The above-described method by which Cirripedia lay their eggs, namely, united together in a common membrane, placed between their old outer and new inner integuments, and the manner in which the lamellæ, when thus formed, are retained for a time fastened to the fræna, and are then cast off, appears to me very curious. In some of the lower Crustacea, it is known, that the ova escape by rupturing the ovisacs formed by the protruded ovarian tubes, and this is the nearest analogy with which I am acquainted. The ova are impregnated (as I infer from the state of the vesiculæ seminales), when first brought into the sack, and whilst the membrane of the lamellæ is very tender: the long probosciformed penis seems well adapted for this end. In the male of _Ibla Cumingii_, which has not a probosciformed penis, the whole flexible body, probably, performs the function of the penis: in _Scalpellum ornatum_, however, the spermatozoa must be brought in by the action of the cirri, or of the currents produced by them. That cross impregnation may and sometimes does take place, I infer from the singular case of an individual, in a group of Balani, in which the penis had been cut off, and had healed without any perforation; notwithstanding which fact, larvæ were included in the ova. _Exuviation; Rate of Growth; Size._--I have had occasion repeatedly to allude to the exuviation of the Lepadidæ: with the exception of the genus Lithotrya,[20] in which the calcareous scales on the peduncle, together with the membrane connecting them, is cast off, neither the valves nor the membrane uniting them, nor that forming the peduncle with its scales and styles, are moulted; but the surface gradually disintegrates and is removed, perhaps sometimes in flakes, whilst new and larger layers are formed beneath. In Scalpellum, I ascertained that the new membrane, connecting together the newly-formed calcified rims under the valves of the capitulum, was formed as a fold, with the articulated spines which it bears, all adpressed in certain definite directions. This fold of new membrane, when the old membrane splits and yields, of course expands, and thus the size of the capitulum is increased. In the peduncle, lines of splitting can seldom be perceived, except, indeed, in the sub-globular, embedded, downward-growing peduncle of Anelasma, as described under that genus. I do not understand what determines the complicated lines of splitting of the old membrane between the several valves of the capitulum,--without it be simply, that along these lines alone, the old membrane is not strengthened by the new membrane being closely applied under it, the new being formed, as we have just said, in a fold, in order to allow of increase in size. Although, as I believe, there is strictly no exuviation in the outer membranes of mature Lepadidæ, it seems that narrow strips of membrane are cast off from between the valves, for the few first moults, after the final metamorphosis of the larva.