express my deliberate opinion, that every part and organ, internal and external, in Cirripedes, is liable to some amount of variation in some of the species. I must now point out the principal changes which supervene during growth, and which cannot properly be called variations. In the first place, I think, it is scarcely possible to recognise a species when under the 1/10th of an inch in diameter. In some cases, as in _var. d'Orbignii_ of _B. tintinnabulum_, the shell is invariably coloured when old, but quite white when very young. Generally the tints become very much darker with age. Some species, which usually or invariably have, when mature, longitudinally folded walls, as _B. flosculus_ and _balanoides_, are perfectly smooth in youth. The walls in _B. eburneus_, when young, have white, hyaline, longitudinal lines, and are naked; whereas, with advancing age, these lines disappear, and the subsequently formed shell becomes covered with membrane. The summits of the radii are apt to be oblique in the young of _B. Capensis_, _psittacus_, and _tintinnabulum_, whereas they are generally quite square in old specimens. In _B. eburneus_, _cariosus_, and in a lesser degree in _B. psittacus_, the scuta become longitudinally striated only with age. On the other hand, in very young specimens of _B. tintinnabulum_, the scuta sometimes are deeply impressed by little pits placed in rows. I have already alluded to the longitudinal furrow on the tergum so entirely changing its character, owing to the edges becoming, during growth, folded inwards; this likewise causes a decrease in the proportional breadth of the spur. In old specimens of _B. flosculus, var. sordidus_, the whole tergum is much more elongated than in young specimens. The basal margin of the sheath is hollow beneath in the young of _B. cariosus_ and of some other species, but in the old it is continuous with the inner surface of the walls. The inner lamina of the parietes generally loses, to a certain extent, its longitudinally ribbed character in old age. The basis is solid, instead of being porose, in very young specimens of _B. improvisus_. In all the species, the carino-lateral compartments, in early age, are very narrow in proportion to the width of the lateral compartments; and in all, at this early period, the operculum is large in proportion to the whole shell. The number of spines on the edge of the maxillæ, the number of segments in all the cirri, and the number of spines on each segment, are few in early youth, and go on increasing with each successive exuviation: the pedicels of the cirri are long in proportion to the rami at this same early age.[84] [84] In some specimens of _Balanus perforatus_ I made the following enumeration of the number of segments in the cirri:--