The Rostrum is shorter, and internally more concave than the carina: I believe it projects more abruptly outwards than is represented in the figure.

The Peduncle commences some little way below the scuta: it is narrow and very short: it is finely villose: it is lined by delicate transverse striæ-less muscles, within which there are the usual stronger, longitudinal muscles. The base is flat and truncated. I examined, and carefully compared, the prehensile antennæ with those of the hermaphrodite, and found every part and every measurement the same. The full importance of this identity will hereafter be more fully insisted on. The antennæ are represented of their proper proportional size in [fig. 3.]

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.

Concluding Remarks.—In comparing the capitulum of the hermaphrodite with that of the complemental male ([Pl. VI], [figs. 6 and 3]), we must be struck with the differences in their shape, in the number, relative sizes, and forms of the several valves. It should, however, be borne in mind, that the scuta and carina in the hermaphrodite at first grow exclusively downwards; so that if we remove the upper portions subsequently added, the difference in shape in these valves is not so great as it at first appears. The rostrum in the male is of much larger relative size; whilst of the upper latera there is not a trace, although in the hermaphrodite these valves are larger than the rostrum. The terga, compared with those of the hermaphrodite, differ more essentially than do the other valves; and the manner in which the primordial valves project, shows that from the first commencement of calcification, the lines of growth have followed an unusual course. The great breadth and shortness of the terga is evidently related to the shortening of the whole capitulum, and the transverse position of the orifice; and this shortening of the capitulum, no doubt, is rendered necessary for its reception and protection within the shallow furrow between the scuta of the hermaphrodite. Finally, if we compare the internal parts of the hermaphrodite and male, the differences are considerable, though partly to be accounted for by the youth of the latter: the form and position of the labrum, and the distance between the first and second pair of cirri, is the same in both; but the mandibles and maxillæ differ considerably.