I may here remark that, in most sessile Cirripedes, the outside membrane connecting the operculum and shell, is regularly moulted. [20] The external integuments being moulted in Crustacea, but not in the Cirripedia, may appear, at first, an important difference: but we here see that non-exuviation is not universal amongst the Lepadidæ, and, on the other hand, according to M. Joly, (‘Annales des Sciences Naturelles,’ 2d series, Zoolog.), there is one true crustacean, the _Isaura cycladoides_, which has a persistent bivalve shell. The delicate tunic lining the sack, (a mere duplicature of that thick one, forming the outside of the capitulum, and generally transformed into valves,) and the integuments of the whole body, are regularly moulted. With these integuments, the membrane lining the oesophagus, the rectum, and the deep olfactory pouches, and the horny apodemes of the maxillæ, are all cast together. I have seen a specimen of Lepas, in which, from some morbid adhesion, the old membrane lining one of the olfactory pouches had not been moulted, but remained projecting from the orifice as a brown shrivelled scroll. The new spines on the cirri (and on the maxillæ) are formed within the old ones; but as they have to be a little longer than the latter, and as they cannot enter these up to their very points, their basal portions are not thus included, but are formed, running obliquely across the segments of the cirri; and what is curious, these same basal portions are turned inside out, like the fingers of a glove when hastily drawn off. After the exuviation of the old spines, the new spines have their inverted basal portions drawn out from within the segments, and turned outside in, so as to assume their proper positions.