Almost every one who has walked over a rocky shore knows that a barnacle or acorn-shell is an irregular cone, formed generally of six compartments, with an orifice at the top, closed by a neatly-fitted, moveable lid, or operculum.[19] Within this shell the animal's body is lodged; and through a slit in the lid, it has the power of protruding six pairs of articulated cirri or legs, and of securing by their means any prey brought by the waters within their reach. The basis is firmly cemented to the surface of attachment. The whole shell, basis, and operculum consists, as we have already seen, of the first three segments of the head, modified into a singularly constructed carapace, which encloses the mouth and rest of the body. The anterior extremity of the shell is situated in the centre of the basis, where indeed, by due care, the antennæ of the pupa may be always detected; and the posterior extremity is directed vertically upwards. [19] The best published description of the structure of the shell of a sessile Cirripede, is given by Dr. Coldstream, in the 'Encyclopædia of Anatomy and Physiology,' article 'Cirrhopoda.' _Structure of Shell._ When the shell of a sessile Cirripede or barnacle, for instance, of a Balanus, is first examined, the structure appears extremely complicated; but this can hardly be considered as really the case. The structure will, I think, be best understood by recalling to mind that of Pollicipes,--the oldest known genus, from which, in one sense, all ordinary Cirripedes, both sessile and pedunculated, seem to radiate. I must premise, and the fact in itself deserves early notice, that the homologies of the several parts in the pedunculated and sessile Cirripedes admits of no doubt,--that is, if amongst the pedunculated, the genus Pollicipes, or certain species of Scalpellum, be taken as a standard of comparison.[20] The peduncle corresponds with the basis, as may be clearly seen, if a Pollicipes with a short peduncle, and a Balanus, with a deep cup-formed or cylindrical basis be compared, for the contained parts are similar, and both grow at their upper edges upwards and outwards. Secondly, the valves round the lower part of the capitulum of a Pollicipes, though generally much more numerous, and forming more than one whorl or circle, and not so closely packed together, answer to the compartments forming the shell of a sessile Cirripede; this is shown by their lateral and downward growth, by their upper ends generally projecting freely above the cavity in which the animal's body is lodged; and in the case _Pollicipes mitella_, by an actual resemblance in outline, some being triangular, some broad at the upper end, and some sub-rhomboidal, and, lastly, in the manner in which they slightly overlap and indent each other: moreover further resemblances in the relative position and even in the size of the several valves, will hereafter be pointed out between certain sessile