[52] I do not understand the development of the spermatozoa in Cirripedia: in a recent Chthamalus and Balanus, I found the greater number had a little filament in front of the head or nodular enlargement, which latter varied in size and in shape from globular to that of a spindle. The filament before the head, also, varied in proportional length; it did not project in exactly the same straight line with the hinder part, and some of the spermatozoa were entirely without this filament in front;--such is the case with the spermatozoa here described. Although I have dissected, at least, thirty specimens, taken at different times of the year, and from different localities, and when many of the specimens were mature and ready for the impregnation of ova, as clearly shown by the presence of innumerable spermatozoa, I have never seen even a trace of an ovum or ovaria. _Antennæ and Attachment._--The prehensile antennæ (Pl. V, fig. 10), are seated a little above the very base of the sack-like animal; and this might have been expected from the antennæ in the larva, being seated on the ventral surface, not at the very extremity of the head. By a very strong light, they can sometimes just be seen whilst the parasite is attached to the hermaphrodite (the scutum of the latter having been cleaned on the under side), and are thus represented in fig. 9. They are formed of thicker membrane than the general integument of the body: the second segment, or disc, is pointed and hoof-like; when seen in profile (fig. 11), the upper convex surface has a uniform slope with the upper surface of the basal segment; it is furnished with a single backward pointing spine, attached, I believe, on the under side, nearly opposite the articulation of the ultimate segment: at the apex, there are some excessively minute hairs or down. The ultimate segment projects rectangularly outwards as usual, and has on its inner side, rather beneath the middle, a conspicuous notch (fig. 12), which bears two or three long, non-plumose spines; on the summit there are three or four rather shorter spines. On the outside of the great basal segment there is a single spine curving backwards. The importance of the following measurements (in fractions of an inch) will hereafter be seen. Length of whole organ, from end of disc to the further } margin of the oblique basal articulation } 38-39/6000 Length of whole organ, to the inner margin of the oblique} basal articulation } 1/6000 Breadth of basal segment, measured half-way between the } basal and second articulations,--the limb being viewed } from vertically above } 8/6000 Length of hoof-like disc, measured from the apex to the } middle of the articulation with the basal segment } 9-10/6000 Breadth of ditto 5/6000