f ova and
ovaria in the assumed males of both species of Ibla, at the period when
their vesiculae seminales were gorged with spermatozoa,--from the close
general resemblance between the parts of the mouth in the parasites and
in the Iblas to which they are attached,--from the differences between
the two parasites being strictly analogous to the differences between
the two species of Ibla,--from the generic character of their prehensile
antennae,--and from other such points,--if from these several
considerations, we admit that these parasites really are the males of
the two species to which they adhere, then in some degree the occurrence
of parasitic males in the allied genus Scalpellum is rendered more
probable. So the absolute similarity in the antennae of the males and
hermaphrodites both in _S. vulgare_ and _S. Peronii_; and such relations
as that of the relative villosity of the several species in this same
genus, all in return strengthen the case in Ibla. Again, the six-valved
parasites of _S. Peronii_ and _S. villosum_ are so closely similar, that
their nature, whatever it may be, must be the same; hence we may add up
the evidence derived from the identity of the antennae in the parasite
and hermaphrodite _S. Peronii_, with that from the antennae in the male
_S. villosum_, approaching in character to Pollicipes, to which genus
the hermaphrodite is so closely allied; and to this evidence, again, may
be added the singular coincident absence of caudal appendages in the
male and hermaphrodite _S. villosum_. If these two six-valved parasites
be received as the complemental males of their respective species, no
one, probably, will doubt regarding the nature of the parasite of _S.
rostratum_, in which the direct evidence is the weakest; but even in
this case, the particular point of attachment, and the state of
development of the valves, form a link connecting in some degree, the
parasites of the first three species with the last two species of
Scalpellum, in accordance with the affinities of the hermaphrodites.

When first examining the parasites of _S. rostratum_, _S. Peronii_, and
_S. villosum_, before the weight of the cumulative evidence had struck
me, and noting their apparent state of immaturity, it occurred to me
that possibly they were the young of their respective species, in their
normal state of development, attached to old individuals, as may often
be seen in Lepas; this, however, would be a surprising fact,