_Concluding Remarks._--If the creature which I have considered as the male of _Ibla Cumingii_ be really so, and the evidence formerly given seems to me amply conclusive, then the animal just described, from its close affinity in every point of structure with the former, assuredly is the male of _Ibla quadrivalvis_. But feeling strongly how improbable it is, that an additional or complemental male should be associated with an hermaphrodite, I will make a few remarks on the only possible hypothesis, if my view be rejected,--namely, that the two parasites considered by me to be exclusively males, are not so, but are independent hermaphrodite Cirripedes, the female organs and ova (which, if present, would have been nearly mature, judging from the presence of spermatozoa in both species) having been overlooked by me in every specimen: and again, that in the animal described as the female _I. Cumingii_, I have, though minutely dissecting several specimens, and finding far smaller parts, such as the organs of sense and nervous system, entirely overlooked all the conspicuous male organs, though when I came to _I. quadrivalvis_, and naturally expected to find it likewise exclusively female, a single glance showed me the great probosciformed penis, and by the simplest dissection the vesiculæ seminales and testes were exhibited. Such an oversight is scarcely credible; but even if assumed, we have to believe in the extraordinary circumstance of the two parasites being species of an independent genus, not only the very next in alliance to the animals to which they are attached, but in certain most important points, 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_.