that I found sessile Cirripedes very sensitive of vibrations in objects adjoining them, though not, apparently, of noises in the air or water. In a group of specimens, I could not touch one even most delicately with a needle, without all the adjoining ones instantly withdrawing their cirri; it made no difference if the one touched had its operculum already closed and motionless. _Reproductive System_,--_Male Organs._--All the Cirripedia which I have hitherto examined, with the exception of certain species of Ibla and Scalpellum, are hermaphrodite or bisexual.[17] I shall so fully describe the sexual relations of the several species of these two genera, under their respective headings, and at the end of the genus of Scalpellum, that I will not here give even an abstract of the grounds on which my firm belief is based, that the masculine power of certain hermaphrodite species of Ibla and Scalpellum, is rendered more efficient by certain parasitic males, which, from their not pairing, as in all hitherto known cases, with females, but with hermaphrodites, I have designated _Complemental Males_. [17] I am compelled to differ greatly from the account given by Prof. Steenstrup of the reproductive system in the Cirripedia, in his ‘Untersuchungen über das Vorkommen des Hermaphroditismus, ch. v, 1846;--a translation of which I have seen, owing to the great kindness of Mr. Busk. Mr. Goodsir has described (‘Edin. New Phil. Journal,’ July 1843,) what he considers the male of Balanus; but I have seen this same parasitic creature charged with ova, including larvæ! From the resemblance of the larvæ to the little crustacean described by Mr. Goodsir, in the same paper, as a distinct parasite, I believe the latter to be the male of his so-called male Balanus, and that all belong to the same species, allied to Bopyrus. This genus, as is well known, is parasitic on other crustacea; and it is a rather interesting fact thus to find, that this new parasite which is allied to Bopyrus, in structure, is likewise allied to it in habits, living attached to Cirripedia, a sub-class of the crustacea.