Calliostoma embryos are very stereotypical spiral cleavers: no polar lobe, moderately unequal divisions, and very orderly arrangement of blastomeres. The embryos don't hatch out until they are fairly well-developed veligers, which will swim for a couple weeks before settling. Like a lot of molluscs the eggs are rather yolky and opaque, so they don't lend themselves well to pictures... the cleavage-stage pictures below look so odd because they were taken using a transmitted scope with side lighing, supplemented by some transmitted light through a phase contrast condenser.

These little snails are quite abundant in some dredging areas and will often spawn in response to the following treatment: the snails are placed in finger bowls of clean seawater that is allowed to warm up to room temperature; then the snails are turned over, and turned over again every time they right themselves. After they've had enough of this the males release streams of sperm and females release anywhere from dozens to hundreds of largish green eggs with very thick jelly coats.