Abstract
In the months of April and May 1871 I obtained a supply of the freshwater bivalve Pisidium pusillum , from a muddy little stream near Jena, in Saxe Weimar. The fact that the development of the eggs of this mollusk takes place within a pair of brood-cavities formed at the root of the inner gill-lamella on each side, enables the observer very readily to obtain embryos in different stages of development.

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