc
but little pointed, and with spines at the further end, departs from
Scalpellum and approaches Pollicipes! Will any one believe that these
several parallel differences, between the Cirripedial parasites and the
Cirripedes to which they are attached, are accidental, and without
signification? yet, this must be admitted, if my view of their male sex
and mature be rejected.

One more, and the most important special relation between the parasites
and the cirripedes to which they are attached, remains to be noticed,
namely that of their prehensile larval antennae. I observed the antennae
more or less perfectly in the males of all, and except in _S. villosum_,
in all the species, though so utterly different in