namely, the organs of the mouth, actually
deserving a place in the very same genus. Moreover, the two parasites
differ from each other, not only in about the same slight degree, but in
a corresponding manner, as do the two Iblas to which they are attached;
thus the mouths of _Ibla quadrivalvis_ and _I. Cumingii_ are closely
similar, (the difference being barely of specific value,) so are the
mouths of the two parasites; but the parts are larger in the
hermaphrodite _I. quadrivalvis_, than in _I. Cumingii_, so are they in
the parasites. Again, the most conspicuous character in _I.
quadrivalvis_, is the number of segments in the caudal appendages, far
exceeding those in the other species of Ibla, as well as of every other
pedunculated Cirripede, and the parasite of this species has articulated
spinose appendages, far larger than the barely visible, non-articulated
pair in _I. Cumingii_.

Considering the whole case, there seems no room to doubt the justness of
the conclusion arrived at, under the former as well as under the present
species, namely, that these little parasites are the males of the two
species of Ibla to which they are attached;--wonderful though the fact
be, that in one case, the male should pair with an hermaphrodite already
provided with efficient male organs. It is to bring this fact
prominently forward, that I have called such males, Complemental Males;
as they seem to form the complement to the male organs in the
hermaphrodite. We look in vain for any, as yet known, analogous facts in
the animal kingdom. In the genus Scalpellum, however, next in alliance
to Ibla, in which, consequently, if anywhere, we might expect to find
such facts, they occur; and until these are fully considered, I hope the
conclusions here arrived at, will not be summarily rejected. Although
the existence of Hermaphrodites and Males within the limits of the same
species, is a new fact amongst animals, it is far from rare in the
Vegetable Kingdom: the male flowers, moreover, are sometimes in a
rudimentary condition compared to the hermaphrodite flowers, exactly in
the same manner as are the male Iblas. If the final cause of the
existence of these Complemental Males be asked, no certain answer can be
given; the vesiculae seminales in the hermaphrodite of _Ibla
quadrivalvis_, appeared to be of small diameter; but on the other hand,
the ova to be impregnated are fewer than in most Cirripedes. No
explanation, as we have seen, can be gi