Page:Natural History, Mollusca.djvu/113

Rh shell or chorion, may be observed changing form. The time required to mature the embryo varies in different species, and probably in the same species under different circumstances; in general from ten to fifteen days is sufficient for this purpose. The development of the embryo is highly interesting, especially as, owing to the transparency of all the organs, nothing is easier than to watch its progress. I have kept many specimens in glass vases of sea water; and as all the species of this Order are hermaphrodite—that is, the sexual functions are united in the same individual—every specimen obtained is pretty sure to spawn during the season. The yolk, which at first nearly fills the egg-shell, soon becomes a little elongated, with one end diagonally truncated, or, as it were, cut off obliquely; the truncated end then becomes two-lobed, "each lobe exhibiting an imperfect spiral, and having its margin ciliated. The now animated being is seen to rotate within its prison. Shortly the lobes enlarge, and a fleshy process, the rudimentary foot, is observed to develop itself a little behind them, on the medial line; a shell closely investing the inferior portion of the embryo, the lobes and rudimentary foot being uppermost. The shell rapidly increases, and assumes a nautiloid form; afterwards the foot displays, attached to its posterior surface, a circular operculum, which is opposed to the mouth of the shell. The lobes now expand into two large, flattened, ovate appendages, with very long vibratile cilia around the margins, and the larvæ are at length mature. The whole mass of spawn now presents the utmost animation. Hundreds of these busy atoms are seen, each within its transparent, membranous cell, rotating with