h and the cirri of these two
complemental males are, than the corresponding parts in the two
hermaphrodites: this no doubt is due to the two males having been
arrested in their development, at a corresponding early period of
growth. Several of the characters, by which the hermaphrodite _S.
villosum_ so closely approaches, and almost blends into the genus
Pollicipes,--such as the thicker cirri, with the intermediate tufts of
bristles, the small second tooth of the mandibles, and the little
brush-like prominence on the maxillae,--are not in the least apparent in
the complemental male.

SUMMARY ON THE NATURE AND RELATIONS OF THE MALES AND COMPLEMENTAL MALES,
IN IBLA AND SCALPELLUM.

Had the question been, whether the parasites which I have now described,
were simply the males of the Cirripedes to which they are attached, the
present summary and discussion would perhaps have been superfluous; but
it is so novel a fact, that there should exist in the animal kingdom
hermaphrodites, aided in their sexual functions by independent and, as I
have called them, Complemental males, that a brief consideration of the
evidence already advanced, and of some fresh points, will not be
useless. These parasites are confined to the allied genera Ibla and
Scalpellum; but they do not occur in Pollicipes,--a genus still more
closely allied to Scalpellum; and it deserves notice, that their
presence is only occasional in those species of Scalpellum which come
nearest to Pollicipes. In the genera Ibla and Scalpellum, the facts
present a singular parallelism; in both we have the simpler case of a
female, with one or more males of an abnormal structure attached to her;
and in both the far more extraordinary case of an hermaphrodite, with
similarly attached Complemental males. In the two species of Ibla, the
complemental and ordinary males resemble each other, as closely as do
the corresponding hermaphrodite and female forms; so it is with two
sets of the species of Scalpellum. But the males of Ibla and the males
of Scalpellum certainly present no special relations to each other, as
might have been expected, had they been distinct parasites independent
of the animals to which they are attached, and considering that they are
all Cirripedes having the same most unusual habits. On the contrary, it
is certain that the animals which I consider to be the males and
complemental males of the two species of Ibla, if classed by their own
characters, would,