[13] Chitine is confined to the Articulata. It was Dr. C. Schmidt (Contributions, &c., being a Physiologico-Chemical investigation: in Taylor’s ‘Scientific Memoirs,’ vol. v), who discovered that the membrane connecting the valves and forming the peduncle, and the tissues of the internal animal, were composed of this substance. But Dr. Schmidt says that the valves in Lepas are composed of 3.09 of albuminates, and 96.81 of incombustible residue; I cannot but think that the existence of the albuminates is an error caused by Dr. Schmidt’s belief that the Cirripedia were intermediate between Crustacea and Mollusca, in the shells of which latter, the animal basis consists of albuminates. For after placing the valves of Lepas and Pollicipes in cold acid, I found that the membrane left could _not be dissolved_ in boiling caustic potash, but could, though slowly, (and without change of colour,) in boiling muriatic acid; and these are the main diagnostic characters of Chitine, compared with albuminous substances. I may add, that Schmidt was also induced to consider the shells of Cirripedia as having the same nature with those of Mollusca, from finding that in the above 96.81 of incombustible matter, 99.3 consisted of carbonate and only 0.7 of phosphate of lime; but Dr. Schmidt’s own analyses prove how extremely variable the proportions of these salts are in the Crustacea, as the following instance shows:-- _Lobster._ _Squilla._ Phosphate of Lime 12.06 47.52 Carbonate of Lime 87.94 52.48 And, therefore, it is not very surprising that Cirripedia should have still less phosphate of lime in their shells, than has a lobster compared with a squilla. Within the capitulum is the sack, which, together with the upper internal part of the peduncle, encloses the animal’s body. The sack is lined by a most delicate membrane of chitine, under which there is a double layer of corium; this double layer is united together by short, strong, transverse bundles of fibres, branched at both ends:[14] in some genera, the ovarian tubes extend between these two layers. We have seen, under the head of the Metamorphoses, that the delicate tunic lining the sack is simply a duplicature of the thick membrane and valves forming the capitulum, the whole being the posterior portion of the carapace of the larva slightly modified. [14] I am much indebted to Mr. Inman of Liverpool for having kindly sent me excellent specimens illustrating this structure.