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The shells of Mollusca appear to be coeval with the first formation of the animal: they may be observed covering the embryo on its first development in the egg, even before it has acquired its proper shape or any of its internal organs. The accurate Swammerdam observed them in the eggs of several of the garden and pond snails. His observations have been recently verified and extended by Pfeiffer, on many species of land and fresh-water Mollusca; and I have myself observed the same fact in the eggs of several animals belonging to the different orders of marine shells: there is reason, therefore, to believe that this circumstance is general throughout the class. These observations are most easily made on the embryo of the fresh-water shells, such as theLymnœœ, Physœ, Ancyli , andBithyniœ, the eggs of these animals being covered with a transparent coat; while the viviparous Mollusca, and especially theLittorinœ, Paludinœ , andCyclades , offer the additional advantage of exhibiting the embryos of their animals in all the different states of development at the same time. The cephalopodous Mollusca form no exception; their bone, composed of two or three calcareous plates, being found fully developed in the egg of the Cuttle-fish some time before the young animal is hatched.

XXX. Some observations on the economy of molluscous animals, and the structure of their shells