The probosciformed penis lies adpressed on the under side of the thorax, with its apex generally projecting between the first and second pairs of cirri. It presents the same ringed or articulated structure as in the Lepadidæ: it arises from an unarticulated projection or support, which also forms the posterior border to the anus. This support often terminates, as first observed by Poli, in a very sharp point; but this point cannot be of much functional importance, for though present in _Balanus balanoides_, it is absent in the closely allied _B. crenatus_; in Tubicinella there is only a rudiment of this point; I have not observed it in any member of the Chthamalinæ. The strong, transverse and longitudinal muscles with which the penis is furnished, are attached to this support. The apex or orifice of the penis is, I believe, invariably surrounded by some bristles. Its length varies much, according to its state of contraction or relaxation; and this again, I believe, is dependent on the condition of the male secreting organs. In a small specimen of _Elminius modestus_, the penis was actually thrice as long as the whole thorax, including the prosoma: in Pachylasma and in _Octomeris angulosa_, the penis is very short, being equal only to once and a half the length of the pedicel of the sixth cirrus: in _Octomeris brunnea_, the unarticulated support is much elongated, being as long as the pedicel of the sixth cirrus, in which respect this organ resembles that of _Ibla quadrivalvis_, and of no other Cirripede. From the attachment of the penis at the posterior end and on the under side of the anus--from the position of the caudal appendages (where such occur) over the anus--from the position of these same appendages in the pupa--and lastly, from the position of the papilla-like penis in the abnormal _Proteolepas_, I infer that, homologically, the penis is situated at the apex of the abdomen, on its ventral surface; and that, consequently, this organ cannot be considered as the abdomen itself in a modified condition. _Female Organs of Generation._