_Mouth._--The labrum, as in the hermaphrodite, is highly bullate, and far removed from the adductor scutorum muscle. The _Palpi_ are small and triangular, with their blunt apices clothed with a very few scattered bristles. _Mandibles_, with only three teeth, and the lower angle minute, slightly pectinated; the first tooth is distant from the second, and larger than it. Width of the whole organ, .0021 of an inch. _Maxillæ_, bearing only a few spines, furnished with a long apodeme; beneath the upper large pair there is a notch, under which there are two spines of considerable size and a small tuft of fine bristles; width .001 of an inch, and therefore only 1/16th of the size of the same organ in the hermaphrodite: the relative sizes of the maxillæ and mandibles are the same in the male and hermaphrodite. _Outer Maxillæ_ blunt, triangular, with a few thinly-scattered bristles on the inner face; those on the outside being longer. _Cirri._--The First pair is far removed from the second; the rami are very short, barely exceeding the pedicel in length; they are formed of only four segments, each bearing a pair of spines; but on the end of the terminal segment, there are three spines, of which the central one is very long. Second pair also short. In the sixth pair there are five or six elongated segments, each bearing three pair of long spines; dorsal tufts large. The cirri are furnished with transversely-striated muscles. The _Caudal Appendages_ exist as two very minute plates, with a few bristles at their apices. The _Penis_ is not acuminated, with four bristles at the end; it is short, equalling only the lower segment of the pedicel of the sixth cirrus. In the one specimen preserved in spirits, I unfortunately omitted to search for the vesiculæ seminales; I cannot doubt that such existed, but it would have been important to have ascertained whether they contained spermatozoa. I made out, most distinctly, that there was no trace of ovarian tubes within the peduncle; and my assertion may be believed when I state, that I traced the two much finer and more transparent cement-ducts, from the prehensile antennæ up to the body of the animal: in Lepas I have _repeatedly_ detected, with ease, the ovarian tubes within the peduncle, before the calcification of the valves had even commenced, and therefore at a much earlier period of growth than in these parasites. Consequently I am prepared to affirm, that these parasites are not females, but that, as far as can be judged, from external organs, they are exclusively males.